Everyone knows it - 24 hours. But why did it happen? Let's take a closer look at the history of the appearance of the main units of time and find out how many hours, seconds and minutes are in a day. And also let's see if it is worth tying these units exclusively to astronomical phenomena.
Where did the day come from? This is the time of one rotation of the earth around its axis. Still knowing little about astronomy, people began to measure time in such ranges, including in each light and dark time.
But there is an interesting feature here. When does the day start? From a modern point of view, everything is obvious - the day begins at midnight. People of ancient civilizations thought otherwise. It is enough to look at the very beginning of the Bible in order to read in the 1st book of Genesis: "... and there was evening, and there was one morning." The day began with There is a certain logic in this. The people of that time were guided by the Sun, the village was over, the day was over. Evening and night is the next day.
But how many hours are there in a day? Why was the day divided into 24 hours, because the decimal system is more convenient, and much more? If there were, say, 10 hours in a day, and 100 minutes in each hour, would something change for us? Actually, nothing but numbers, on the contrary, it would even be more convenient to make calculations. But the decimal system is far from the only one used in the world.
They used the sexagesimal counting system. And the bright half of the day was well divided in half, for 6 hours each. In total, there were 24 hours in a day. This rather convenient division was taken from the Babylonians and other peoples.
Among the ancient Romans, counting time was even more interesting. The countdown started at 6 am. So they counted further from this moment - the first hour, the third hour. Thus, it can be easily calculated that the "eleventh hour workers" commemorated by Christ are those who start work at five o'clock in the evening. Indeed, too late!
At six o'clock in the evening came the twelfth hour. That's how many hours in a day were counted in ancient Rome. But it was still night time! The Romans did not forget about them either. After the twelfth hour, the night watch began. The attendants changed at night every 3 hours. Evening and night time was divided into 4 guards. The first evening watch began at 6 pm and lasted until 9. The second, midnight watch, lasted from 9 to 12 o'clock. The third watch, from 12 at night to 3 in the morning, ended when the roosters sang, which is why it was called “rooster crow”. The last, fourth watch was called "morning" and ended at 6 in the morning. And everything started all over again.
The need to divide watches into component parts also arose much later, but they did not retreat from the sexagesimal system even then. And then the minute was divided into seconds. True, it later became clear that it was impossible to rely only on the determination of the duration of seconds and days. For a century, the length of the day increases by 0.0023 seconds - it seems to be very little, but enough to get confused about how many seconds are in a day. And that's not all the difficulties! Our Earth does not make one revolution around the Sun in an even number of days, and this also affects the solution of the question of how many hours are in a day.
Therefore, to simplify the situation, the second was equated not to the movement of celestial bodies, but to the time of the processes inside the cesium-133 atom at rest. And to match the actual state of affairs with the revolution of the Earth around the Sun twice a year - on December 31 and June 30 - 2 extra leap seconds are added, and once every 4 years - an additional day.
In total, it turns out that there are 24 hours in a day, or 1440 minutes, or 86400 seconds.
All people interested in astronomy know that the word "day" has many different meanings. For example, sidereal day, solar day. But recently many new concepts have arisen for which the same word is used. In this article, we will give more precise definitions.
1. Day as a unit of time
First of all, we recall that the unit of time in astronomy, as in other sciences, is the second of the international system of units SI - the atomic second. Here is the definition of the second as given by the 13th General Conference of Weights and Measures in 1967:
If the word "day" is used to denote a unit of time, it should be understood as 86400 atomic seconds. In astronomy, larger units of time are also used: the Julian year is 365.25 days exactly, the Julian century is 36525 days exactly. The International Astronomical Union (a public organization of astronomers) in 1976 recommended that astronomers use just such units of time. The main time scale, the International Atomic Time (Time Atomic International, TAI), is based on the readings of many atomic clocks in different countries. Hence, from a formal point of view, the basis for measuring time has gone out of astronomy. The old units "mean solar second", "sidereal second" should not be used.
2. A day as a period of rotation of the Earth around its axis
It is somewhat more difficult to define this use of the word "day". There are many reasons for this.
