Astronomical day on earth. Sunny day
This concept arose in ancient times. The length of the day was beyond doubt, which was even expressed in the proverb: “Day and night - a day away.” The time taken as the beginning of the day varied from people to people and from era to era. Now midnight is considered the end of the previous day and the beginning of the next. In Ancient Egypt, the day was counted from dawn to dawn, among the ancient Jews - from evening to evening (now this counting has been preserved in the Orthodox Church).
Day on Earth
The development of science has clarified the concept of a 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. This intersection is called the upper culmination, 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 average Sun (an imaginary point that during the tropical year makes a full revolution around the vernal equinox point, moving evenly along the equator) and the vernal equinox point or a specific star. In the first case they talk about true solar days, in the second - about average solar days, in the third - about sidereal days.
The length of the sidereal day differs from the duration of the solar day. The Earth not only rotates around its axis, it also rotates around the Sun. For the Sun to appear in the sky, the Earth has to make a little more than a full 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 length of the true solar day constantly fluctuates due to the Earth's orbit, therefore, for convenience, the basis for calculating time is the average solar day, the duration of which is 24 hours.
Days 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 axis tilt. For example, on the Moon, the average solar day lasts according to earthly reckoning 29 days 44 minutes 2.82 seconds, and the deviation of the true solar day from this indicator can reach 13 hours.
Except for the Moon, Phobos, Deimos and Charon, all satellites in the Solar System revolve around giant planets. The gravity of these colossal planets slows down the rotation of the satellites, so for most of them the day turns out to be 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 satellite, Titan, its rotation speed is constantly changing. One day on Hyperion can differ from others by several tens of percent!
Among the planets, Mars is closest to Earth in terms of day length: a Martian day lasts 24 hours 39 minutes 35.244 seconds.
Venus and Jupiter can be considered “record holders” for the length of the day. On Venus, the day is the longest - 116 Earth days, and on Jupiter - the shortest, just under 10 hours. However, in relation to Jupiter and other gas giants, the length of the day is spoken of only as an average. The substance that makes up the gas ball rotates at different speeds at different latitudes. For example, the exact length of a day at Jupiter’s equator is 9 hours 50 minutes 30 seconds, and at the poles it is one second less.
Here on Earth, we tend to take time for granted, never considering that the increments in which we measure it are quite relative.
For example, the way we measure our days and years is actually a result of our planet's distance from the Sun, the time it takes to revolve around it, and to rotate on its own axis. The same is true for other planets in our solar system. While we Earthlings calculate the day in 24 hours from dawn to dusk, the length of one day on another planet differs significantly. 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 (closest distance to the Sun) to 69,816,900 km at aphelion (farthest). Mercury takes 58.646 Earth days to rotate around its axis, meaning that a day on Mercury takes approximately 58 Earth days from dawn to dusk.
However, it takes Mercury only 87,969 Earth days to circle the Sun once (aka its 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, Mercury's northern polar regions are constantly in shadow.
This is due to its axial tilt of 0.034° (compared to Earth's 23.4°), meaning Mercury does not experience extreme seasonal changes, with days and nights lasting for months, depending on the season. It is always dark at the poles of Mercury.
A 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, a fact that is obvious when you look at its poles. Whereas the planets in the solar system experienced flattening at the poles due to their rotational speed, Venus did not survive it.
Venus rotates at a speed of 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., spinning in the opposite direction of its orbital path around the Sun).
Nevertheless, Venus still rotates around its axis in 243 Earth days, that is, many days pass between its sunrise and sunset. This may seem strange until you know that one Venusian year lasts 224,071 Earth days. Yes, Venus takes 224 days to complete its orbital period, but more than 243 days to go from dawn to dusk.
Thus, one Venus day is slightly more than a Venusian year! It's good that Venus has other similarities with Earth, but it's clearly not a daily cycle!
Day on Earth:
When we think of a day on Earth, we tend to think of it as simply 24 hours. In truth, the sidereal rotation period of the Earth is 23 hours 56 minutes and 4.1 seconds. So one day on Earth is equivalent to 0.997 Earth days. It's strange, but then 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. Due to the tilt of the Earth's axis, the amount of sunlight received in some hemispheres will vary. The most striking 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 during winter, one night can last up to six months, known as the "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 simple as I would like to imagine.
