The moon moves around the earth in the same direction as the earth rotates around its axis. The reflection of this movement, as we know, is the apparent movement of the Moon against the background of the stars towards the rotation of the sky. Every day, the Moon moves to the east relative to the stars by about 13 °, and after 27.3 days it returns to the same stars, having described a full circle on the celestial sphere.

Period of revolution of the moon around the earth relative to the stars(in inertial reference frame) called stellar or sidereal(from lat. sidus - star) month. It is 27.3 days.

The apparent movement of the moon is accompanied by a continuous change in its appearance - phase change. This happens because the Moon occupies different positions relative to the Sun and the Earth that illuminates it. A diagram explaining the change in the phases of the moon is shown in Figure 20.

When the Moon is visible to us as a narrow crescent, the rest of its disk is also slightly glowing. This phenomenon is called ashen light and is explained by the fact that the Earth illuminates the night side of the Moon by reflected sunlight.

The time interval between two successive identical phases of the moon is called the synodic month.(from Greek synodos - connection); is the period of revolution of the moon around the earth relative to the sun. It is (as observations show) 29.5 days.

Thus, the synodic month is longer than the sidereal month. This is easy to understand, knowing that the same phases of the Moon occur at the same positions relative to the Earth and the Sun. In Figure 21, the relative position of the Earth T and the Moon L corresponds to the moment of the new moon. Moon L after 27.3 days, having made a full revolution, will take its previous position relative to the stars. During this time, the Earth T, together with the Moon, will pass along its orbit relative to the Sun an arc TT 1 equal to almost 27 °, since every day it shifts by about 1 °. In order for the Moon L 1 to take its former position relative to the Sun and the Earth T 1 (came to the new moon), it will take another two days. Indeed, the Moon passes 360 ° in a day: 27.3 days = 13 ° / day, in order to pass an arc of 27 °, it needs. 27°: 13°/day=2 days. So it turns out that the synodic month of the Moon is about 29.5 Earth days.

We always see only one hemisphere of the Moon. This is sometimes perceived as the absence of its axial rotation. In fact, this is due to the equality of the periods of rotation of the Moon around its axis and its revolution around the Earth.

Check this by circling an object around you and at the same time o rotating it around an axis with a period equal to the period of the circle.

Rotating around its axis, the Moon alternately turns its different sides towards the Sun. Therefore, on the Moon there is a change of day and night, and the solar day is equal to the synodic period (its revolution relative to the Sun). Thus, on the Moon, the length of a day is equal to two Earth weeks, and our two weeks make up a night there.

It is easy to understand that the phases of the Earth and the Moon are mutually opposite. When the Moon is almost full, the Earth is visible from the Moon as a narrow crescent. Figure 42 shows a photograph of the sky and the lunar horizon with the Earth, in which only its illuminated part is visible - less than a semicircle.

Exercise 5

1. The crescent of the Moon in the evening is bulging to the right and close to the horizon. On which side of the horizon is it?

2. Today the upper climax of the Moon occurred at midnight. When is the next upper climax of the moon?

3. At what time intervals do stars culminate on the Moon?

2. Lunar and solar eclipses

The Earth and the Moon, illuminated by the Sun (Fig. 22), cast cones of shadow (convergent) and cones of penumbra (divergent). When the Moon falls into the shadow of the Earth, in whole or in part, complete or partial eclipse of the moon. From the Earth, it can be seen simultaneously from everywhere where the Moon is above the horizon. The phase of a total eclipse of the Moon continues until the Moon begins to emerge from the Earth's shadow, and can last up to 1 hour 40 minutes. The sun's rays, refracted in the Earth's atmosphere, fall into the cone of the earth's shadow. In this case, the atmosphere strongly absorbs the blue and neighboring rays (see Fig. 40), and transmits mainly red ones into the cone, which are absorbed more weakly. That is why the Moon, during a large phase of the eclipse, turns reddish, and does not disappear altogether. In the old days, an eclipse of the moon was feared as a terrible omen, it was believed that "the month bleeds." Lunar eclipses occur up to three times a year, separated by almost half a year intervals, and, of course, only on a full moon.

A solar eclipse can only be seen as a total eclipse when a spot of the moon's shadow falls on Earth.. The spot diameter does not exceed 250 km, and therefore a total eclipse of the Sun is simultaneously visible only on a small part of the Earth. When the Moon moves in its orbit, its shadow moves across the Earth from west to east, drawing a successively narrow band of total eclipse (Fig. 23).

Where the Moon's penumbra falls on the Earth, there is a partial eclipse of the Sun.(Fig. 24).

Due to a slight change in the distances of the Earth from the Moon and the Sun, the apparent angular diameter of the Moon is either slightly larger, or slightly less than the solar one, or equal to it. In the first case, the total eclipse of the Sun lasts up to 7 min 40 s, in the third - only one instant, and in the second case, the Moon does not completely cover the Sun at all, it is observed annular eclipse. Then, around the dark disk of the Moon, a shining rim of the solar disk is visible.

Based on an accurate knowledge of the laws of motion of the Earth and the Moon, the moments of eclipses and where and how they will be visible are calculated for hundreds of years ahead. Maps have been compiled showing the band of a total eclipse, lines (isophases) where the eclipse will be visible in the same phase, and lines relative to which for each locality one can count the moments of the beginning, end and middle of the eclipse.

Solar eclipses per year for the Earth can be from two to five, in the latter case, certainly private. On average, in the same place, a total solar eclipse is seen extremely rarely - only once in 200-300 years.

Of particular interest to science are total eclipses of the Sun, which previously inspired superstitious horror in ignorant people. Such eclipses were considered an omen of war, the end of the world.

Astronomers undertake expeditions to the total eclipse band in order to study the outer rarefied shells of the Sun, invisible directly outside the eclipse, for seconds, rarely minutes of the total phase. During a total solar eclipse, the sky darkens, a glowing ring burns along the horizon - the glow of the atmosphere illuminated by the rays of the Sun in areas where the eclipse is incomplete, the pearl rays of the so-called solar corona stretch around the black solar disk (see Fig. 69).

If the plane of the lunar orbit coincided with the plane of the ecliptic, then on every new moon there would be a solar eclipse, and on every full moon a lunar eclipse. But the plane of the lunar orbit crosses the plane of the ecliptic at an angle of 5 ° 9. Therefore, the Moon usually passes north or south of the ecliptic plane, and eclipses do not occur. , an eclipse is possible.

The plane of the lunar orbit rotates in space (this is one of the types of perturbations in the motion of the Moon produced by the attraction of the Sun) * and makes a complete turn in 18 years. Therefore, the periods of possible eclipses are shifted according to the dates of the year. Scientists of antiquity noticed the periodicity in eclipses associated with this 18-year period, and could therefore approximately predict the onset of eclipses. Now the errors in the prediction of the eclipse moments are less than 1 s.

Information about upcoming eclipses and the conditions of their visibility are given in the "School Astronomical Calendar".

Exercise 6

1. There was a full moon yesterday. Could there be a solar eclipse tomorrow? a week later?

2. The day after tomorrow there will be a solar eclipse. Will there be a moonlit night tonight?

3. Is it possible to observe the solar eclipse on November 15 from the North Pole of the Earth? April 15? Explain the answer.

4. Is it possible to see lunar eclipses in June and November from the North Pole of the Earth? Explain the answer.

5. How to distinguish the phase of the eclipse of the Moon from one of its usual phases?

6. What is the duration of solar eclipses on the Moon compared to their duration on Earth?

Young or old month?

Seeing the incomplete disk of the Moon in the sky, not everyone will accurately determine whether it is a young month or whether it is already in decline. The narrow crescent of the newly born month and the crescent of the old Moon differ only in that they are bulging in opposite directions. In the northern hemisphere, the young month is always directed with its convex side to the right, the old one to the left. How to remember reliably and accurately where which month looks?

Let me suggest such an omen.

By the similarity of the sickle or crescent with letters R or WITH it is easy to determine whether the month before us is growing (i.e., young) or old .

The French also have a mnemonic sign. They advise mentally attaching a straight line to the horns of the crescent; get latin letters d or p. Letter d- initial in the word "dernier" (last) - indicates the last quarter, i.e. the old month. Letter R - the initial in the word "premier" (first) - indicates that the Moon is in the phase of the first quarter, in general - young. The Germans also have a rule that associates the shape of the moon with certain letters.

These rules can only be used in the northern hemisphere of the Earth. For Australia or the Transvaal, the meaning will be just the opposite. But even in the northern hemisphere, they may not be applicable - namely in the southern latitudes.

Already in the Crimea and Transcaucasia, the sickle and crescent tilt strongly to one side, and further south they lie down completely. Near the equator, the crescent of the moon hanging on the horizon seems either to be a gondola swaying on the waves (“the shuttle of the moon” of Arabic fairy tales) or a bright arch. Neither Russian nor French signs are suitable here - both pairs of letters can be made from a recumbent bow if desired: R And C, r And d.

In order not to be mistaken in the age of the Moon in this case, one must turn to astronomical signs: the young month is visible in the evening in the western part of the sky; old - in the morning in the eastern part of the sky.

moon on flags

On fig. 30 in front of us is the flag of Turkey (former). It has an image of a crescent moon and a star. This leads us to the following questions:

1. Which month's sickle is depicted on the flag - young or old?

2. Can the lunar crescent and star be observed in the sky in the form in which they are shown on the flag?

Rice. 30. Flag of Turkey (former).

1. Remembering the sign just mentioned and taking into account that the flag belongs to the country of the northern hemisphere, we establish that the month on the flag old.

Rice. 31. Why the star cannot be seen between the horns of the month

2. The star cannot be seen inside the disk of the Moon, completed to a circle (Fig. 31, A). All heavenly bodies are much further than the Moon and, therefore, must be obscured by it. They can only be seen beyond the edge of the dark part of the Moon, as shown in Fig. 31,6.

It is curious that on the modern flag of Turkey, which also contains the image of a lunar crescent and a star, the star is moved away from the crescent exactly as in Fig. 31, b.

Riddles of the lunar phases

The moon receives its light from the sun, and therefore the convex side of the crescents must, of course, be turned towards the sun. Artists often forget this. At art exhibitions, it is not uncommon to see a landscape with a crescent moon facing the Sun with its straight side; there is also a lunar sickle, turned to the Sun with its horns (Fig. 32).

Rice. 32. An astronomical error has been made on the landscape. Which? (Answer in text).

However, it should be noted that drawing a young month correctly is not as easy as it seems. Even experienced artists draw the outer and inner arcs of the crescent moon in the form of semicircles (Fig. 33, b). Meanwhile, only the outer arc has a semicircular shape, while the inner one is a semi-ellipse, because it is a semicircle (the border of the illuminated part), visible in perspective (Fig. 33, A).

Rice. 33. How to (a) and how not to (b) depict a crescent moon

It is not easy to give the crescent moon and the correct position in the sky. The crescent and crescent moon are often placed in relation to the Sun in a rather puzzling way. It would seem that since the Moon is illuminated by the Sun, then the straight line connecting the ends of the month should form a right angle with the beam going from the Sun to its middle (Fig. 34).

Rice. 34. The position of the crescent moon relative to the Sun

In other words, the center of the Sun must be on a perpendicular drawn through the middle of the straight line connecting the ends of the month. However, this rule is observed only for a narrow crescent located near the Sun. On fig. 35 shows the position of the month in different phases relative to the rays of the Sun. The impression is obtained such as if the rays of the Sun are bent before reaching the Moon.

Rice. 35. In what position relative to the Sun do we see the Moon in different phases.

The solution lies in the following. The ray going from the Sun to the Moon is actually perpendicular to the line connecting the ends of the month, and in space is a straight line. But our eye draws in the sky not this straight line, but its projection onto the concave firmament, that is, a curved line. That is why it seems to us that the Moon in the sky is "wrongly hung." The artist must study these features and be able to transfer them to the canvas.

double planet

The double planet is the Earth with the Moon. They have the right to this name because our satellite stands out sharply among the satellites of other planets with a significant size and mass in relation to its central planet. There are absolutely larger and heavier satellites in the solar system, but compared to their central planet, they are much smaller than our Moon in relation to the Earth. In fact, the diameter of our Moon is more than a quarter of the earth's, and the diameter relative to the largest satellite of other planets is only 10th of the diameter of its planet (Triton is a satellite of Neptune). Further, the mass of the Moon is 1/81 of the mass of the Earth; meanwhile, the heaviest of the satellites that exists in the solar system, the third satellite of Jupiter, is less than 10,000th of the mass of its central planet.

What fraction of the mass of the central planet is the mass of large satellites is shown by the plate on page 86. You can see from this comparison that our Moon, in terms of its mass, makes up the largest fraction of its central planet.

The third thing that gives the Earth-Moon system the right to claim the name of a "double planet" is the close proximity of both celestial bodies. Many satellites of other planets circle at much greater distances: some satellites of Jupiter (for example, the ninth, Fig. 36) circle 65 times further.

Rice. 36. The Earth-Moon system compared to the Jupiter system (the sizes of the celestial bodies themselves are shown not to scale)

In connection with this is the curious fact that the path described by the Moon around the Sun differs very little from the path of the Earth. This will seem incredible if you remember that the Moon moves around the Earth at a distance of almost 400,000 km. Let us not forget, however, that while the Moon makes one revolution around the Earth, the Earth itself manages to be transported along with it by about the 13th part of its annual path, i.e., by 70,000,000 km. Imagine a circular path of the Moon - 2,500,000 km - stretched along a distance 30 times greater. What will be left of its circular shape? Nothing. That is why the path of the Moon near the Sun almost merges with the orbit of the Earth, deviating from it only by 13 barely noticeable protrusions. It can be proved by a simple calculation (with which we will not burden the presentation here) that the path of the Moon in this case is everywhere turned towards the Sun of its concavity . Roughly speaking, it looks like a convex thirteen-sided triangle with softly rounded corners.

On fig. 37 you see an accurate depiction of the paths of the Earth and the Moon over the course of one month. The dotted line is the path of the Earth, the solid line is the path of the Moon. They are so close to each other that for their separate image it was necessary to take a very large scale of the drawing: the diameter of the earth's orbit is equal here? If we take 10 cm for it, then the largest distance in the drawing between both paths would be less than the thickness of the lines depicting them. Looking at this drawing, you are clearly convinced that the Earth and the Moon move around the Sun along almost the same path and that the name of a double planet is appropriated to them by astronomers quite rightly.

Rice. 37. Monthly path of the Moon (solid line) and the Earth (dotted line) around the Sun

So, for an observer placed on the Sun, the path of the Moon would appear to be a slightly wavy line, almost coinciding with the orbit of the Earth. This does not in the least contradict the fact that the Moon moves in a small ellipse with respect to the Earth.

The reason, of course, is that, looking from the Earth, we do not notice the portable movement of the Moon along with the Earth along the Earth's orbit, since we ourselves participate in it.

Why doesn't the moon fall on the sun?

The question may seem naive. Why would the moon fall on the sun? After all, the Earth attracts it stronger than the distant Sun and, naturally, makes it revolve around itself.

Readers who think so will be surprised to learn that the opposite is true: the Moon is more strongly attracted by the Sun than by the Earth!

That this is so, the calculation shows. Let's compare the forces that attract the Moon: the force of the Sun and the force of the Earth. Both forces depend on two circumstances: on the magnitude of the attracting mass and on the distance of this mass from the Moon. The mass of the Sun is 330,000 times greater than the mass of the Earth; the Sun would attract the Moon more strongly than the Earth if the distance to the Moon were the same in both cases.

But the Sun is about 400 times farther from the Moon than the Earth. The force of attraction decreases with the square of the distance; therefore, the attraction of the Sun must be reduced by a factor of 400 2, i.e., by a factor of 160,000. This means that the solar attraction is stronger than the earth's one by 330,000/160,000, that is, more than two times.

So, the Moon is attracted by the Sun twice as much as the Earth. Why, then, in fact, the Moon does not collapse on the Sun? Why does the Earth still make the Moon revolve around it, and not the action of the Sun takes over?

The Moon does not fall on the Sun for the same reason that the Earth does not fall on it; The moon revolves around the sun together with the earth, and the attractive action of the sun is spent without a trace in constantly transferring both of these bodies from a straight path to a curved orbit, i.e., turning a rectilinear motion into a curvilinear one. It suffices to glance at Fig. 38 to verify what has been said.

Other readers may have some doubts. How does it come out anyway? The earth is pulling the moon towards it. The sun pulls the moon with more force, and the moon, instead of falling on the sun, circles around the earth? This, indeed, would be strange if the Sun attracted only the Moon to itself. But it attracts the Moon along with the Earth, the entire "double planet", and, so to speak, does not interfere with the internal relations of the members of this pair with each other. Strictly speaking, the common center of gravity of the Earth-Moon system is attracted to the Sun; this center (called the barycenter) revolves around the Sun under the influence of solar attraction. It is located at a distance of 2/3 of the earth's radius from the center of the earth towards the moon. The Moon and the center of the Earth revolve around the barycenter, making one revolution every month.

The visible and invisible sides of the moon

Among the effects delivered by a stereoscope, nothing is more striking than the sight of the moon. Here you see with your own eyes that the moon is really spherical, while in the real sky it seems flat, like a tea tray.

But how difficult it is to get such a stereoscopic photograph of our satellite, many do not even suspect. To make it, one must be well acquainted with the peculiarities of the capricious movements of the night luminary.

The fact is that the Moon bypasses the Earth in such a way that it is turned to it all the time with the same side. Running around the Earth, the Moon rotates at the same time around its axis, and both movements are completed in the same period of time.

On fig. 38 you see an ellipse, which should visually depict the orbit of the moon. The drawing deliberately enhances the elongation of the lunar ellipse; in fact, the eccentricity of the lunar orbit is 0.055 or 1/18. It is impossible to represent the lunar orbit exactly on a small drawing so that the eye distinguishes it from a circle: with a major semi-axis even a whole meter, the minor semi-axis would be shorter than it by only 1 mm; The earth would be only 5.5 cm from the center. To make it easier to understand the further explanation, a more elongated ellipse is drawn in the figure.

Rice. 38. How the Moon moves around the Earth in its orbit (details in the text)

So imagine that the ellipse in Fig. 38 is the path of the Moon around the Earth. The earth is placed at a point ABOUT - at one of the foci of the ellipse. Kepler's laws apply not only to the movements of the planets around the Sun, but also to the movements of satellites around the central planets, in particular to the revolution of the Moon. According to Kepler's second law, the moon travels this way in a quarter of a month AE, what area OABCDE equals 1/4 of the area of ​​the ellipse, i.e. the area MABCD(equality of areas UAE And M.A.D. in our drawing is confirmed by the approximate equality of the areas MOQ And EQD). So, in a quarter of a month, the moon travels from A before E. The rotation of the Moon, as well as the rotation of the planets in general, in contrast to their circulation around the Sun, occurs evenly: in 1/4 of a month it rotates exactly 90 °. So when the moon is in E, the radius of the moon facing the earth at a point A, will describe an arc of 90°, and will be directed not to a point M, and to some other point, to the left M, close to another focus R lunar orbit. Because the moon slightly turns its face away from the earthly observer, he will be able to see on the right side a narrow strip of its previously invisible half. At the point Elupa shows the earthly observer an already narrower strip of its usually invisible side, because the angle OFP less than an angle OEP. At the point G- at the apogee of the orbit - the Moon occupies the same position in relation to the Earth as at perigee A. With its further movement, the Moon turns away from the Earth in the opposite direction, showing our planet another strip of its invisible side: this strip first expands, then narrows, and at the point A The moon is in its original position.

We have seen that, due to the elliptical shape of the lunar path, our satellite does not face the Earth with its strictly one and the same half. The moon invariably faces the same side not to the Earth, but to another focus of its orbit. For us, it sways about the middle position like a balance; hence the astronomical name for this wiggle: "libration" - from the Latin word "libra", meaning "scales". The amount of libration at each point is measured by the corresponding angle; for example, at the point is, the libration is equal to the angle OEP. The largest libration is 7°53?, i.e., almost 8°.

It is interesting to follow how the angle of libration increases and decreases with the movement of the Moon in its orbit. Let's put in D the tip of the compass and describe the arc passing through the foci ABOUT And R. It will cross the orbit at points B and F. corners OVR And OFP as inscribed equal to half the central angle ODP. From this we deduce that when the Moon moves from A before D libration grows rapidly at first, at the point IN reaches half the maximum, then continues to increase slowly; on the way from D before F libration decreases slowly at first, then rapidly. On the second half of the ellipse, the libration changes its value at the same rate, but in the opposite direction. (The amount of libration at each point in the orbit is approximately proportional to the distance of the Moon from the major axis of the ellipse.)

That wobble of the Moon, which we have now considered, is called libration in longitude. Our satellite is also subject to another libration - in latitude. The plane of the lunar orbit is inclined to the plane of the Moon's equator by 6°. Therefore, we see the Moon from the Earth in some cases a little from the south, in others - from the north, looking a little into the "invisible" half of the Moon through its poles. This libration in latitude reaches 6°.

Let us now explain how the astronomer-photographer makes use of the described slight wobbles of the Moon about its mean position in order to obtain stereoscopic pictures of it. The reader probably guesses that for this it is necessary to watch for two such positions of the Moon, in which in one it would be rotated with respect to the other by a sufficient angle. At points A and B, B and C, C and D and etc. The Moon occupies positions so different from the Earth that stereoscopic images are possible. But here we face a new difficulty: in these positions, the difference in the age of the Moon, 1? -2 days, is too great, so that the strip of the lunar surface near the circle of illumination in one picture is already emerging from the shadow. This is unacceptable for stereoscopic images (the strip will shine like silver). A difficult task arises: to watch for the same phases of the moon, which differ in the amount of libration (in longitude) so that the circle of illumination passes through the same details of the lunar surface. But even this is not enough: in both positions there must still be the same librations in latitude.

Our reader is unlikely to produce lunar stereophotographs. The method of obtaining them is explained here, of course, not with a practical purpose, but only in order to consider the features of the lunar movement, which give astronomers the opportunity to see a small strip of the side of our satellite that is usually inaccessible to the observer. Thanks to both lunar librations, we see, in general, not half of the entire lunar surface, but 59% of it. Before the launch of the third space rocket in the direction of the Moon in the Soviet Union, 41% of the lunar surface was inaccessible to study.

How this part of the surface of the moon is arranged, no one knew. Witty attempts were made, by continuing backwards parts of the lunar ridges and light stripes, passing from the invisible part of the Moon to the visible, to sketch some guesswork of the details of the half inaccessible to us. As a result of the launch of the Luna-3 automatic interplanetary station on October 4, 1959, photographs of the far side of the Moon were obtained. Soviet scientists were given the right to give names to newly discovered lunar formations. The craters are named after prominent figures of science and culture - Lomonosov, Tsiolkovsky, Joliot-Curie and others, and named after two new seas - the Sea of ​​Moscow and the Sea of ​​Dreams. The far side of the Moon was photographed for the second time by the Soviet Zond-3 station, launched on July 18, 1965.

In 1966, Luna 9 landed softly on the moon and transmitted back to Earth an image of the lunar landscape. In 1969, the lunar Sea of ​​Tranquility had to be disturbed. The landing cabin of the American spacecraft Apollo 11 landed on the dry bottom of this "sea". Astronauts Neil Armstrong and Edwin Aldrin became the first humans to walk on the moon. They installed several instruments, took samples of the lunar soil and returned to the ship, which was waiting for them in orbit. Apollo 11 was piloted by Michael Collins. Until the end of 1972, five more American expeditions visited the Moon.

At the same time, automatic stations were launched to the Moon in the USSR. In 1970, Luna 16, having landed on the surface of the Moon, took samples of lunar soil for the first time and delivered them to Earth. In the same year, Luna-17 launched the self-propelled Lunokhod-1 onto the surface of our satellite. This eight-wheeled robot, which looks like a turtle and an army field kitchen at the same time, traveled almost 11 kilometers in 301 days and transmitted 20,000 images, 200 panoramas and conducted soil research at 500 points to Earth.

A little later, Luna-20 brought soil samples to Earth from the mountainous region of the Moon, inaccessible to astronauts. In 1973, Luna-21 sent Lunokhod-2 on a campaign, which traveled 37 km in 4.5 months, exploring the terrain and soil composition. Both wheeled robots were controlled from the Earth by radio and systematically transmitted to the MCC pictures of lunar landscapes, the results of soil analysis. Automatic station "Luna-24" (1976) drilled lunar soil to a depth of 2 m and delivered 170 g of its samples to Earth.

The often expressed idea about the existence of atmosphere and water on the far side of the Moon is not justified and contradicts the laws of physics: if there is no atmosphere and water on one side of the Moon, then there cannot be them on the other (we will return to this issue).

The second moon and the moon of the moon

Reports appear in the press from time to time that this or that observer managed to see the second satellite of the Earth, its second Moon.

The question of the existence of a second satellite of the Earth is not new. It has a long history behind it. Anyone who has read Jules Verne's novel "From the Cannon to the Moon" probably remembers that the second moon is already mentioned there. It is so small and its speed is so great that the inhabitants of the Earth cannot observe it. The French astronomer Petit, - says Jules Berne, - suspected its existence and determined the period of its revolution around the Earth at 3 hours 20 m. Its distance from the Earth's surface is 8140 km. It is curious that the English journal Znanie, in an article on astronomy by Jules Verne, considers the reference to Petit, as well as Petit himself, to be simply fictitious. This astronomer is not really mentioned in any encyclopedia. Yet the novelist's message is not fictitious. In the 1950s, the director of the Toulouse Observatory, Petit, really defended the existence of a second moon, a meteorite with an orbital period of 3 hours 20 meters, circling, however, not 8,000, but 5,000 km from the earth's surface. This opinion was shared even then by only a few astronomers, but later it was completely forgotten. Theoretically, there is nothing unscientific in assuming the existence of a second, very small satellite of the Earth. But such a celestial body would have to be observed not only in those rare moments when it passes (seemingly) across the disk of the Moon or the Sun. Even if it turns so close to the Earth that it must plunge into the wide earth's shadow with each revolution, then even in this case it would be possible to see it in the morning and evening sky shining as a bright star in the rays of the Sun. With its rapid movement and frequent returns, this star would have attracted the attention of many observers. At the moments of a total solar eclipse, the second moon would also not have escaped the gaze of astronomers. In a word, if the Earth really had a second satellite, it would happen to be observed quite often. Meanwhile, there were no indisputable observations.

Strictly speaking, the Earth has, in addition to the Moon, two more satellites. Not artificial, but completely natural. And not tiny, but the same size as the moon itself. But, although these "Moons" were discovered a long time ago (in 1956, by the Polish astronomer Kordylewski), very few people managed to see them. The thing is that these satellites are entirely composed of dust. These dusty "Moons" move among the stars along the same track as the real Moon, and at the same speed. One is 60 degrees ahead of the Moon, the other is 60 degrees behind. And they are separated from the Earth by the same distance as the Moon. The edges of these "Moons" are blurred, making it very difficult to see.

Along with the problem of the second Moon, the question was also raised whether our Moon has its own small satellite - the “moon of the Moon”.

But it is very difficult to directly ascertain the existence of such a lunar satellite. The astronomer Multon expresses the following considerations about this:

“When the Moon shines with full light, its light or the light of the Sun does not make it possible to distinguish a very small body in its neighborhood. Only at the moments of lunar eclipses could the Moon's satellite be illuminated by the Sun, while neighboring parts of the sky would be free from the influence of the scattered light of the Moon. Thus, only during lunar eclipses could one hope to discover a small body orbiting the moon. Studies of this kind have already been carried out, but have not yielded real results.”

Why is there no atmosphere on the moon?

This question belongs to those which are cleared up if they are first, so to speak, reversed. Before we talk about why the Moon does not hold an atmosphere around it, let's pose the question: why does the atmosphere around our own planet hold? Recall that air, like any gas, is a chaos of unrelated molecules rapidly moving in different directions. Their average speed at t = 0 °C - about? km per second (speed of a rifle bullet). Why don't they scatter into the world space? For the same reason that a rifle bullet does not fly into space. Having exhausted the energy of their movement to overcome gravity, the molecules fall back to the Earth. Imagine a molecule near the earth's surface, flying vertically upward with speed? km per second. How high can she fly? It is easy to calculate: speed v, lift height h and the acceleration of gravity g linked by the following formula:

v 2 = 2gh.

Let us substitute instead of v its value - 500 m/s, instead of g- 10 m / s 2, we have

h = 12 500 m = 12 km.

But if air molecules cannot fly above 12? km, then where do the air molecules above this boundary come from? After all, oxygen, which is part of our atmosphere, was formed near the earth's surface (from carbon dioxide as a result of plant activity). What force has lifted and keeps them at an altitude of 500 kilometers or more, where the presence of traces of air has been unconditionally established? Physics gives here the same answer that we would hear from a statistician if we asked him: “The average duration of human life is 70 years; Where do 80 year olds come from? The thing is that our calculation refers to an average, not a real molecule. The average molecule has a second speed of ? km, but real molecules move some more slowly, others faster than the average. True, the percentage of molecules whose velocity deviates noticeably from the average is small and rapidly decreases with increasing magnitude of this deviation. Of the total number of molecules contained in a given volume of oxygen at 0°, only 20% have a speed of 400 to 500 meters per second; approximately the same number of molecules move at a speed of 300-400 m/s, 17% - at a speed of 200-300 m/s, 9% - at a speed of 600-700 m/s, 8% - at a speed of 700-800 m/s, 1% - at a speed of 1300–1400 m/s. A small part (less than a millionth) of the molecules has a speed of 3500 m/s, and this speed is sufficient for the molecules to fly even to a height of 600 km.

Really, 3500 2 = 20h, where h=12250000/20 i.e. over 600 km.

The presence of oxygen particles at an altitude of hundreds of kilometers above the earth's surface becomes clear: this follows from the physical properties of gases. The molecules of oxygen, nitrogen, water vapor, carbon dioxide, however, do not have speeds that would allow them to completely leave the globe. This requires a speed of at least 11 km per second, and only single molecules of these gases have such speeds at low temperatures. That is why the Earth holds its atmospheric shell so firmly. It is calculated that for the loss of half of the supply of even the lightest of the gases of the earth's atmosphere - hydrogen - a number of years, expressed in 25 digits, must pass. Millions of years will not make any change in the composition and mass of the earth's atmosphere.

In order to explain now why the Moon cannot keep a similar atmosphere around it, it remains to say a little.

The pull of gravity on the Moon is six times weaker than on Earth; accordingly, the speed required to overcome the force of gravity there is also less and is only 2360 m/s. And since the speed of oxygen and nitrogen molecules at a moderate temperature can exceed this value, it is clear that the Moon would have to continuously lose its atmosphere if it were to form one.

When the fastest of the molecules escape, other molecules will acquire a critical speed (this is a consequence of the law of distribution of velocities between gas particles), and more and more particles of the atmospheric shell must irrevocably escape into the world space.

After a sufficient period of time, negligible on the scale of the universe, the entire atmosphere will leave the surface of such a weakly attracting celestial body.

It can be proved mathematically that if the average velocity of molecules in the planet's atmosphere is even three times less than the limiting one (i.e., it is 2360: 3 = 790 m/s for the Moon), then such an atmosphere should dissipate by half within a few weeks. (The atmosphere of a celestial body can be sustained only if the average speed of its molecules is less than one-fifth of the maximum speed.) The idea was expressed - or rather, the dream - that in time, when earthly humanity visits and conquers the Moon, it will surround it with an artificial atmosphere and make it habitable. After what has been said, the unrealizability of such an enterprise should be clear to the reader.

The absence of an atmosphere in our satellite is not an accident, not a whim of nature, but a natural consequence of physical laws.

It is also clear that the reasons for which the existence of an atmosphere on the Moon is impossible should determine its absence in general on all world bodies with a weak gravity force: on asteroids and on most satellites of the planets.

Dimensions of the lunar world

This, of course, is indicated with complete certainty by numerical data: the magnitude of the diameter of the Moon (3500 km), surface, volume. But numbers, indispensable in calculations, are powerless to give that visual representation of the dimensions that our imagination requires. It will be useful to refer to specific comparisons for this.

