All people interested in astronomy know that the word "day" has many meanings. 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, let us recall that the unit of time in astronomy, as in other sciences, is the second of the international system of SI units - 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, Time Atomic International (TAI), is based on the readings of many atomic clocks in different countries. Consequently, from a formal point of view, the basis for measuring time has left astronomy. The old units "mean solar second", "sidereal second" should not be used.
2. A day as the period of rotation of the Earth around its axis
Defining this use of the word “day” is somewhat more difficult. There are many reasons for this.
Firstly, the Earth's rotation axis, or, scientifically speaking, its angular velocity vector, 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 vector angular velocity. This phenomenon is called pole movement. Therefore, the radius vector (a 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 its previous direction. Thirdly, the speed of rotation of the Earth, i.e. The absolute value of the angular velocity vector also does not remain constant. So, strictly speaking, there is no specific period of rotation of the Earth. 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, we must indicate the direction relative to which we will count the Earth's revolutions. There are currently three such directions in astronomy. This is the direction to the vernal equinox, to the Sun and the celestial ephemeris.
The period of rotation of the Earth relative to the vernal equinox is called the sidereal day. It is equal to 23 h 56 m 04.0905308 s. Please note that the sidereal day is a period relative to the spring point, not the stars.
The vernal equinox point itself undergoes 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 using the "celestial ephemeris origin". We will not give its definition (it is quite complicated). It was chosen so that the period of rotation of the Earth relative to it was 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 was adjusted to the period of rotation of the Earth relative to the Sun, with averaging done around the 19th century. This period is equal to 86,400 units of time, which were called mean solar seconds. The adjustment occurred 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 atomic clocks are a million times more accurate than “earthly clocks.”
The rotation period of the Earth does not remain constant. There are many reasons for this. These include seasonal changes in the distribution of temperature and air pressure around the globe, internal processes, and external influences. There are secular slowdowns, decadal (over decades) unevenness, seasonal and sudden. In Fig. 1 and 2 show graphs showing the change in the length of the day in 1700-2000. and in 2000-2006. In Fig. 1 there is a tendency for the day to increase, and in Fig. 2 - seasonal unevenness. Graphs based on materials from the International Earth Rotation Service and support systems(International Earth Rotation and Reference Systems Service, IERS, http://www.iers.org/).
Is it possible to return the basis of time measurement to astronomy and is it worth doing? This possibility exists. These are pulsars whose rotation periods are preserved with great accuracy. In addition, many of them are known. It is possible that over long periods of time, for example, decades, observations of pulsars will serve to clarify 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 Earth's rotation. And its features carry information about internal structure Earth. This complex problem awaits its researchers.
Rice. 1. The difference between the Earth’s rotation period and 86400 SI, in milliseconds. Data before the beginning of the 20th century. are not very reliable, but the tendency towards an increase in the length of the day is clearly visible.
More strictly, this is the period of time between two climaxes of the same name (upper or lower) (passing through the meridian) of the center of the Sun at a given point on the Earth (or other celestial body).
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Leap year and sidereal day problems
More about time
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Solar day on Earth
Fluctuations in the duration of sunny days
Average solar days are not affected periodic changes, like true solar days, but their duration changes monotonically due to changes in the period of the Earth’s axial rotation and (to a lesser extent) with changes in the length of the tropical year, increasing by approximately 0.0017 seconds per century. Thus, the duration of the average solar day at the beginning of 2000 was equal to 86400.002 SI seconds. It should be noted that here the SI second, determined using the intra-atomic periodic process, is indicated as a unit of measurement, and not the average solar second, which by definition is equal to 1/86,400 of the average solar day and, therefore, is also not constant.
Introduction of amendments
Although the average solar day is not, strictly speaking, a constant unit of time, it is everyday life people are connected with them. Due to the accumulation of corrections to the length of the day in mean solar time in relation to uniform atomic time, it is sometimes necessary to add a so-called leap second to the UTC atomic scale in order to restore the reference of this scale to the solar time scale. Theoretically, it is also possible to subtract a leap second, since the rotation of the Earth, in principle, does not have to constantly slow down.
Solar days on other planets and satellites
Moon
The average solar day on the Moon is equal to the average synodic month (the average interval between two identical phases of the Moon, for example, full moons) - 29 days 12 hours 44 minutes 2.82 seconds. True solar days can deviate from the average by 13 hours in both directions, which is associated both with the unevenness of the Earth's orbital motion and with the inclination of the Moon's orbit to the ecliptic, with the ellipticity of its orbit and with the inclination of the Moon's rotation axis to the orbital plane (see also libration).
Others On gas giants that do not have a solid surface, solar days depend on latitude - the atmosphere rotates at different speeds at different latitudes.