A day on Mars:
In many ways, Mars can also be called “Earth’s twin.” Add seasonal variations and water (albeit frozen) to the polar ice cap, and a day on Mars is pretty close to a day on Earth. Mars makes one revolution around its axis in 24 hours. 
37 minutes and 22 seconds. This means that one day on Mars is equivalent to 1.025957 Earth days.
Seasonal cycles on Mars are similar to ours on Earth, more than on any other planet, due to its 25.19° axial tilt. As a result, Martian days experience similar changes with the Sun, which rises early and sets late in the summer and vice versa in the winter.
However, seasonal changes last twice as long on Mars because the Red Planet is at a greater distance from the Sun. This results in a Martian year lasting twice as long as an Earth year—686.971 Earth days or 668.5991 Martian days, or sols.
Day on Jupiter:
Given the fact that it is the largest planet in the solar system, one would expect the day on Jupiter to be long. But, as it turns out, a day on Jupiter officially 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 rotation speed of approximately 45,300 km/h. This high rotation rate is also one of the reasons why the planet has such strong storms.
Note the use of the word formal. Since Jupiter is not a solid body, its upper atmosphere moves at a different speed than 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 reference frames.
System I is used in latitudes from 10°N to 10°S, where its rotation period is 9 hours 50 minutes and 30 seconds. System II is applied 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 the official rotation of Jupiter (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 approximately 10,476 times.
Day on Saturn:
The situation of Saturn is very similar to Jupiter. Despite its large size, the planet has an estimated rotation speed of 35,500 km/h. One sidereal rotation of Saturn takes approximately 10 hours 33 minutes, making one day on Saturn less than half an Earth day.
Saturn's orbital period is equivalent to 10,759.22 Earth days (or 29.45 Earth years), with a year lasting approximately 24,491 Saturn days. However, like Jupiter, Saturn's atmosphere rotates at different speeds depending on latitude, requiring astronomers to use three different reference frames.
System I covers the equatorial zones of the South Equatorial Pole and the North Equatorial Belt, and has a period of 10 hours 14 minutes. System II covers all other latitudes of Saturn except the north and south poles, with a rotation period of 10 hours 38 minutes and 25.4 seconds. System III uses radio emissions to measure Saturn's internal rotation rate, which resulted in a rotation period of 10 hours 39 minutes 22.4 seconds.
Using these different systems, scientists have obtained various data from Saturn over the years. For example, data obtained 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 in UCLA's 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 stems from the fact that different parts rotate at different speeds.
Day on Uranus:
As we approached Uranus, the question of how long a day lasts became more complex. 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 if it were not for the extreme tilt of the axis of this gas-ice giant.
With an axial tilt of 97.77°, Uranus essentially revolves around the Sun on its side. This means that its north or south points directly toward the Sun at different times in its orbital period. When it is summer at one pole, the sun will shine continuously there for 42 years. When the same pole is turned away from the Sun (that is, it is winter on Uranus), there will be darkness there for 42 years.
Therefore, we can say that one day on Uranus, from sunrise to sunset, lasts as long 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 planet's rotation at the equator, approximately 60° south latitude, is 17 hours and 14.5 minutes, the visible features of the atmosphere move much faster, completing a complete rotation in just 14 hours.
Day on Neptune:
Finally, we have Neptune. Here, too, measuring 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 replace each other faster than the equator.
Considering that the planet's magnetic field rotates at a rate of 16.1 hours, the equatorial zone rotates approximately 18 hours. Meanwhile, the polar regions rotate within 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 axial tilt of 28.32° leads to seasonal variations similar to those on Earth and Mars. Neptune's long orbital period means that a season lasts for 40 Earth years. But since its axial tilt is comparable to Earth's, the change in the length of its day during its long year is not so extreme.
As you can see from this summary of the various planets in our solar system, the length of the day depends entirely on our frame of reference. In addition, the seasonal cycle varies depending on the planet in question and where on the planet the measurements are taken.