Let's compare the lunar continent (after all, the Moon is a continuous continent) with the continents of the globe (Fig. 39). This will tell us more than the abstract statement that the total surface of the lunar globe is 14 times smaller than the earth's surface. In terms of the number of square kilometers, the surface of our satellite is only slightly smaller than the surface of both Americas. And that part of the moon that faces the Earth and is available to our observation is almost exactly equal to the area of ​​South America.

Rice. 39. Dimensions of the Moon compared to the mainland of Europe (it should not, however, be concluded that the surface of the lunar ball is smaller than the surface of Europe)

To visualize the dimensions of the lunar "seas" compared to those of the earth, in Fig. 40 the contours of the Black and Caspian Seas are superimposed on the map of the Moon on the same scale. It is immediately clear that the lunar "seas" are not particularly large, although they occupy a noticeable part of the disk. Sea of ​​Clarity, for example (170,000 km 2 ), at about 2? times smaller than the Caspian.

But among the ring mountains of the Moon there are genuine giants, which are not on Earth. For example, the circular rampart of Mount Grimaldi covers a surface larger than the area of ​​Lake Baikal. Inside this mountain, a small state, for example, Belgium, or Switzerland, could fit entirely.

Rice. 40. Terrestrial seas compared with the lunar. The Black and Caspian Seas, transferred to the Moon, would be there more than all the lunar seas (the numbers indicate: 1 - the Sea of ​​\u200b\u200bRains, 2 - the Sea of ​​\u200b\u200bClarity, 3 - the Sea of ​​\u200b\u200bTranquility, 4 - the Sea of ​​​​Plenty, 5 - the Sea of ​​\u200b\u200bNectar)

Lunar landscapes

Photographs of the lunar surface are reproduced in books so often that the appearance of the characteristic features of the lunar relief - ring mountains (Fig. 41), "craters" - is probably familiar to each of our readers. It is possible that others observed the lunar mountains through a small tube; a tube with a 3 cm lens is sufficient for this.

Rice. 41. Typical Ring Mountains of the Moon

But neither photographs nor telescope observations give an idea of ​​what the lunar surface would look like to an observer on the Moon itself. Standing directly next to the lunar mountains, the observer would see them in a different perspective than through a telescope. It is one thing to look at an object from a great height, and quite another to look at it from the side. Let us show by several examples how this difference manifests itself. Mount Eratosthenes appears from Earth as an annular shaft with a peak inside. In a telescope, it appears in relief and sharply thanks to clear, unblurred shadows. Take a look, however, at its profile (Fig. 42): you see that compared to the huge diameter of the crater - 60 km - the height of the shaft and the inner cone is very small; the gentleness of the slopes even more hides their height.

Rice. 42. Profile of the big ring mountain

Imagine yourself now wandering inside this crater and remember that its diameter is equal to the distance from Lake Ladoga to the Gulf of Finland. You can hardly catch then the annular shape of the shaft; moreover, the convexity of the soil will hide its lower part from you, since the lunar horizon is twice as narrow as the earth's (correspondingly, the diameter of the lunar ball is four times smaller). On Earth, a person of average height, standing on a flat area, can see around him no further than 5 km. This follows from the horizon distance formula

Where D- distance in km, h- eye height in km, R- radius of the planet in km.

Substituting the data for the Earth and the Moon into it, we find out that for a person of average height, the range of the horizon

on Earth………,4.8 km,

on the Moon……….2.5 km.

What kind of picture would appear to an observer inside a large lunar crater, Fig. 43. (The landscape is depicted for another large crater - Archimedes.) Isn't it true: a vast plain with a chain of hills on the horizon bears little resemblance to what is usually imagined with the words "lunar crater"?

Rice. 43. What picture would an observer standing in the center of a large ring mountain on the Moon see?

Finding himself on the other side of the shaft, outside the crater, the observer would also see not what he expects. The outer slope of the ring mountain (cf. Fig. 42) rises so gently that it does not appear to the traveler as a mountain at all, and most importantly, he will not be able to make sure that the hilly ridge he sees is a ring mountain with a round basin. To do this, you will have to get over its crest, and here, as we have already explained, the lunar climber does not expect anything remarkable.

In addition to the huge annular lunar mountains, there are, however, many small craters on the Moon, which are easy to capture with a glance, even standing in close proximity. But their height is insignificant; the observer will hardly be struck by anything extraordinary here. On the other hand, the lunar mountain ranges, which bear the name of the earthly mountains: the Alps, the Caucasus, the Apennines, etc., compete with the earthly ones in height and reach 7–8 km. On a relatively small moon, they look quite impressive.

Rice. 44. Half a pea casts a long shadow in oblique lighting

The absence of an atmosphere on the Moon and the resulting sharpness of the shadows create a curious illusion when viewed through a chimney: the slightest irregularities in the soil are intensified and appear to be very prominent. Lay half of the pea with the bulge up. Is she big? And look what a long shadow it casts (Fig. 44). With side illumination on the Moon, the shadow is 20 times the height of the body that casts it, and this has served astronomers well: thanks to long shadows, objects 30 m high can be observed with a telescope on the Moon. But the same circumstance makes us exaggerate the irregularities of the lunar soil. Mount Pico, for example, is so sharply outlined through a telescope that one involuntarily imagines it as a sharp and steep rock (Fig. 45). This is how she was portrayed in the past. But, observing it from the lunar surface, you would see a completely different picture - what is shown in Fig. 46.

But other features of the lunar relief, on the contrary, are underestimated by us. Through a telescope, we observe thin, barely noticeable cracks on the surface of the Moon, and it seems to us that they cannot play a significant role in the lunar landscape.

Rice. 45. Mount Pico used to be considered steep and sharp.

Rice. 46. ​​In fact, Mount Pico has very gentle slopes.

Rice. 47. The so-called "Straight Wall" on the Moon; view through a telescope

But transferred to the surface of our satellite, we would see in these places at our feet a deep black abyss, stretching far beyond the horizon. Another example. There is a so-called "Straight Wall" on the Moon - a sheer ledge that cuts through one of its plains. Seeing this wall through a telescope (Fig. 47), we forget that it is 300 m high; being at the base of the wall, we would be overwhelmed by its immensity. On fig. 48 the artist tried to depict this sheer wall, visible from below: its end is lost somewhere beyond the horizon: after all, it stretches for 100 km! In the same way, thin cracks, discerned in a strong telescope on the lunar surface, should in nature represent huge dips (Fig. 49).

Rice. 48. What should the “Straight Wall” look like to an observer located near its base

Rice. 49. One of the lunar "cracks", observed in close proximity.

moon sky

black firmament

If an inhabitant of the Earth could find himself on the Moon, three extraordinary circumstances would attract his attention first of all.

The strange color of the daytime sky on the Moon would immediately catch your eye: instead of the usual blue dome, a completely black sky would be spread, dotted with the bright radiance of the Sun! - a lot of stars that stand out clearly, but do not twinkle at all. The reason for this phenomenon is the absence of an atmosphere on the Moon.

“The blue vault of a clear and pure sky,” Flammarion says in his characteristic picturesque language, “the gentle blush of dawn, the majestic glow of evening twilight, the enchanting beauty of deserts, the foggy distance of fields and meadows, and you, the mirror waters of lakes, reflecting distant azure skies from ancient times containing a whole infinity in their depths - your existence and all your beauty depend solely on that light shell that extends over the globe. Without her, none of these paintings, none of these magnificent colors would exist. Instead of an azure blue sky, you would be surrounded by boundless black space; instead of majestic sunrises and sunsets, days would abruptly, without transitions, be replaced by nights and nights - days. Instead of a gentle half-light that reigns everywhere where the dazzling rays of the Sun do not directly fall, there would be bright light only in places directly illuminated by the daylight, and in all the rest a thick shadow would reign.

Earth in the sky of the moon

The second attraction on the Moon is a huge disk of the Earth hanging in the sky. It will seem strange to the traveler that the globe, which, when flying to the moon, was left at the bottom , unexpectedly found myself here up .

There is no one up and down in the universe for all worlds, and you should not be surprised that, leaving the Earth below, you would see it above, being on the Moon.

The disk of the Earth hanging in the lunar sky is huge: its diameter is approximately four times larger than the diameter of the lunar disk familiar to us in the terrestrial sky. This is the third startling fact that awaits the lunar traveler. If on moonlit nights our landscapes are sufficiently well lit, then the nights on the Moon, with the rays of the full Earth with a disk 14 times larger than the moon, should be unusually bright. The brightness of a star depends not only on its diameter, but also on the reflectivity of its surface. In this respect, the earth's surface is six times larger than the moon's; therefore, the light of a full Earth must illuminate the Moon 90 times more than a full moon illuminates the Earth. On "Earth nights" on the Moon one could read fine print. The illumination of the lunar soil by the Earth is so bright that it allows us, from a distance of 400,000 km, to distinguish the night part of the lunar ball in the form of an indistinct shimmer inside a narrow crescent; it is called the "ash light" of the moon. Imagine 90 full moons pouring their light from the sky, and take into account the absence of an atmosphere on our satellite that absorbs part of the light, and you will get some idea of ​​\u200b\u200bthe enchanting picture of lunar landscapes flooded in the middle of the night with the radiance of a full Earth.

Could a lunar observer distinguish the outlines of continents and oceans on the Earth's disk? A common misconception is that the Earth in the sky of the Moon is something like a school globe. This is how artists depict it when they have to draw the globe in world space: with the contours of the continents, with a snow cap in the polar regions, etc. in detail. All this must be attributed to the realm of fantasy. On the globe, when observed from the outside, it is impossible to distinguish such details. Not to mention the clouds that usually cover half of the earth's surface, our atmosphere itself greatly scatters the sun's rays; therefore the earth must appear as bright and as opaque to the eye as Venus. The Pulkovo astronomer G.A. Tikhov wrote:

“Looking at the Earth from space, we would see a disk the color of a very whitish sky and hardly distinguish any details of the surface itself. A significant proportion of the sunlight falling on the Earth manages to be scattered in space by the atmosphere and all its impurities before it reaches the surface of the Earth itself. And what is reflected by the surface itself will again have time to greatly weaken due to new scattering in the atmosphere.

So, while the Moon clearly shows us all the details of its surface, the Earth hides her face from the Moon, and from the whole universe under a radiant veil of the atmosphere.

But this is not the only difference between the lunar night star and the earthly one. In our sky, the moon rises and sets, describing its path along with the starry dome. In the lunar sky, the Earth does not make such a movement. It does not rise there and does not set, does not take part in the harmonious, extremely slow procession of the stars. It hangs almost motionless in the sky, occupying a certain position for each point of the moon, while the stars slowly slide behind it. This is a consequence of the peculiarity of the lunar motion we have already considered, which consists in the fact that the Moon always faces the Earth with the same part of its surface. For a lunar observer, the Earth hangs almost motionless in the sky. If the Earth stands at the zenith of some lunar crater, then it never leaves its zenith position. If from any point it is visible on the horizon, it remains forever on the horizon of that place. Only the lunar librations, of which we have already discussed, somewhat disturb this immobility. The starry sky makes its slow rotation behind the earth's disk, at 27 1/3 of our days, the Sun goes around the sky at 29? days, the planets make similar movements, and only one Earth rests almost motionless in the black sky.