First, the axis of rotation of the Earth, or, scientifically speaking, the vector of its angular velocity, does not maintain a constant direction in space. This phenomenon is called precession and nutation. Secondly, the Earth itself does not maintain a constant orientation relative to its angular velocity vector. This phenomenon is called the movement of the poles. Therefore, the radius vector (the segment from the center of the Earth to a point on the surface) of an observer on the Earth's surface will not return after one revolution (and never at all) to the previous direction. Thirdly, the speed of the Earth's rotation, i.e. the absolute value of the angular velocity vector does not remain constant either. So, strictly speaking, there is no definite period of the Earth's rotation. But with a certain degree of accuracy, a few milliseconds, we can talk about the period of rotation of the Earth around its axis.
In addition, it is necessary to indicate the direction relative to which we will count the revolutions of the Earth. There are currently three such directions in astronomy. This is the direction to the vernal equinox, to the Sun and the celestial ephemeris beginning.
The period of rotation of the Earth relative to the vernal equinox is called a sidereal day. It is equal to 23 h 56 m 04.0905308 s . Note that a sidereal day is a period relative to the spring point, not the stars.
The vernal equinox itself makes a complex movement on the celestial sphere, so this number should be understood as an average value. Instead of this point, the International Astronomical Union proposed to use the "celestial ephemeris". We will not give its definition (it is rather complicated). It is chosen so that the period of the Earth's rotation relative to it is close to the period relative to the inertial reference frame, i.e. relative to stars, or more precisely, extragalactic objects. The angle of rotation of the Earth relative to this direction is called the sidereal angle. It is equal to 23 h 56 m 04.0989036 s , slightly more than a sidereal day by the amount by which the spring point shifts in the sky due to precession per day.
Finally, consider the rotation of the Earth relative to the Sun. This is the most difficult case, since the Sun moves in the sky not along the equator, but along the ecliptic and, moreover, unevenly. But these sunny days are obviously the most important for people. Historically, the atomic second has been adjusted to the period of the Earth's rotation relative to the Sun, with the averaging done around the 19th century. This period is equal to 86400 units of time, which were called mean solar seconds. The adjustment took place in two steps: first, "ephemeris time" and "ephemeris second" were introduced, and then the atomic second was set equal to the ephemeris second. Thus, the atomic second still "comes from the Sun", but the atomic clock is a million times more accurate than the "terrestrial clock".
The rotation period of the Earth does not remain constant. There are many reasons for this. These are seasonal changes in the distribution of temperature and air pressure around the globe, and internal processes, and external influences. Distinguish secular slowdown, decade (for decades) irregularities, seasonal and sudden. On fig. 1 and 2 are graphs showing the change in the length of the day in 1700-2000. and in 2000-2006. On fig. 1, there is a trend towards an increase in the day, and in Fig. 2 - seasonal unevenness. The graphs are based on the materials of the International Earth Rotation and Reference Systems Service (IERS, http://www.iers.org/).
Is it possible to return the basis of measuring time to astronomy and is it worth it? Such a possibility exists. These are pulsars whose rotation periods are conserved with great accuracy. Besides, there are a lot of them. It is possible that over long time intervals, for example, decades, observations of pulsars will serve to refine atomic time and a "pulsar time" scale will be created.
The study of the uneven rotation of the Earth is very important for practice and interesting from a scientific point of view. For example, satellite navigation is impossible without knowledge of the rotation of the Earth. And its features carry information about the internal structure of the Earth. This complex problem awaits its researchers.
Rice. 1. Difference of the Earth's rotation period from 86400 s SI, in milliseconds. Data up to the beginning of the 20th century. are not very reliable, but the trend towards an increase in the length of the day is clearly visible.
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A day in astronomy
The length of a day on a planet depends on the angular velocity of its own rotation. In astronomy, several types of day are distinguished, depending on the reference system. If a distant star is chosen as the rotation reference point, then, unlike the central luminary of the planetary system, such a day will have a different duration. For example, on Earth, a mean solar day (24 hours) and sidereal or sidereal days (approximately 23 hours 56 minutes 4 seconds) are distinguished. They are not equal to each other, because, due to the orbital motion of the Earth around the Sun, for an observer located on the surface of the Earth, the Sun moves against the background of distant stars.