Time is the most important philosophical, scientific and practical category. The choice of a method for measuring time has interested 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, the sundial, appeared three and a half millennia BC, this problem remains quite complex. Often answering the simplest question related to it, for example, “how many hours are there in a day,” is not so simple.
History of time calculation
The alternation of light and dark times of the day, periods of sleep and wakefulness, work and rest began to mean the passage of time for people back in primitive times. Every day the sun moved across the sky during the day, from sunrise to sunset, and the moon moved at night. It is logical that the period between identical phases of the movement of the luminaries became a unit of time calculation. Day and night gradually formed into a day - a concept that defines 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 60 minutes and 24 hours?
To count something, ancient man used what was usually always at hand - his fingers. This is where the decimal number system adopted in most countries originates. Another method, based on the phalanges of the four fingers of the open palm of the left hand, reached its peak 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, the presence of many divisors, one of which is 12, made it possible to divide it without a remainder.
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 throughout the year, so for a long time there were day and night hours, which were the same only on the days of the equinox.
From the Sumerians, the Greeks also adopted the division of a circle into 360 degrees, on the basis of which a system of geographical coordinates and the division of the hour into minutes (minuta prima (Latin) - “reduced first part” (of the hour)) and seconds (secunda divisio (Latin)) were developed. - “second division” (of the hour)).
Sunny day
The meaning of a day in relation to the interaction of celestial objects is the period of time during which the Earth makes a complete revolution around its axis of rotation. Astronomers usually make several clarifications. They distinguish solar days - the beginning and end of a revolution are calculated by the location of the Sun at the same point on the celestial sphere - and divide them into true and average.
It is impossible to say, down to the second, how many hours there are in a day, which 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 unevenness and complex trajectory of the movement of the star along 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.
We can more or less accurately say how many hours and minutes there are in a day, which experts call average solar. These are the usual calendar periods of time used in everyday life that define a specific date. It is believed that their duration is constant, that they are exactly 24 hours, or 1440 minutes, or 86,400 seconds. But this statement is conditional. It is known that the speed of rotation of the Earth decreases (a day lengthens by 0.0017 seconds per 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 purpose, more stable reference points are selected than nearby celestial bodies. If you calculate the complete revolution of the globe, taking as the initial moment its position relative to the point of the vernal equinox, you can obtain the length of the day, called sidereal.
Modern science establishes exactly how many hours there are in a day that bears the beautiful name of sidereal hours - 23 hours 56 minutes 4 seconds. Moreover, in some cases their duration is further specified: the true number of seconds is 4.0905308333. But this scale of refinement is also insufficient: the constancy of the reference point is affected by the unevenness of the orbital motion of the planet. To exclude this factor, a special, ephemeris origin associated with extragalactic radio sources is selected.
Time and calendar
The final version of determining how many hours there are in a day, close to the modern one, was adopted in Ancient Rome, with the introduction of the Julian calendar. Unlike the ancient Greek system of time calculation, the day was divided into 24 equal intervals, regardless of the time of day or season.
Different cultures use their own calendars, which have specific events, most often of a religious nature, as their starting point. But the length of the average solar day is the same throughout the Earth.
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 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 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 started at 6 am. So they counted from that moment forward - 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 were counted in a day 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 guards changed at night every 3 hours. 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 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, the second was equated not to the movement of celestial bodies, but to the time of processes occurring inside the cesium-133 atom at 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.
Every day the Earth's rotation slows down by 3-4 centimeters, so the length of the day increases. Scientists name several reasons influencing this process. One of them is the ocean tides, due to which huge masses of water rub against the surface of the earth’s crust and imperceptibly but surely slow down its rotation.
An international team of scientists became interested in the magnitude of these deviations. To achieve maximum measurement accuracy, the location of the most distant but clearly visible stars was chosen, and using telescopes, changes in their location were noted over a given time interval. The result obtained was processed on a correlator - a special computer as tall as a person.
The source of information for the correlator is three regions of the planet - the town of Wetzel in Bavaria, