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. International Astronomical Union ( public organization 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, its vector 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 (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 the previous direction. Thirdly, the speed of the Earth's rotation, i.e. 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 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 inertial system reference, 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 is interesting with scientific point vision. 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.

This concept arose in antiquity. The length of the day was not in doubt, which even found expression in the proverb: "Day and night - a day away." The time taken as the beginning of the day changed from people to people and from era to era. Now the end of the previous day and the beginning of the next is considered midnight. AT Ancient Egypt the day was counted from dawn to dawn, among the ancient Jews - from evening to evening (now such an account has been preserved in the Orthodox Church).

Days on Earth

The development of science has clarified the concept of day: the time during which the planet makes a complete revolution around its axis. This movement is determined by the position of the luminaries in the sky.

In astronomy, the day is counted from the intersection of the meridian by the luminary. Such an intersection is called the upper climax, and the Greenwich meridian is traditionally taken as the starting point. What matters is the intersection of the meridian by the center of the visible solar disk (this is called the true Sun), the middle Sun (an imaginary point that, during the tropical year, makes a complete revolution around the vernal equinox point, moving evenly along the equator) and the vernal equinox point or a certain star. In the first case, they speak of true solar days, in the second, of mean solar days, and in the third, of stellar days.

The length of a sidereal day is different from the length of a solar day. The Earth not only rotates on its axis, it also rotates around the Sun. In order for the Sun to appear in the sky, the Earth has to make a little more than a complete revolution around its axis. Therefore, the duration of a solar day used in everyday life is 24 hours, and a sidereal day is 23 hours 56 minutes 4 seconds. This period of time is taken into account when solving astronomical problems.

The duration of a true solar day is constantly fluctuating due to the Earth's orbit, therefore, for convenience, the average solar day, the duration of which is 24 hours, is taken as the basis for calculating time.

Day on other objects of the solar system

Even more striking phenomena concerning the length of the day can be observed on other planets and satellites. As for the latter, not only rotation around its axis and movement around the Sun is important, but also rotation around its planet and tilt of the axis. For example, on the Moon, the average solar day lasts 29 days 44 minutes 2.82 seconds on the earth, and the deviation of the true solar day from this indicator can reach 13 hours.

In addition to the Moon, Phobos, Deimos and Charon, all satellites in the solar system revolve around the giant planets. The gravity of these colossal planets slows down the rotation of satellites, so most of them have a day equal to the period of revolution around the planet. But there is one celestial body that stands out from the overall picture - Hyperion, one of the satellites of Saturn. Due to orbital resonance with another moon, Titan, its rotational speed is constantly changing. One day on Hyperion can differ from others by several tens of percent!

Among the planets, Mars is closest to the Earth in terms of the length of the day: the Martian day lasts 24 hours 39 minutes 35.244 seconds.
"Record holders" for the duration of the day can be considered Venus and Jupiter. On Venus, the day is the longest - 116 Earth days, and on Jupiter - the shortest, a little less than 10 hours. However, in relation to Jupiter and other gas giants, the length of the day is only spoken of as an average. The substance that makes up the gas ball rotates at different speeds at different geographical latitudes. For example, the exact duration of a day at Jupiter's equator is 9 hours 50 minutes 30 seconds, and at the poles it is one second less.

How long are the days? You probably think that exactly 24 hours? Depends on the circumstances. A day is a period of time during which the Earth makes one rotation around its axis.

So how long are the days?

In fact, one rotation of the Earth on its axis is never exactly twenty-four hours.

There are 23 hours 56 minutes and 4 seconds in a day. All my life I've lived a lie!

Amazingly, this indicator can fluctuate in one direction or another for as much as fifty seconds! This is because the speed of the Earth's rotation changes all the time - due to friction caused by synoptic situations, tides, and geological events.

On an average year, a day is a fraction of a second shorter than twenty-four hours.

When these discrepancies were revealed with the help of atomic clocks, it was decided to redefine the second as a fixed fraction of a "solar" day, - more precisely, one million six hundred and forty thousandth.

The new second came into use in 1967 and is defined as "a time interval equal to 9,192,631,770 radiation periods corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom in the absence of disturbance by external fields." You can’t say more precisely - it’s just too dreary to pronounce all this at the end of a long day.

The new definition of the second means that the solar day gradually shifts in relation to the atomic. As a result, scientists had to introduce the so-called "leap second" (or "jump second") into the atomic year in order to align the atomic year with the solar year.

Since 1972, leap seconds have been added 23 times. Imagine, otherwise our day would have increased by almost half a minute. And the Earth continues to slow down its rotation. And, according to scientists, in the 23rd century there will be 25 hours in our day.

The last time a "leap second" was added was on December 31, 2005, at the direction of the International Service for Estimating the Parameters of the Rotation and Coordinates of the Earth, based at the Paris Observatory.