A true solar day is the time interval between two upper culminations (successive passages of the center of the Sun through the southern part of the meridian (for the northern hemisphere); in other words, the time between two true noons); the beginning of this day is taken as the moment of passage of the center of the Sun through the southern part of the meridian; the hour angle of the center of the Sun is called true time (see Equation of time). True solar days are longer than sidereal days and their duration varies throughout the year, which comes from the inclination of the ecliptic to the plane of the equator and from the uneven movement of the Earth around the Sun.
International System of Units (SI)
The unit of time measurement day (Russian designation: days; international: d) is one of the off-system units of measurement and is not included in the SI. However, in the Russian Federation, it is approved for use without limitation of validity with the scope of "all areas". In this case, 1 day is taken to be exactly 86,400 seconds. In SI, a second is defined as 9,192,631,770 periods of radiation corresponding to a transition between two hyperfine levels of the ground state of a cesium-133 atom. Accordingly, 794,243,384,928,000 such periods can be considered as the definition of a day in the SI.
In astronomy, a day measured in SI seconds is called a Julian day.
The average solar day does not contain an integer number of seconds (for example, their duration at epoch 2000.0 was 86400.002 s), and the duration of the average solar day is also not constant due to the secular change in the angular velocity of the Earth's rotation (see ).
In other languages
As mentioned above, in everyday life the term day often replaced by the word day, but in any case, in Russian there are words for the unambiguous separation of the concepts of "day" (light day) and "day" (24 hours). A separate word for the concept of "day" also occurs in the following languages:
In Islam, a day is counted from sunset to sunset, that is, the complete disappearance of the sun on the horizon marks the beginning of a new day, regardless of the glow.
Division of the day
The number of parts into which the day was divided, or separately night and day, depended on the degree of development of a given people and increased gradually with the development of mankind. Most of the peoples of the New World divided the day only into four parts, corresponding to the rising of the sun, the highest point of its daytime journey, the setting of the sun, and, finally, the middle of the night. According to the traveler Gorrebow, who described Iceland in the middle of the 18th century, the Icelanders divided the day into 10 parts. The Arabs distinguished only the rising of the sun, its rise and fall, the setting of the sun, twilight, night, the first cock crow and dawn. However, among some, in the past, uncivilized peoples, one could find a relatively accurate division of the day, as, for example, among the natives of the islands of the Society, who in Cook's time had a division of the day into 18 parts, the length of which was, however, unequal; the shortest time intervals corresponded to morning and evening, the longest - to midnight and noon.
In Babylon, there was also a division of day and night into 12 hours. According to the "History" (II, 109) of Herodotus, the Greeks adopted this system from the Babylonians, later, probably from the Egyptians or Greeks, the Romans adopted. For example, in winter, the duration of the “daytime hour” in Rome was about 45 minutes.
| Period | Number of daylight hours | Beginning of the first hour of the day in modern reckoning | Number of night hours | The beginning of the first hour of the night in modern reckoning |
|---|---|---|---|---|
| November 27 - January 1 | 7 | 8:30 | 17 | 15:30 |
| January 2-16; November 11-26 | 8 | 7:21 | 16 | 15:21 |
| January 17 - February 1; October 26 - November 10 |
9 | 7:30 | 15 | 16:30 |
| February 2-17; October 10-25 | 10 | 6:21 | 14 | 16:21 |
| February 18 - March 5; September 24 - October 9 |
11 | 6:30 | 13 | 17:30 |
| March 6-20; September 8-23 | 12 | 5:21 | 12 | 17:21 |
| March 21 - April 5; August 23 - September 7 |
13 | 5:30 | 11 | 18:30 |
| April 6-22; August 7-22 | 14 | 4:21 | 10 | 18:21 |
| April 23 - May 8; July 23 - August 6 |
15 | 4:30 | 9 | 19:30 |
| May 9-24; July 6-22 | 16 | 3:21 | 8 | 19:21 |
| May 25 - July 5 | 17 | 3:30 | 7 | 20:30 |
Division into 12 main parts
| Times of Day | Name | Name meaning |
|---|---|---|
| 23:00-01:00 | Hour of the Rat | The time when rats are most active looking for food. Rats also have a different number of fingers on their front and hind legs, thanks to which these rodents have become a symbol of a “reversal”, a “new beginning”. |
| 01:00-03:00 | Hour of the Ox | The time when the oxen begin to chew the cud, slowly and with pleasure. |