But, remaining in one place, the Earth quickly, in 24 hours, rotates around its axis, and if our atmosphere were transparent, the Earth could serve as the most convenient heavenly clock for future passengers of interplanetary spacecraft. In addition, the Earth has the same phases as the Moon shows in our sky. This means that our world does not always shine in the lunar sky with a full disk: it appears either in the form of a semicircle, or in the form of a crescent, more or less narrow, or in the form of an incomplete circle, depending on which part of the half of the Earth illuminated by the Sun is facing the Moon. Having drawn the relative positions of the Sun, Earth and Moon, you can easily see that the Earth and Moon should show opposite phases to each other.

When we observe the new moon, the lunar observer should see the full disk of the Earth - "full earth"; on the contrary, when we have a full moon, there is a “new earth” on the moon (Fig. 50). When we see the narrow crescent of the new month, from the Moon one could admire the Earth in detriment, and just such a crescent is missing until the full disk, which the Moon is showing us at this moment. However, the phases of the Earth are not as sharply outlined as those of the moon: the Earth's atmosphere blurs the boundary of light, creates that gradual transition from day to night and back, which we observe on Earth in the form of twilight.

Rice. 50. New Earth on the Moon. The black disk of the Earth is surrounded by a bright border of the radiant terrestrial atmosphere

Another difference between the terrestrial and lunar phases is as follows. On Earth, we never see the Moon at the very moment of the new moon. Although it usually stands above or below the Sun (sometimes by 5 °, i.e., 10 of its diameters), so that the narrow edge of the lunar ball illuminated by the Sun could be seen, it is still inaccessible to our vision: the brilliance of the Sun clogs the modest radiance of the silver thread of the new moon. We usually notice a new Moon only at the age of two days, when it manages to move a sufficient distance from the Sun, and only in rare cases (in spring) - at the age of one day. This is not the case when observing the "new earth" from the Moon: there is no atmosphere there, scattering a radiant halo around the daylight. Stars and planets are not lost there in the rays of the Sun, but stand out clearly in the sky in its immediate vicinity. Therefore, when the Earth is not directly in front of the Sun (i.e., not at the moments of eclipses), but slightly above or below it, it is always visible in the black, star-studded sky of our satellite in the form of a thin crescent with horns facing away from the Sun (Fig. 51). As it moves away from the Earth to the left of the Sun, the sickle seems to roll to the right.

Rice. 51. "Young" Earth in the sky of the Moon. White circle under the earth crescent - the Sun

A phenomenon corresponding to the one just described can be seen by observing the Moon through a small tube: on a full moon, the disk of the night star is not seen by us in the form of a complete circle; since the centers of the Moon and the Sun do not lie on the same line with the observer's eye, the lunar disk lacks a narrow crescent, which slides in a dark strip near the edge of the illuminated disk to the left as the Moon moves to the right. But the Earth and the Moon always show opposite phases to each other; therefore, at the moment described, the lunar observer should have seen a thin crescent of "new earth".

Rice. 52. Slow motions of the Earth near the lunar horizon due to libration. Dashed lines - the path of the center of the earth's disk

We have already noticed in passing that the librations of the Moon must be reflected in the fact that the Earth is not completely stationary in the lunar sky: it oscillates about the average position in the north-south direction by 14 °, and in the west-east direction by 16 °. For those points of the Moon where the Earth is visible on the very horizon, our planet must therefore appear sometimes to set and soon then to rise again, describing strange curves (Fig. 52). Such a peculiar rising or setting of the Earth in one place on the horizon, without bypassing the entire sky, can last many Earth days.

Eclipses on the Moon

Let us supplement the picture of the lunar sky sketched now with a description of those celestial spectacles which are called eclipses. There are two types of eclipses on the Moon: solar and terrestrial. The former are not like solar eclipses familiar to us, but are extremely spectacular in their own way. They occur on the Moon at those moments when there are lunar eclipses on the Earth, since then the Earth is placed on the line connecting the centers of the Sun and the Moon. Our satellite plunges at these moments into the shadow cast by the globe. Whoever has seen the Moon at such moments knows that it does not completely lose its light, does not disappear from the eye; it is usually visible in the cherry-red rays penetrating into the cone of the earth's shadow. If we were transported at that moment to the surface of the Moon and looked from there at the Earth, we would clearly understand the reason for the red illumination: in the sky of the Moon, the globe, placed in front of the bright, albeit much smaller Sun, appears as a black disk surrounded by a crimson border of its atmosphere. It is this border that illuminates the Moon, immersed in shadow, with a reddish light (Fig. 53).

Rice. 53. The course of a solar eclipse on the Moon: Sun C gradually sets behind the earth's disk 3, which is motionless in the lunar sky.

Solar eclipses last on the Moon not for several minutes, as on Earth, but for more than 4 hours, as long as we have lunar eclipses, because, in essence, these are our lunar eclipses, only observed not from the Earth, but from the Moon.

As for the "earthly" eclipses, they are so scanty that they hardly deserve the name of eclipses. They occur at those moments when solar eclipses are visible on Earth. On the large disk of the Earth, lunar observers would then see a small moving black circle - that is, happy parts of the earth's surface, from where one can admire the eclipse of the Sun.

It should be noted that such eclipses as our solar eclipses cannot be observed at all in any other place in the planetary system. We owe this exceptional spectacle to a random circumstance: the Moon, which obscures the Sun from us, is exactly as many times closer to us than the Sun, by how many times the lunar diameter is smaller than the solar one - a coincidence that does not repeat on any other planet.

Why do astronomers observe eclipses?

Thanks to the now noted accident, the long cone of shadow, which our satellite constantly drags behind it, reaches just to the earth's surface (Fig. 54). As a matter of fact, the average length of the lunar shadow cone is less than the average distance of the Moon from the Earth, and if we were dealing only with average values, we would come to the conclusion that we never have total solar eclipses. They actually happen because the Moon moves around the Earth in an ellipse and in some parts of the orbit is 42,200 km closer to the surface of the Earth than in others: the distance of the Moon varies from 363,300 to 405,500 km.

Rice. 54. The end of the cone of the lunar shadow slides over the earth's surface; in places covered with it, a solar eclipse is observed

Sliding along the earth's surface, the end of the lunar shadow draws on it a "visibility band of a solar eclipse." This strip is not wider than 300 km, so that the number of inhabited places rewarded with the spectacle of a solar eclipse is rather limited each time. If we add to this that the duration of a total solar eclipse is calculated in minutes (no more than eight), then it becomes clear that a total solar eclipse is an extremely rare sight. For any given point on the globe, it occurs once every two or three centuries.

Therefore, scientists literally hunt for solar eclipses, equipping special expeditions to those places on the globe, sometimes very remote for them, from where this phenomenon can be observed. The solar eclipse of 1936 (June 19) was visible as total only within the Soviet Union, and 70 foreign scientists from ten different countries came to us to observe it for two minutes. At the same time, the works of four expeditions were wasted due to cloudy weather. The scope of the work of Soviet astronomers to observe this eclipse was extremely large. About 30 Soviet expeditions were sent to the total eclipse.

In 1941, despite the war, the Soviet government organized a number of expeditions along the total eclipse from the Sea of ​​Azov to Alma-Ata. And in 1947, a Soviet expedition went to Brazil to observe the total eclipse on May 20th. Observations of solar eclipses on February 25, 1952, June 30, 1954, and February 15, 1961 took on a particularly large scale in the USSR. On May 30, 1965, a Soviet expedition observed an eclipse on the tiny island of Manuae in the southwestern Pacific Ocean.

Lunar eclipses, although they occur one and a half times less often than solar ones, are observed much more often. This astronomical paradox is explained very simply.

A solar eclipse can be observed on our planet only in a limited zone for which the Sun is obscured by the Moon; within this narrow strip, it is full for some points, and partial for others (i.e., the Sun is obscured only partially). The moment of the onset of a solar eclipse is also not the same for different points of the band, not because there is a difference in the calculation of time, but because the lunar shadow moves along the earth's surface and different points are covered by it at different times.

A lunar eclipse proceeds quite differently. It is observed immediately on the entire half of the globe, where at this time the Moon is visible, that is, it stands above the horizon.

Successive phases of a lunar eclipse occur for all points on the earth's surface at the same moment; the difference is due only to the difference in the account of time.

That is why the astronomer does not have to “hunt” for lunar eclipses: they come to him on their own. But in order to “catch” a solar eclipse, sometimes one has to make very long journeys. Astronomers send out expeditions to tropical islands, far to the west or east, only to observe for a few minutes the covering of the solar disk by the black circle of the Moon.

Is there any point in equipping expensive expeditions for the sake of such fleeting observations? Is it not possible to make the same observations without waiting for the Sun to be accidentally obscured by the Moon? Why don't astronomers artificially produce a solar eclipse by obscuring the image of the Sun in a telescope with an opaque circle? Then it will be possible, it would seem, to observe without trouble those neighborhoods of the Sun that are so interesting to astronomers during eclipses.

Such an artificial solar eclipse, however, cannot give what is observed when the Sun is obscured by the Moon. The fact is that the rays of the Sun, before reaching our eyes, pass through the earth's atmosphere and are scattered here by air particles. That is why the sky during the day seems to us a bright blue vault, and not black, dotted with stars, as it would appear to us even during the day in the absence of an atmosphere. Covering the Sun in a circle, but remaining at the bottom of the ocean of air, although we protect the eye from the direct rays of the daylight, the atmosphere above us is still flooded with sunlight and continues to scatter the rays, eclipsing the stars. This does not happen if the shielding screen is outside the atmosphere. The moon is just such a screen, located a hundred times further than the perceptible boundary of the atmosphere. The rays of the Sun are stopped by this screen before they penetrate the earth's atmosphere, and therefore no light is scattered in the shaded band. True, not completely: nevertheless, few rays penetrate the shadow region, scattered by the surrounding light regions, and therefore the sky at the time of a total solar eclipse is never as black as at midnight; Only the brightest stars are visible.

What tasks do astronomers set for themselves when observing a total solar eclipse? Let's note the main ones.

The first is the observation of the so-called "reversal" of spectral lines in the outer shell of the Sun. The lines of the solar spectrum, under normal conditions, are dark on a light spectrum ribbon, become bright for a few seconds on a dark background after the moment the Sun is completely covered by the Moon's disk: the absorption spectrum turns into an emission spectrum. This is the so-called "flare spectrum". Although this phenomenon, which provides precious material for judging the nature of the outer shell of the Sun, can under certain conditions be observed and not only during an eclipse, it is revealed during eclipses so clearly that astronomers strive not to miss such an opportunity.

Rice. 55. At the time of a total solar eclipse, a “solar corona” flashes around the black disk of the Moon.

The second task is research solar corona . The crown is the most remarkable of the phenomena observed at the moments of a total solar eclipse: around the completely black circle of the Moon, bordered by fiery projections (prominences) of the outer shell of the Sun, a pearl halo of various sizes and shapes shines in different eclipses (Fig. 55). The long rays of this aurora are often several times the diameter of the sun, and the brightness is usually only half the brightness of the full moon.

During the eclipse of 1936, the solar corona was exceptionally bright, brighter than the full moon, which is rare. The long, somewhat blurred rays of the corona extended three or more solar diameters; the entire crown was represented as a five-pointed star, the center of which was occupied by the dark disk of the moon.

Astronomers take pictures of the corona during eclipses, measure its brightness, and study its spectrum. All this helps to study its physical structure.

Rice. 56. One of the consequences of the general theory of relativity is the deflection of light rays under the influence of the gravitational force of the Sun. According to the theory of relativity, a terrestrial observer in D sees the star at point E in the direction of the straight line TDFE, while in reality the star is at point E and sends its rays along the curved path EBFDT. In the absence of the Sun, the light beam from the star is to the Earth T would be directed in a straight line

The third task, put forward only in recent decades, is to test one of the consequences of the general theory of relativity. According to the theory of relativity, the rays of the stars, passing by the Sun, are influenced by its powerful attraction and undergo a deflection, which should be revealed in the apparent displacement of the stars near the solar disk (Fig. 56). Verification of this consequence is possible only at the moments of a total solar eclipse.

Measurements during the eclipses of 1919, 1922, 1926 and 1936 did not give, strictly speaking, decisive results, and the question of experimental confirmation of the indicated consequence from the theory of relativity remains open to this day.

These are the main goals for which astronomers leave their observatories and go to remote, sometimes very inhospitable places to observe solar eclipses.

As for the very picture of a total solar eclipse, in our fiction there is an excellent description of this rare natural phenomenon (V.G. Korolenko "On the eclipse"; the description refers to the eclipse in August 1887; the observation was made on the banks of the Volga, in the city of Yuryevets .) Here is an excerpt from Korolenko's story with minor omissions:

“The sun sinks for a minute in a wide hazy spot and appears from the cloud already significantly damaged ...

Now it is already visible to the naked eye, helped by a thin vapor that still smokes in the air, softening the dazzling brilliance.

Silence. Somewhere you can hear nervous, heavy breathing ...

Half an hour passes. The day shines almost the same, the clouds cover and open the sun, now floating in the sky in the form of a sickle.

Among the youth there is a careless revival and curiosity.

The old men sigh, the old women somehow hysterically groan, and some even scream and groan, as if from a toothache.

The day begins to fade noticeably. The faces of people take on a frightened tone, the shadows of human figures lie on the ground, pale, indistinct. The steamboat going down floats by some kind of ghost. Its outlines became lighter, lost the certainty of colors. The amount of light apparently decreases, but since there are no condensed shadows of the evening, there is no play of light reflected on the lower layers of the atmosphere, these twilights seem unusual and strange. The landscape seems to blur in something; the grass loses its greenery, the mountains seem to lose their heavy density.

However, while the thin crescent-shaped rim of the sun remains, the impression of a very pale day still reigns, and it seemed to me that the stories of darkness during an eclipse were exaggerated. “Really,” I thought, “this still insignificant spark of the sun, burning like the last forgotten candle in a vast world, means so much? .. Really, when it goes out, night should suddenly come?”

But that spark is gone. It somehow impetuously, as if escaping with an effort from behind a dark shutter, flashed with another golden spray and went out. And at the same time, a thick darkness fell on the earth. I caught the moment when a full shadow came running through the twilight. It appeared in the south and, like a huge blanket, quickly flew over the mountains, along the rivers, across the fields, fanning the entire heavenly space, wrapped us up and in an instant closed in the north. I was now standing below, on the bank, and looked back at the crowd. Deathly silence reigned in it... The figures of people merged into one dark mass...

But this was no ordinary night. It was so bright that the eye involuntarily searched for the silvery moonlight that pierced through the blue darkness of an ordinary night. But nowhere was there a radiance, there was no blue. It seemed that thin, indistinguishable to the eye ashes scattered from above the ground, or as if the thinnest and thickest net hung in the air. And there, somewhere on the sides, in the upper layers, one can feel the illuminated air distance, which sees through into our darkness, merging the shadows, depriving the darkness of its form and density. And over all the confused nature, clouds are running in a wonderful panorama, and among them there is an exciting struggle ... A round, dark, hostile body, like a spider, has stuck into the bright sun, and they are rushing together in the transcendental heights. Some kind of radiance, pouring in changeable tints from behind a dark shield, gives movement and life to the spectacle, and the clouds further enhance the illusion with their disturbing silent run.

Lunar eclipses are not of the exceptional interest for modern astronomers that is associated with solar eclipses. Our ancestors saw lunar eclipses as opportunities to verify the spherical shape of the Earth. It is instructive to recall the role played by this proof in the history of Magellan's circumnavigation of the world. When, after a tedious long journey through the desert waters of the Pacific Ocean, the sailors fell into despair, deciding that they had irretrievably retired from solid land into a water expanse that would never end, Magellan alone did not lose courage. “Although the church constantly asserted on the basis of Holy Scripture that the Earth is a vast plain surrounded by water,” says the companion of the great navigator, “Magellan drew firmness from the following consideration: during eclipses of the Moon, the shadow cast by the Earth is round, and what a shadow, such should be the object that throws it ... ". In old books on astronomy, we even find drawings explaining the dependence of the shape of the lunar shadow on the shape of the Earth (Fig. 57).

Rice. 57. An old drawing explaining the idea that the shape of the Earth can be judged by the appearance of the earth's shadow on the disk of the moon

Now we no longer need such proofs. But lunar eclipses make it possible to judge the structure of the upper layers terrestrial atmosphere by the brightness and color of the moon. As you know, the Moon does not disappear without a trace in the earth's shadow, but continues to be visible in the sun's rays, bending inside the shadow cone. The strength of the illumination of the moon at these moments and its color shades are of great interest to astronomy and are found to be in an unexpected relationship with the number of sunspots. In addition, the phenomena of lunar eclipses have recently been used to measure the rate of cooling of the lunar soil when it is deprived of solar heat (we will return to this later).

Why do eclipses repeat after 18 years?

Long before our era, Babylonian skywatchers noticed that a series of eclipses - both solar and lunar - repeat every 18 years and 10 days. This period was called "Saros". Using it, the ancients predicted the onset of eclipses, but they did not know what caused such a correct periodicity and why the “saros” had exactly this and not another duration. The rationale for the periodicity of eclipses was found much later, as a result of a thorough study of the movement of the moon.

What is the time it takes for the moon to orbit in its orbit? The answer to this question may be different depending on at what moment the revolution of the Moon around the Earth is considered completed. Astronomers distinguish between five kinds of months, of which we are now interested in only two:

1. The so-called "synodic" month, that is, the period of time during which the Moon makes a complete revolution in its orbit, if you follow this movement from the Sun. This is the period of time between two identical phases of the moon, for example, from new moon to new moon. It is equal to 29.5306 days.

2. The so-called draconic month, that is, the interval after which the Moon returns to the same "node" of its orbit ( knot - the intersection of the lunar orbit with the plane of the earth's orbit). The duration of such a month is 27.2122 days.

Eclipses, as it is easy to understand, occur only at the moments when the Moon in the phase of a full moon or a new moon is in one of its nodes: then its center is on the same line with the centers of the Earth and the Sun. It is obvious that if an eclipse happened today, then it should come again after such a period of time that concludes integer number of synodic and draconian months : then the conditions under which there are eclipses will be repeated.

How to find such intervals? To do this, we need to solve the equation

Where X And y - whole numbers. Presenting it as a proportion

we see that the smallest accurate the solutions to this equation are:

x = 272 122………. y = 295 306.

It turns out a huge, tens of millennia, period of time, practically useless. The ancient astronomers were content with the decision approximate . The most convenient means for finding approximations in such cases is given by continued fractions. Expand the fraction

into continuous. It is done like this. Eliminating the integer, we have

In the last fraction, we divide the numerator and denominator by the numerator:

Numerator and denominator of a fraction

divide by the numerator and do so in the future. We end up getting

From this fraction, taking its first links and discarding the rest, we obtain the following successive approximations:

The fifth fraction in this series already gives sufficient accuracy. If you stop at it, i.e. accept x = 223, and y = 242, then the period of recurrence of eclipses will be equal to 223 synodic months, or 242 draconian.

This is 6585 1/3 days, i.e. 18 years 11.3 days (or 10.3 days).

This is the origin of saros. Knowing where it came from, we can also be aware of how accurately it can be used to predict eclipses. We see that, considering saros equal to 18 years 10 days, 0.3 days are discarded. This should affect the fact that the eclipses provided for such a shortened period will occur in other watches days than the previous time (about 8 hours later), and only when using a period equal to the triple exact saros, the eclipses will be repeated at almost the same moments of the day. In addition, saros does not take into account changes in the distance of the Moon from the Earth and the Earth from the Sun, changes that have their own periodicity; it depends on these distances whether the solar eclipse will be total or not. Therefore, saros makes it possible to predict only that an eclipse should occur on a certain day, but whether it will be total, partial or annular, and whether it will be possible to observe it in the same places as the previous time, cannot be asserted.

Finally, it also happens that an insignificant partial eclipse of the Sun after 18 years reduces its phase to zero, i.e., is not observed at all; and, conversely, sometimes small partial eclipses of the Sun, previously not observed, become visible.

Today, astronomers do not use saros. The capricious motions of the earth's satellite have been studied so well that eclipses are now predicted to the nearest second. If the predicted eclipse had not happened, modern scientists would be ready to admit anything, but not the erroneous calculations. This is aptly noted by Jules Verne, who in the novel "The Land of Furs" tells about an astronomer who went on a polar journey to observe a solar eclipse. Contrary to prediction, it did not happen. What conclusion did the astronomer draw from this? He announced to those around him that the ice field on which they were located was not a mainland, but a floating ice floe, carried by the sea current beyond the eclipse band. This assertion was soon justified. Here is an example of deep faith in the power of science!

Is it possible to?

Eyewitnesses say that during a lunar eclipse they happened to observe the disk of the Sun on one side of the sky near the horizon and at the same time on the other side - a darkened disk of the Moon.

Similar phenomena were also observed in 1936, on the day of a partial lunar eclipse on July 4th. July 4 in the evening at 20 o'clock. 31 min. The moon rose, and at 20 o'clock. 46 min. the sun was setting, and at the moment of moonrise there was a lunar eclipse, although the moon and sun were visible simultaneously above the horizon. I was very surprised by this, because the rays of light propagate in fact in a straight line, ”one of the readers of this book wrote to me.