Good news for astronomers and those of us who love clocks to keep pace with the Earth's movement around the Sun, but a headache for computer programs and all the hardware that runs on space satellites.

The idea of ​​introducing a "leap second" met with a strong rebuff from the International Telecommunications Union, which even made a formal proposal to completely abolish it back in December 2007.

Of course, you can wait until the difference between Coordinated Universal Time (UTC) and Greenwich Mean Time (GMT) reaches exactly one hour (somewhere in 400 years) and even then put everything in order. In the meantime, the debate around what counts as "real" time continues.

Here on Earth, we tend to take time for granted, never thinking that the step in which we measure it is rather relative.

For example, how we measure our days and years is the actual result of our planet's distance from the Sun, the time it takes to orbit it, and rotate around its own axis. The same is true for other planets in our solar system. While we earthlings calculate a day in 24 hours from dawn to dusk, the length of one day on another planet is significantly different. In some cases, it is very short, while in others, it can last more than a year.

Day on Mercury:

Mercury is the closest planet to our Sun, ranging from 46,001,200 km at perihelion (the closest distance to the Sun) to 69,816,900 km at aphelion (farthest). Mercury rotates on its axis in 58.646 Earth days, which means that a day on Mercury takes about 58 Earth days from dawn to dusk.

However, it takes Mercury only 87,969 Earth days to go around the Sun once (in other words, the orbital period). This means that a year on Mercury is equivalent to approximately 88 Earth days, which in turn means that one year on Mercury lasts 1.5 Mercury days. Moreover, the northern polar regions of Mercury are constantly in shadow.

This is due to its 0.034° axial tilt (Earth's is 23.4° by comparison), meaning that Mercury does not experience extreme seasonal changes where days and nights can last for months, depending on the season. It is always dark at the poles of Mercury.

Day on Venus:

Also known as Earth's twin, Venus is the second closest planet to our Sun, ranging from 107,477,000 km at perihelion to 108,939,000 km at aphelion. Unfortunately, Venus is also the slowest planet, this fact is obvious when you look at its poles. Whereas the planets in the solar system experienced flattening at the poles due to rotational speed, Venus did not survive it.

Venus rotates at only 6.5 km/h (compared to Earth's rational speed of 1670 km/h), which results in a sidereal rotation period of 243.025 days. Technically, this is minus 243.025 days, since Venus's rotation is retrograde (i.e. rotation in the opposite direction of its orbital path around the Sun).

Nevertheless, Venus still rotates around its axis in 243 Earth days, that is, a lot of days pass between its sunrise and sunset. This may seem strange until you know that one Venusian year is 224.071 Earth days long. Yes, Venus takes 224 days to complete its orbital period, but more than 243 days to go from dawn to dusk.

So one day of Venus is a little more than a Venusian year! It is good that Venus has other similarities with the Earth, but this is clearly not a daily cycle!

Day on Earth:

When we think of a day on Earth, we tend to think it's just 24 hours. In truth, the sidereal period of the Earth's rotation is 23 hours 56 minutes and 4.1 seconds. So one day on Earth is equivalent to 0.997 Earth days. Oddly enough, again, people prefer simplicity when it comes to time management, so we round up.

At the same time, there are differences in the length of one day on the planet depending on the season. In view of the inclination earth's axis, the amount of sunlight received in some hemispheres will vary. Most bright cases occur at the poles, where day and night can last for several days and even months, depending on the season.

At the North and South Poles in winter, one night can last up to six months, known as " polar night". In summer, the so-called “polar day” will begin at the poles, where the sun does not set for 24 hours. It's actually not as easy as one would like to imagine.

Day on Mars:

In many ways, Mars can also be called Earth's twin. Add seasonal fluctuations and water (albeit in frozen form) to the polar ice cap, and a day on Mars is pretty close to Earth. Mars makes one revolution on its axis in 24 hours.
37 minutes and 22 seconds. This means that one day on Mars is equivalent to 1.025957 Earth days.

The seasonal cycles on Mars are more similar to ours than on any other planet, due to its 25.19° axial tilt. As a result, Martian days experience similar changes with the Sun rising early and setting late in the summer and vice versa in the winter.

However, seasonal changes last twice as long on Mars because the Red Planet is on greater distance from the sun. This results in a Martian year being twice as long as an Earth year - 686.971 Earth days or 668.5991 Martian days or Sol.

Day on Jupiter:

Given the fact that it is the largest planet in the solar system, one would expect a day on Jupiter to be long. But as it turns out, officially a day on Jupiter lasts only 9 hours 55 minutes and 30 seconds, which is less than a third of the length of an Earth day. This is due to the fact that the gas giant has a very high rotational speed of approximately 45,300 km / h. Such a high rotation speed is also one of the reasons why the planet has such violent storms.