| 03:00-05:00 | Hour of the Tiger | The time when tigers are most ferocious, roaming in search of prey. |
| 05:00-07:00 | Hour of the Rabbit | The time when the fabulous Jade Rabbit on the Moon prepares herbal elixirs to help people. |
| 07:00-09:00 | Hour of the Dragon | The time when dragons soar in the sky to make it rain. |
| 09:00-11:00 | Hour of the Snake | The time when the snakes leave their shelters. |
| 11:00-13:00 | Hour of the Horse | The time when the sun is high at its zenith, and while other animals lie down to rest, the horses are still on their feet. |
| 13:00-15:00 | Hour of the Sheep | The time when sheep and goats eat grass and urinate frequently. |
| 15:00-17:00 | Hour of the Monkey | Time of active life of monkeys |
| 17:00-19:00 | Hour of the Rooster | The time when roosters begin to gather in their communities. |
| 19:00-21:00 | Hour of the Dog | Time for the dogs to do their duty of guarding the buildings. |
| 21:00-23:00 | Hour of the Pig | The time when pigs sleep peacefully. |
Division into 30 main parts
Division into 22 main parts
Division into 10 main parts
| Time | Geological period | Number of days in a year | Day length |
|---|---|---|---|
| Today | Quaternary | 365 | 24 hours |
| 100 million years ago | Yura | 380 | 23 hours |
| 200 million years ago | Permian | 390 | 22.5 hours |
| 300 million years ago | Carbon | 400 | 22 hours |
| 400 million years ago | Silurus | 410 | 21.5 hours |
| 500 million years ago | Cambrian | 425 | 20.5 hours |
To find out the length of the day before the era of the appearance of corals, scientists had to resort to the help of blue-green algae. Since 1998, Chinese researchers Zhu Shixing, Huang Xueguang, and Xin Houtian of the Tianjin Institute of Geology and Mineral Resources have analyzed more than 500 1.3 billion-year-old fossil stromatolites that once grew near the equator and were buried in the Yanshan Mountains. Blue-green algae react to the change of light and dark times of the day by the direction of their growth and the depth of color: during the day they are colored in light colors and grow vertically, at night they are dark in color and grow horizontally. According to the appearance of these organisms, taking into account the rate of their growth and the accumulated scientific data on geology and climatology, it turned out to be possible to determine the annual, monthly and daily rhythms of the growth of blue-green algae. According to the results obtained, scientists concluded that 1.3 billion years ago (in the Precambrian epoch) the earth day lasted 14.91-16.05 hours, and the year consisted of 546-588 days.
There are also opponents of this assessment, indicating that the data of studies of ancient tidal deposits, tidalites, contradict it.
In addition to a change in the speed of the Earth's rotation over a long period of time (and the resulting change in the length of the day), insignificant changes in the speed of the planet's rotation occur from day to day, associated with the distribution of masses, for example, due to a decrease in the volume of the world's oceans or atmosphere from fluctuations in their average temperature. . When the world ocean or atmosphere cools, the Earth rotates faster (and vice versa), because as a result, the law conservation momentum momentum operates. Also, a change in the average length of the day can be caused by geological events, for example, strong earthquakes. So, as a result of the 2004 earthquake in the Indian Ocean, the length of the day decreased by about 2.68 microseconds. Such changes are noted and can be measured by modern methods.
In 1967, the International Committee for Weights and Measures adopted a fixed second, without reference to the current duration of a solar day on Earth. A new second became equal to 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels of the ground state
Time is the most important philosophical, scientific and practical category. The choice of a method for measuring time has been of interest to man since ancient times, when practical life began to be associated with the periods of revolution of the sun and moon. Despite the fact that the first clock - solar - appeared three and a half millennia BC, this problem remains quite complicated. Often, answering the simplest question related to it, for example, "how many hours are there in a day," is not so simple.
History of timekeeping
The alternation of daylight and darkness, periods of sleep and wakefulness, work and rest began to mean for people the passage of time even in primitive times. Every day the sun moved across the sky during the day, from sunrise to sunset, and the moon - at night. It is logical that the period between the same phases of the movement of the luminaries has become a unit of time calculation. Day and night gradually formed into a day - a concept that determines the change of date. On their basis, shorter units of time appeared - hours, minutes and seconds.
For the first time, they began to determine how many hours there are in a day in ancient times. The development of knowledge in astronomy led to the fact that day and night began to be divided into equal periods associated with the rise of certain constellations to the celestial equator. And the Greeks adopted the sexagesimal number system from the ancient Sumerians, who considered it the most practical.
Why exactly 60 minutes and 24 hours?
To count something, the ancient man used what is usually always at hand - fingers. From here originates the decimal number system adopted in most countries. Another method, based on the phalanges of the four fingers of the open palm of the left hand, flourished in Egypt and Babylon. In the culture and science of the Sumerians and other peoples of Mesopotamia, the number 60 became sacred. In many cases, it was possible to divide it without a trace by the presence of many divisors, one of which is 12.
The mathematical concept of how many hours there are in a day originates in Ancient Greece. The Greeks at one time took into account only the daylight hours in the calendar and divided the time from sunrise to sunset into twelve equal intervals. Then they did the same with night time, resulting in a 24-part division of the day. Greek scientists knew that the length of the day changes during the year, so for a long time there were day and night hours that were the same only on the days of the equinox.
From the Sumerians, the Greeks also adopted the division of the circle into 360 degrees, on the basis of which a system of geographical coordinates was developed and the division of the hour into minutes (minuta prima (lat.) - "reduced first part" (of the hour)) and seconds (secunda divisio (lat.) - "second division" (hours)).
solar day
The meaning of the day regarding the interaction of celestial objects is the length of time during which the Earth makes a complete revolution around the axis of rotation. It is customary for astronomers to make several clarifications. They single out a solar day - the beginning and end of a revolution is considered by the location of the Sun at the same point in the celestial sphere - and divide them into true and average.
It is impossible to say to the nearest second how many hours in a day that are called true solar hours without specifying a specific date. During the year, their duration periodically changes by almost a minute. This is due to the irregularity and complex trajectory of the luminary in the celestial sphere - the axis of rotation of the planet has an inclination of about 23 degrees relative to the plane of the celestial equator.
More or less accurately, you can say how many hours and minutes are in a day, which experts refer to as average solar. This is the usual calendar time intervals used in everyday life that determine a specific date. They are considered to be of constant duration, that they are exactly 24 hours, or 1440 minutes, or 86,400 seconds. But this statement is also conditional. It is known that the speed of rotation of the Earth is decreasing (a day lengthens by 0.0017 seconds in a hundred years). The intensity of the planet's rotation is influenced by complex gravitational cosmic interactions and spontaneous geological processes within it.
sidereal day
Modern requirements for calculations in space ballistics, navigation, etc. are such that the question of how many hours a day lasts requires a solution with an accuracy of nanoseconds. For this, more stable reference points are chosen than nearby celestial bodies. If we calculate the full revolution of the globe, taking its position relative to the vernal equinox as the initial moment, we can get the duration of the day, called sidereal.
Modern science accurately determines how many hours in a day that bear the beautiful name of stellar - 23 hours 56 minutes 4 seconds. Moreover, in some cases, their duration is even more specified: the true number of seconds is 4.0905308333. But this scale of refinements is also insufficient: the non-uniformity of the planet's orbital motion affects the constancy of the reference point. To eliminate this factor, a special, ephemeris origin of coordinates is chosen, associated with extragalactic radio sources.
Time and calendar
The final version of determining how many hours in a day, close to modern, was adopted in ancient Rome, with the introduction of the Julian calendar. Unlike the ancient Greek time system, the day was divided into 24 equal intervals, regardless of the time of day and season.
Different cultures use their own calendars, which have specific events as a starting point, most often of a religious nature. But the duration of the average solar day is the same throughout the Earth.
1. Day as a unit of time
First of all, we recall that the unit of time in astronomy, as in other sciences, is the second of the international system of units SI - the atomic second. Here is the definition of the second as given by the 13th General Conference of Weights and Measures in 1967:
A second is the duration of 9 192 631 770 periods of radiation of the cesium 133 atom, emitted by it during the transition between two hyperfine levels of the ground state (see the page of the International Bureau of Weights and Measures, some clarifications are also given there).