Mercury circles the Sun in 87.97 days, and makes a full revolution around its axis in 58.65 days (these periods are in the ratio 3:2). The average time interval between the two upper culminations of the Sun on this planet is 176 days, which is equal to two of its years. Interestingly, when it is near perihelion, the Sun for an observer on the surface of the planet can move at reverse direction, therefore, strictly speaking, linking the definition of a solar day to the culmination in this case is not entirely correct.
On Venus, whose sidereal period of rotation around its axis is 243 days - longer than the orbital period (224.7 days), the average solar day is approximately 116.7 days (due to rotation in the opposite direction)
On Mars, the average solar day is only slightly longer than on Earth. They are equal to 24 hours 39 minutes 35.244 seconds.
On Jupiter, a day is 9 hours 55 minutes 40 seconds, on Saturn it is 10 hours 34 minutes 13 seconds. A day on Uranus is 17 hours 14 minutes 24 seconds, and on Neptune it is 15 hours 57 minutes 59 seconds.
For Pluto, due to its extreme distance from the Sun (and, therefore, the small angular orbital velocity), the average solar day is almost equal to the rotation period: 6 days 9 hours 17 minutes 36 seconds.
Notes
Everyone knows this - 24 hours. But why did this happen? Let's take a closer look at the history of the appearance of the basic units of time and find out how many hours, seconds and minutes there are in a day. We’ll also see whether it’s worth linking these units exclusively to astronomical phenomena.
Where did the day come from? This is the time of one revolution of the earth around its axis. Still knowing little about astronomy, people began to measure time in such ranges, including light and dark times at each time.
But there is interesting feature. When does the day start? From a modern point of view, everything is obvious - the day begins at midnight. People of ancient civilizations thought differently. It is enough to look at the very beginning of the Bible 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 to this. People of that time were guided by the sun setting, the day was over. Evening and night are already the next day.
But how many hours are there in a day? Why was the day divided into 24 hours, since the decimal system is more convenient, and much more convenient? If there were, say, 10 hours in a day, and 100 minutes in each hour, would anything change for us? Actually, nothing but numbers; on the contrary, it would even be more convenient to carry out calculations. But the decimal system is far from the only one used in the world.
They used the sexagesimal counting system. And the light half of the day was well divided in half, 6 hours each. In total, there were 24 hours in a day. This rather convenient division was taken from the Babylonians by other peoples.
The ancient Romans counted time in an even more interesting way. The countdown began at 6 o'clock in the morning. So they counted from that moment on - hour one, hour three. Thus, one can easily consider that the “eleventh hour workers” remembered by Christ are those who begin work at five o’clock in the evening. It's really too late!
At six o'clock in the evening it was twelve o'clock. This is how many hours in a day were counted in ancient Rome. But there were still night hours left! The Romans did not forget about them. After the twelfth hour the night watches began. The duty officers changed every 3 hours at night. Evening and night time was divided into 4 watches. The first evening watch began at 6 pm and lasted until 9. The second, midnight, lasted from 9 to 12. The third watch, from 12 at night to 3 in the morning, ended when the roosters crowed, which is why it was called “rooster crowing.” The last, fourth watch was called “morning” and ended at 6 am. And it all started all over again.
The need to divide the clock into its component parts also arose much later, but they did not deviate from the sexagesimal system even then. And then the minute was divided into seconds. True, it later became clear that it is impossible to rely only on the duration of seconds and days to determine the duration of seconds and days. Over the course of a century, the length of the day increases by 0.0023 seconds - it seems like very little, but enough to get confused about the question of how many seconds there are in a day. And that's not all the difficulties! Our Earth does not complete one revolution around the Sun in exactly the same number of days, and this also affects the solution to the question of how many hours there are in a day.
Therefore, to simplify the situation, a second was not equated to movement. celestial bodies, and to the time of processes occurring inside the cesium-133 atom in a state of rest. And to correspond to the actual state of affairs with the Earth’s revolution around the Sun, 2 extra leap seconds are added twice a year - on December 31 and June 30, and an additional day is added once every 4 years.
In total, it turns out that there are 24 hours in a day, or 1440 minutes, or 86400 seconds.
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Day in astronomy
The length of a day on a planet depends on the angular speed of its own rotation. In astronomy, there are several types of days, depending on the reference system. If you choose a distant star as the reference point for rotation, then, unlike the central star of the planetary system, such days will have a different duration. For example, on Earth, a distinction is made between an average solar day (24 hours) and a sidereal or sidereal day (approximately 23 hours 56 minutes 4 seconds). 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 is displaced against the background of distant stars.