The picture is really mysterious: although, contrary to the belief of the Chekhov girl, it is impossible to “see the line connecting the center of the Sun and the Moon” through sooty glass, but it is quite possible to mentally draw it past the Earth with such an arrangement. Can an eclipse occur if the Earth does not shield the Moon from the Sun? Can such eyewitness testimony be trusted?

In reality, however, there is nothing incredible about such an observation. The fact that the Sun and the darkened Moon are visible in the sky at the same time is due to the curvature of light rays in the earth's atmosphere. Due to this curvature, called "atmospheric refraction", each luminary seems to us higher his true position (p. 48, fig. 15). When we see the Sun or Moon near the horizon, they are geometrically below horizon. There is, therefore, nothing impossible in the fact that the disk of the Sun and the obscured Moon are both visible above the horizon at the same time.

“Usually,” Flammarion says on this occasion, “they point to the eclipses of 1666, 1668 and 1750, when this strange feature manifested itself most sharply. However, there is no need to go that far. February 15, 1877 The moon rose in Paris at 5 o'clock. 29 min. The sun was setting at 5 o'clock. 39 min., and, meanwhile, the total eclipse has already begun. On December 4, 1880, there was a total lunar eclipse in Paris: on this day the Moon rose at 4 o'clock, and the Sun set at 4 o'clock 2 minutes, and this was almost in the middle of the eclipse, which lasted from 3 o'clock. 3 min. up to 4 o'clock. 33 min. If this is not observed much more often, then only due to the lack of observers. To see the Moon in a total eclipse before sunset or after sunrise, you just need to choose a place on Earth so that the Moon is on the horizon near the middle of the eclipse.

What Not Everyone Knows About Eclipses

1. How long can solar and how long lunar eclipses last?

2. How many eclipses can happen in one year?

3. Are there years without solar eclipses? And without moons?

4. When will the next total solar eclipse be visible in Russia?

5. From which side during an eclipse does the black disk of the Moon approach the Sun - on the right or on the left?

6. On which edge does the eclipse of the Moon begin - on the right or on the left?

7. Why do spots of light in the shade of foliage have the shape of crescents during a solar eclipse (Fig. 58)?

8. What is the difference between the shape of a solar crescent during an eclipse and the shape of an ordinary crescent moon?

9. Why is a solar eclipse viewed through smoked glass?

1. Longest duration full phase solar eclipse 7 3/4 m (at the equator; at higher latitudes - less). Yet the eclipse phases can capture up to 3? hours (at the equator).

Duration of all phases lunar eclipse - up to 4 hours; the time of complete darkening of the moon lasts no more than 1 hour 50 m.

2. The number of all eclipses during the year - both solar and lunar - cannot be more than 7 and less than 2. (In 1935 there were 7 eclipses: 5 solar and 2 lunar.)

Rice. 58. Spots of light in the shadow of the foliage of a tree during the partial phase of the eclipse are crescent-shaped.

3. Without solar Eclipses do not pass a single year: every year there are at least 2 solar eclipses. Years without lunar Eclipses happen quite often, about every 5 years.

4. The next total solar eclipse visible in Russia will take place on August 1, 2008. The band of total eclipse will pass through Greenland, the Arctic, Eastern Siberia, and China.

5. In the northern hemisphere of the Earth, the disk of the Moon is moving towards the Sun from the right to the left. The first contact of the Moon with the Sun should always be expected with right sides. In the southern hemisphere, with left (Fig. 59).

Rice. 59. Why for an observer in the northern hemisphere of the Earth, the disk of the Moon during an eclipse approaches the Sun on right, and for an observer in the southern hemisphere - left?

6. In the northern hemisphere, the moon enters the earth's shadow with its leftist edge, in the south - right.

7. Spots of light in the shadow of foliage are nothing but images of the Sun. During an eclipse, the Sun looks like a crescent, and its images in the shade of foliage should have the same look (Fig. 58).

8. Lunar the crescent is bounded from the outside by a semicircle, from the inside by a semi-ellipse. Solar the crescent is bounded by two arcs of a circle of the same radius (see p. 59, "Mysteries of the Moon Phases").

9. The Sun, even if partially obscured by the Moon, cannot be looked at with unprotected eyes. The sun's rays burn the most sensitive part of the retina, significantly reducing visual acuity for a long time, and sometimes for life.

Even at the beginning of the XIII century. the Novgorod chronicler noted: "From this sign in Veliky Novgorod, hardly anyone from the person lost to see." Avoiding burns, however, is easy if you stock up on thickly smoked glass. It must be smoked on a candle so thickly that the disk of the Sun appears through such glass. sharply defined circle , without rays and halo; for convenience, the smoked side is covered with another, clean glass and pasted over with paper around the edges. Since it is impossible to predict in advance what the conditions for the visibility of the Sun will be during the hours of the eclipse, it is useful to prepare several glasses with different opacity.

You can also use colored glasses if you put together two glasses of different colors (preferably "additional"). Ordinary canned sunglasses are insufficient for this purpose.

What is the weather like on the moon?

Strictly speaking, there is no weather on the Moon, if this word is understood in the usual sense. What can be the weather where there is absolutely no atmosphere, clouds, water vapor, precipitation, wind? The only thing that can be discussed is the temperature of the soil.

So how hot is the Moon's soil? Astronomers now have at their disposal an instrument that makes it possible to measure the temperature not only of distant luminaries, but also of their individual sections. The design of the device is based on the phenomenon of thermoelectricity: in a conductor soldered from two dissimilar metals, an electric current runs when one junction is warmer than the other; the strength of the resulting current depends on the temperature difference and allows you to measure the amount of absorbed heat.

The sensitivity of the device is amazing. With microscopic dimensions (the critical part of the device is no more than 0.2 mm and weighs 0.1 mg), it responds even to the heating effect of stars of the 13th magnitude, which increases the temperature ten millionths of a degree . These stars are not visible without a telescope; they shine 600 times fainter than stars that are on the borderline of visibility with the naked eye. Capturing such an insignificant amount of heat is like detecting the warmth of a candle from a distance of several kilometers.

With such an almost miraculous measuring device at their disposal, astronomers introduced it into certain sections of the telescopic image of the Moon, measured the heat it received, and on this basis estimated the temperature of various parts of the Moon (with an accuracy of 10 °). Here are the results (Fig. 60): in the center of the disk of the full moon, the temperature is above 100°; water poured here on lunar soil would boil even under normal pressure. “On the Moon, we wouldn’t have to cook our dinner on the stove,” writes one astronomer, “any nearby rock could fill its role.” Starting from the center of the disk, the temperature decreases uniformly in all directions, but even 2700 km from the central point it is not lower than 80°. Then the temperature drops faster, and near the edge of the illuminated disk frost prevails at -50°. It is even colder on the dark side of the Moon, turned away from the Sun, where frost reaches -170 °.

Rice. 60. The temperature on the Moon reaches +125 ° C in the center of the visible disk on the full moon and quickly drops to the edges to -50 ° and below

It was previously mentioned that during eclipses, when the lunar sphere plunges into the earth's shadow, the soil of the moon, deprived of sunlight, cools rapidly. It was measured how great this cooling was: in one case, a drop in temperature during an eclipse was found from +125 to -115 °, i.e., almost 240 ° during some I 1 /-2 hours. Meanwhile, on Earth, under similar conditions, i.e. during a solar eclipse, there is a decrease in temperature by only two, a lot - by three degrees. This difference must be attributed to the earth's atmosphere, which is comparatively transparent to the visible rays of the Sun and blocks the invisible "thermal" rays of the heated soil.

The fact that the soil of the Moon so quickly loses the heat it has accumulated indicates both the low heat capacity and poor thermal conductivity of the lunar soil, as a result of which, when it is heated, only a small supply of heat has time to accumulate.

In middle latitudes, the Sun always rises in the eastern side of the sky, gradually rises above the horizon, reaches its highest position in the sky at noon, then begins to descend towards the horizon and sets in the western part of the sky. In the Northern Hemisphere, this movement occurs from left to right, and in the Southern Hemisphere, from right to left. An observer in the Northern Hemisphere of the Earth will see the Sun in the south, and an observer in the Southern Hemisphere will see the Sun in the north. The daytime path of the Sun in the sky is symmetrical with respect to the north-south direction.

2. Can the Sun be observed at its zenith in Belarus? Why?

The sun is observed at the zenith in a belt bounded by the following interval of geographic latitude: $-23°27" \le φ \le 23°27".$ Belarus is located to the north, so the Sun at the zenith cannot be observed in our country.

3. Why is the Moon always facing the Earth with the same side?

The Moon makes a complete revolution in its orbit around the Earth in 27.3 days. (sidereal month). And for the same time it makes one revolution around its axis, so the same hemisphere of the Moon always faces the Earth.

4. What is the difference between sidereal and synodic months? What is the reason for their different duration?

A synodic month is a period of time between two consecutive moon phases of the same name (for example, new moons), and it lasts 29.5 days.

A sidereal month is the period of the Moon's orbit around the Earth relative to the stars, and it lasts 27.3 days.

The different duration of these months is due to the fact that the Earth does not rest in one place, but moves in its orbit. Therefore, in order to repeat the previous configuration and complete the synodic month, the Moon has to travel a greater distance in its orbit than to complete the sidereal month.

5. What is meant by the lunar phase? Describe the phases of the moon.

The lunar phase is the part of the lunar disk visible in sunlight.

Consider the phases of the moon, starting with new moon. This phase occurs when the Moon passes between the Sun and the Earth and its dark side is facing us. The moon is not visible from Earth at all. After one or two days, a narrow bright crescent appears in the western part of the sky and continues to grow. "young" moon. After 7 days, the entire right half of the lunar disk will already be visible - it is coming first quarter phase. Further, the phase increases, and 14-15 days after the new moon, the Moon comes into opposition with the Sun. Her phase becomes complete, comes full moon. The sun's rays illuminate the entire lunar hemisphere facing the Earth. After the full moon, the moon gradually approaches the sun from the west and is illuminated by it from the left. Approximately a week later last quarter phase. Then the new moon comes again.

6. The crescent of the Moon is bulging to the right and close to the horizon. On which side of the horizon is it?

The moon is observed in the western part of the horizon.

7. Why do solar and lunar eclipses occur?

During their movement in orbits, the Earth and the Moon from time to time line up with the Sun. If the Moon is near the plane of the Earth's orbit, an eclipse occurs. When the Moon comes between the Earth and the Sun, there is a solar eclipse, and when the Earth comes between the Sun and the Moon, there is a lunar eclipse.

8. Describe total, partial and annular solar eclipses.

Passing between the Sun and the Earth, the small Moon cannot completely obscure the Earth. The solar disk will be entirely closed only to observers located inside the cone of the lunar shadow, the maximum diameter of which on the Earth's surface does not exceed 270 km. Only from here, from this relatively narrow region of the earth's surface, where the shadow of the moon falls, will it be possible to see total solar eclipse. In the same place where the penumbra from the Moon falls, inside the so-called cone of the lunar penumbra, it will be visible partial solar eclipse. If at the time of the eclipse the Moon, moving along its elliptical orbit, will be at a considerable distance from the Earth, then the visible disk of the Moon will be too small to completely cover the Sun. Then a shining rim of the solar disk will be observed around the dark disk of the Moon. This - annular eclipse.

The starry sky would have lost much of its attractiveness if sometimes such a magnificent night luminary as the Moon did not appear against its background. In the past, some authors of astronomical treatises have even expressed regret that the inhabitants of other planets are deprived of such a spectacle. Today we know that there is no one to address this regret: we humans are the only intelligent inhabitants of the solar system.

As a luminary, the Moon is primarily characterized by its impermanence. Its visible appearance, its phases are constantly changing and, accordingly, the illumination created by the Moon on the earth's surface also changes.

When the Moon is between the Earth and the Sun, but does not obscure the solar disk, the Moon is invisible to the Earth observer. This phase of the moon is called the new moon. 1-2 days after the new moon, a narrow crescent of the “young”, growing moon appears in the rays of the evening dawn. Every evening this sickle thickens, and about a week after the new moon comes the first quarter. In this phase, the Moon looks like a light semicircle, bulging to the right. Further, the Moon continues to grow, and a full moon comes a week later, when the entire illuminated hemisphere of the Moon becomes visible to the earthly observer.

After the full moon, the lunar phases change in reverse order. The moon is “damaged” on the right, a week later the last quarter comes (a light half-disk with a bulge turned to the left), and then the “old” Moon becomes like the letter “C” and, every day approaching the Sun in the sky, is finally lost in the rays of the morning dawn.

When the crescent of the moon is narrow enough, it is often possible to observe the ashen light of the moon - a faint glow of its unlit part. Actually, in this case, we see the light not of the Moon, but of the Earth, scattered by the lunar surface. It is curious that when the Pacific Ocean faces the Moon, the ashy light takes on a noticeable bluish tint, and when the Earth is turned towards the Moon by the Asian continent, the ashy light becomes yellowish. So our planet rich in colors is reflected in the “crooked” rough lunar mirror!

Orbiting around the Earth, the Moon moves against the background of the constellations, moving to the east by about 13 ° per day. The time it takes for the moon to complete one revolution around the earth is called a sidereal month. It is equal to 27.3 Earth days. A full cycle of changes in the lunar phases takes a slightly longer period of time. It is called the synodic month and is equal to 29.5 Earth days.

The reason why the sidereal month is not equal to the synodic month is clear enough. When the Moon, having completed its complete revolution around the Earth, returns again to its previous position relative to the stars, the Sun (due to the orbital motion of the Earth) will shift in the sky to the east, therefore, the phase of the Moon will be different than at the beginning of the sidereal month. Only after a little more than two Earth days, the Moon, catching up with the Sun in their apparent movement across the sky, will again reach the initial phase, and thus the synodic month will be completed.

If there were no lunar inequalities, the path of the moon against the background of the starry sky would always be the same. In fact, strictly speaking, it never repeats itself, and only that belt of constellations can be noted through which the Moon can (and sometimes does) pass. This belt, in addition to the zodiac constellations (Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius), also includes some constellations bordering them.

The conditions for the visibility of the moon depend on the season. For example, in winter, when the daily path of the Sun in northern latitudes is low above the horizon, the full Moon, which opposes the Sun in the sky, on the contrary, shines brightly high in the sky around midnight. In summer, the opposite is observed - the apparent path of the full moon above the horizon is very low. For all seasons, the rising of the full moon coincides with the setting of the sun and, conversely, with the rising of the sun, the full moon goes under the horizon.

Knowing how the apparent annual path of the Sun and the apparent monthly path of the Moon are located in the sky, one can, for example, realize that the “young” Moon is best seen on spring evenings - then its daily path above the horizon is high and long. On the contrary, on autumn evenings, the “young” Moon rises low above the horizon and sets early. Those who wish to independently observe the Moon will be able to find information about this visibility not only in astronomical yearbooks, but also in the usual tear-off calendar, where the phase of the moon and the time of its rise and set are indicated for each day.

On the bright surface of the lunar disk, the eye easily distinguishes grayish spots of constant outlines - the so-called lunar "seas". The fact that they are always the same was noticed in antiquity. This fact indicates that the Moon always faces us with the same hemisphere. The second hemisphere, not visible from the Earth, became available for study only with the help of spacecraft.

While maintaining a constant orientation with respect to the Earth, the lunar ball at the same time rotates around its axis so that the time of rotation of the Moon around the axis is exactly equal to the period of its revolution around the Earth. Such motion is called synchronous, and it seems to be characteristic of some satellites of other planets as well. Note that the Moon's axis of rotation is almost perpendicular to the plane of the Earth's orbit.

The period of a complete revolution of the Moon around its axis can be called a lunar sidereal day, since the movement of the Moon in this case is considered relative to the stars. The solar day on the Moon is somewhat longer, and, as you can easily figure out, they are equal to the synodic month (29 Earth days). Indeed, in this case, at the end of the solar day, the terminator returns to its original position, which means that the initial lunar phase is repeated. Thus, a solar day on the Moon lasts almost a month, and day and night are two Earth weeks each. This feature of the lunar world leads to the fact that the surface of the Moon periodically experiences prolonged heating, followed by an equally prolonged cooling.

It would seem that from the synchronous motion of the Moon it inevitably follows that only half of the lunar surface is always accessible to the earthly observer. Actually this is not true. For some reason, which we will now consider, the Moon “sways” slightly, slightly revealing to us part of its invisible hemisphere. Thanks to this "wiggle" or libration, the earthly observer sees not half, but about 60% of the entire lunar surface. There are four types of libration.

Longitude libration. It is caused by the fact that the rotation of the Moon around its axis is uniform, and the rotation of the Moon around the Earth along an ellipse is uneven (Kepler's second law). Because of this, it seems that the Moon is slightly swaying, alternately revealing to the earthly observer either the eastern or the western part of its invisible hemisphere. During the sidereal month, the appearance of the marginal zones of the Moon noticeably changes, which is easy to verify by observing the Moon even with binoculars.

Moonlight and moonlit nights are enthusiastically described by many poets and prose writers. And it's hard to disagree with them - the Moon as a luminary is amazingly beautiful. But it seems dazzlingly bright on dark nights only in contrast to the black background of the night sky - during the day the Moon looks much less spectacular.

The most paradoxical thing, perhaps, is that in fact the Moon is a very bad "mirror". It reflects only 7% of the sunlight falling on the dog. In terms of its reflectivity, the Moon resembles dry black earth, wet loam, and very dark rocks such as basalt and diabase. In other words, in general, the Moon is dark gray, and not dazzling silver, which seems to our eyes, subject to various optical illusions.

If we study in more detail how the Moon reflects rays of different colors, it turns out that with increasing wavelength, the reflectivity of the lunar surface increases. So, for example, the Moon reflects 4% of the violet rays falling on it, 7% of the yellow and 9% of the red. A substance with such optical properties is perceived by your eye as dark gray with a brownish tint.

The first photographs of the Moon were taken shortly after the invention of photography. Later, the Moon was photographed through different filters. In color photographs of the Moon, color contrasts are enhanced - when observing the Moon through a telescope, it is only sometimes possible to distinguish a very faint color of some part of the Moon. In general, the lunar surface, in contrast to the earth, is distinguished by the uniformity of color. The more unusual is the appearance of the multi-colored Moon, created by means of chemistry.

However, even faint color shades of lunar objects indicate their different nature and, possibly, different origins. But this applies already to the details of the lunar world, and not to the properties of the Moon as a night star.

Consistent change of the visible moon in the sky

The moon goes through the following phases of illumination:

• new moon- a state when the moon is not visible. New Moon is the phase of the Moon when its ecliptic longitude is the same as that of the Sun. Thus, at this time, the Moon is between the Earth and the Sun approximately on the same straight line with them. If they are exactly on the same straight line, a solar eclipse occurs. The moon at the new moon is not visible in the night sky, since at that time it is very close to the Sun in the celestial sphere (no further than 5 °) and at the same time turned to us by the night side. But sometimes it can be seen against the background of the solar disk (solar eclipse). In addition, after some time (usually about two days) after or before the new moon, with a very clear atmosphere, you can still see the disk of the Moon, illuminated by weak light reflected from the Earth (ash light of the Moon). The interval between new moons averages 29.530589 days (synodic month). On the new moon, the Jewish New Year and the Chinese (Japanese, Korean, Vietnamese) New Year of the 60-year cycle begin.
• young moon- the first appearance of the moon in the sky after the new moon in the form of a narrow sickle.
• first quarter- a state when half of the moon is illuminated.
• waxing moon
• full moon- a state when the entire moon is illuminated as a whole. The full moon is the phase of the moon when the difference between the ecliptic longitudes of the sun and the moon is 180°. This means that the plane through the Sun, Earth and Moon is perpendicular to the plane of the ecliptic. If all three objects are on the same line, a lunar eclipse occurs. The moon in the full moon looks like a regular luminous disk. In astronomy, the moment of a full moon is calculated to within a few minutes; in everyday life, the full moon is usually called the period of several days, during which the moon visually almost does not differ from the full one. During a full moon, the so-called opposition effect can occur for several hours, in which the brightness of the disk noticeably increases, despite its unchanged size. The effect is explained by the complete disappearance (for a terrestrial observer) of the shadows on the surface of the Moon at the moment of opposition. The maximum brightness of the Moon during a full moon is -12.7m.
• waning moon
• last quarter- a state when half of the moon is again illuminated.
• old moon

Mnemonic rule for determining the phases of the moon

To distinguish the first quarter from the last, an observer located in the northern hemisphere can use the following mnemonic rules. If the lunar crescent in the sky looks like the letter "C (d)", then this is the moon "Aging" or "Descending", that is, this is the last quarter (in French dernier). If it is turned in the opposite direction, then, mentally putting a wand to it, you can get the letter “P (p)” - the moon is “Growing”, that is, this is the first quarter (in French premier).

The growing month is usually observed in the evening, and the aging month is usually observed in the morning.

It should be noted that near the equator the moon is always seen "lying on its side", and this method is not suitable for determining the phase. In the Southern Hemisphere, the orientation of the crescent in the corresponding phases is opposite: the growing month (from new moon to full moon) looks like the letter "C" (Crescendo,<), а убывающий (от полнолуния до новолуния) похож на букву «Р» без палочки (Diminuendo, >). Interesting Facts Usually, there is one full moon for each calendar month, but since the phases of the moon change a little faster than 12 times a year, sometimes the second full moon in a month occurs, called the blue moon.