Note the use of the word formal. Since Jupiter is not solid, his upper atmosphere moving at a speed different from the speed at its equator. Basically, the rotation of Jupiter's polar atmosphere is 5 minutes faster than that of the equatorial atmosphere. Because of this, astronomers use three frames of reference.

System I is used at latitudes from 10°N to 10°S, where its rotation period is 9 hours 50 minutes and 30 seconds. System II applies at all latitudes north and south of them, where the rotation period is 9 hours 55 minutes and 40.6 seconds. System III corresponds to the rotation of the planet's magnetosphere, and this period is used by the IAU and IAG to determine Jupiter's official rotation (i.e. 9 hours 44 minutes and 30 seconds)

So, if you could theoretically stand on the clouds of a gas giant, you would see the Sun rise less than once every 10 hours at any latitude of Jupiter. And in one year on Jupiter, the Sun rises about 10,476 times.

Day on Saturn:

The situation of Saturn is very similar to Jupiter. Despite its large size, the planet has an estimated rotational speed of 35,500 km/h. One sidereal rotation of Saturn takes approximately 10 hours and 33 minutes, making one day on Saturn less than half an Earth day.

The orbital period of Saturn's rotation is equivalent to 10,759.22 Earth days (or 29.45 Earth years), and a year lasts approximately 24,491 Saturn days. However, like Jupiter, Saturn's atmosphere rotates at different rates depending on latitude, requiring astronomers to use three different frames of reference.

System I covers the equatorial zones of the South Equatorial Pole and the North Equatorial Belt, and has a period of 10 hours and 14 minutes. System II covers all other latitudes of Saturn, with the exception of the northern and south poles, a rotation period of 10 hours 38 minutes and 25.4 seconds. System III uses radio emission to measure Saturn's internal rotation rate, which resulted in a rotation period of 10 hours 39 minutes 22.4 seconds.

Using these various systems, scientists have obtained various data from Saturn over the years. For example, data acquired during the 1980s by the Voyager 1 and 2 missions indicated that a day on Saturn is 10 hours 45 minutes and 45 seconds (± 36 seconds).

In 2007 this was revised by researchers at the UCLA Department of Earth, Planetary and Space Sciences, resulting in the current estimate of 10 hours and 33 minutes. Much like Jupiter, the problem with accurate measurements is that different parts rotate at different speeds.

Day on Uranus:

As we approached Uranus, the question of how long a day lasts became more difficult. On the one hand, the planet has a sidereal rotation period of 17 hours 14 minutes and 24 seconds, which is equivalent to 0.71833 Earth days. Thus, we can say that a day on Uranus lasts almost as long as a day on Earth. This would be true were it not for the extreme axial tilt of this gas-ice giant.

With an axial tilt of 97.77°, Uranus essentially orbits the Sun on its side. This means that its north or south faces directly towards the Sun at different times of the orbital period. When it is summer at one pole, the sun will shine there continuously for 42 years. When the same pole is turned away from the Sun (that is, it is winter on Uranus), there will be darkness for 42 years.

Therefore, we can say that one day on Uranus from sunrise to sunset lasts as much as 84 years! In other words, one day on Uranus lasts as long as one year.

Also, as with other gas/ice giants, Uranus rotates faster at certain latitudes. Therefore, while the rotation of the planet at the equator, approximately 60° south latitude, is 17 hours and 14.5 minutes, the visible features of the atmosphere move much faster, making a full revolution in just 14 hours.

Day on Neptune:

Finally, we have Neptune. Here, too, the measurement of one day is somewhat more complicated. For example, Neptune's sidereal rotation period is approximately 16 hours 6 minutes and 36 seconds (equivalent to 0.6713 Earth days). But due to its gas/ice origin, the planet's poles rotate faster than the equator.

Bearing in mind that the rotation speed magnetic field planets 16.1 hours, the equatorial zone rotates approximately 18 hours. Meanwhile, the polar regions rotate for 12 hours. This differential rotation is brighter than any other planet in the solar system, resulting in strong latitudinal wind shear.

In addition, the planet's 28.32° axial tilt results in seasonal fluctuations similar to those on Earth and Mars. Neptune's long orbital period means the season lasts for 40 Earth years. But because its axial tilt is comparable to Earth's, the variation in its day length over its long year is not as extreme.

As you can see from this summary about the various planets in our solar system, the length of the day depends entirely on our frame of reference. In addition to that, the seasonal cycle varies, depending on the planet in question, and from where on the planet measurements are taken.

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 light and dark times of the day, 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 anything ancient man I 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 calculated 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, used in Everyday life calendar periods of time that define 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 inside 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 even this scale of refinements is sometimes insufficient: the constancy of the reference point is affected by the unevenness of the planet's orbital motion. To eliminate this factor, a special, ephemeris origin of coordinates is chosen, associated with extragalactic radio sources.

Time and calendar

The final version of the definition of 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.