A true solar day is the time interval between two upper culminations (successive passages of the Sun’s center through southern part meridian (for northern hemisphere); in other words, the time between two true noons); the beginning of this day is taken to be the moment the center of the Sun passes 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 occurs from the inclination of the ecliptic to the equatorial plane and from the uneven movement of the Earth around the Sun.
International System of Units (SI)
The unit of measurement of time - day (Russian designation: day; international: d) is one of the non-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 period with the scope of “all areas”. In this case, 1 day is taken to be equal to exactly 86,400 seconds. In the SI, a second is defined as 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom. Accordingly, the definition of a day in SI can be considered 794,243,384,928,000 such periods.
In astronomy, a day defined 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 equal to 86400.002 s), and the duration of the average solar day is also variable due to the secular change in the angular velocity of the Earth's rotation (see).
In other languages
As mentioned above, in common parlance the term day often replaced by the word day, but in any case, in the Russian language there are words for unambiguously separating the concepts of “day” (daylight) 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 night and day separately, depended on the degree of development of a given people and increased gradually with the development of mankind. Most of peoples of the New World divided the day into only four parts corresponding to sunrise, highest point his day's journey, sunset and, finally, the middle of the night. According to the traveler Gorrebow, who described Iceland in the middle XVIII century, the Icelanders divided the day into 10 parts. The Arabs distinguished only the sunrise, its rise and fall, sunset, twilight, night, the first crow of a rooster and dawn. However, among some formerly uncivilized peoples one could find a comparatively precise division of the day, as, for example, among the natives of the Society Islands, who in Cook's time had a division of the day into 18 parts, the length of which was, however, unequal; the shortest periods of time corresponded to morning and evening, the longest - midnight and noon.
In Babylon there was also a 12-hour division of day and night. According to Herodotus’ “History” (II, 109), the Greeks adopted this system from the Babylonians, and later, probably from the Egyptians or Greeks, the Romans adopted it. For example, in winter the duration of the “day hour” in Rome was about 45 minutes.
| Period | Number of daytime hours | The beginning of the first hour of the day according to modern calculus | Number of night hours | Beginning of the first hour of the morning according to modern calculus |
|---|---|---|---|---|
| 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 |
Divided 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 in search of food. Rats also have different numbers of toes on their front and hind feet, making these rodents a symbol of “turnaround,” “new beginning.” |
| 01:00-03:00 | Hour of the Ox | The time when oxen begin to chew the cud, slowly and happily. |
| 03:00-05:00 | Hour of the Tiger | The time when tigers are at their most ferocious, prowling for 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 fly in the sky to make it rain. |
| 09:00-11:00 | Hour of the Snake | The time when snakes leave their shelters. |
| 11:00-13:00 | Hour of the Horse | The time when the sun is high in the zenith, and while other animals lie down to rest, horses are still on their feet. |
| 13:00-15:00 | Hour of the Sheep | A time when sheep and goats eat grass and urinate frequently. |
| 15:00-17:00 | Hour of the Monkey | Monkeys' active life time |
| 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 | It's time for the dogs to do their building security duties. |
| 21:00-23:00 | Hour of the Pig | The time when pigs sleep peacefully. |
Divided into 30 main parts
Division into 22 main parts
Divided into 10 main parts
| Time | Geological period | Number of days in a year | Length of day |
|---|---|---|---|
| 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 | Silur | 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 emergence 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 fossilized stromatolites, 1.3 billion years old, that once grew near the equator and are buried in the Yanshan Mountains. Blue-green algae react to the change in light and dark times of day by the direction of their growth and depth of color: during the day they are colored bright hues and grow vertically, at night they are dark in color and grow horizontally. By appearance of these organisms, taking into account the speed 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 growth rhythms of blue-green algae. According to the results obtained, scientists concluded that 1.3 billion years ago (in the Precambrian era), the earth's day lasted 14.91-16.05 hours, and the year consisted of 546-588 days.
There are also opponents to this assessment, pointing out that research data on ancient tidal deposits, tidalites, contradict it.
In addition to changes in the speed of rotation of the Earth over a long period of time (and the resulting change in the length of the day), from day to day there are minor changes in the speed of rotation of the planet associated with the distribution of masses, for example, due to a decrease in the volume of the world's oceans or atmosphere due to fluctuations in their average temperature . When the world's oceans or atmosphere cool, the Earth rotates faster (and vice versa), since as a result the law of conservation of angular momentum operates. Also, changes in the average length of the day can be caused by geological events, for example, strong earthquakes. Thus, as a result of the 2004 earthquake in the Indian Ocean, the length of the day decreased by approximately 2.68 microseconds. Such changes have been noted and can be measured with modern methods.
In 1967, the International Committee of Weights and Measures adopted a fixed second, without reference to the current length of the 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
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