If the word "day" is used to denote a unit of time, it should be understood as 86400 atomic seconds. In astronomy, larger units of time are also used: the Julian year is 365.25 days exactly, the Julian century is 36525 days exactly. The International Astronomical Union (a public organization of astronomers) in 1976 recommended that astronomers use just such units of time. The main time scale, the International Atomic Time (Time Atomic International, TAI), is based on the readings of many atomic clocks in different countries. Hence, from a formal point of view, the basis for measuring time has gone out of astronomy. The old units "mean solar second", "sidereal second" should not be used.
2. A day as a period of rotation of the Earth around its axis
It is somewhat more difficult to define this use of the word "day". There are many reasons for this.
First, the axis of rotation of the Earth, or, scientifically speaking, the vector of its angular velocity, does not maintain a constant direction in space. This phenomenon is called precession and nutation. Secondly, the Earth itself does not maintain a constant orientation relative to its angular velocity vector. This phenomenon is called the movement of the poles. Therefore, the radius vector (the segment from the center of the Earth to a point on the surface) of an observer on the Earth's surface will not return after one revolution (and never at all) to the previous direction. Thirdly, the speed of the Earth's rotation, i.e. the absolute value of the angular velocity vector does not remain constant either. So, strictly speaking, there is no definite period of the Earth's rotation. But with a certain degree of accuracy, a few milliseconds, we can talk about the period of rotation of the Earth around its axis.
In addition, it is necessary to indicate the direction relative to which we will count the revolutions of the Earth. There are currently three such directions in astronomy. This is the direction to the vernal equinox, to the Sun and the celestial ephemeris beginning.
The period of rotation of the Earth relative to the vernal equinox is called a sidereal day. It is equal to 23h 56m 04.0905308s. Note that a sidereal day is a period relative to the spring point, not the stars.
The vernal equinox itself makes a complex movement on the celestial sphere, so this number should be understood as an average value. Instead of this point, the International Astronomical Union proposed to use the "celestial ephemeris". We will not give its definition (it is rather complicated). It is chosen so that the period of the Earth's rotation relative to it is close to the period relative to the inertial reference frame, i.e. relative to stars, or more precisely, extragalactic objects. The angle of rotation of the Earth relative to this direction is called the sidereal angle. It is equal to 23h 56m 04.0989036s, slightly more than a sidereal day by the amount by which the point of spring is shifted in the sky due to precession per day.
Finally, consider the rotation of the Earth relative to the Sun. This is the most difficult case, since the Sun moves in the sky not along the equator, but along the ecliptic and, moreover, unevenly. But these sunny days are obviously the most important for people. Historically, the atomic second has been adjusted to the period of the Earth's rotation relative to the Sun, with the averaging done around the 19th century. This period is equal to 86400 units of time, which were called mean solar seconds. The adjustment took place in two steps: first, "ephemeris time" and "ephemeris second" were introduced, and then the atomic second was set equal to the ephemeris second. Thus, the atomic second still "comes from the Sun", but the atomic clock is a million times more accurate than the "terrestrial clock".
The rotation period of the Earth does not remain constant. There are many reasons for this. These are seasonal changes in the distribution of temperature and air pressure around the globe, and internal processes, and external influences. Distinguish secular slowdown, decade (for decades) irregularities, seasonal and sudden. On fig. 1 and 2 are graphs showing the change in the length of the day in 1700-2000. and in 2000-2006. On fig. 1, there is a trend towards an increase in the day, and in Fig. 2 - seasonal unevenness. Graphs are based on materials from the International Earth Rotation and Reference Systems Service (IERS).
Is it possible to return the basis of measuring time to astronomy and is it worth it? Such a possibility exists. These are pulsars whose rotation periods are conserved with great accuracy. Besides, there are a lot of them. It is possible that over long time intervals, for example, decades, observations of pulsars will serve to refine atomic time and a "pulsar time" scale will be created.
The study of the uneven rotation of the Earth is very important for practice and interesting from a scientific point of view. For example, satellite navigation is impossible without knowledge of the rotation of the Earth. And its features carry information about the internal structure of the Earth. This complex problem awaits its researchers.
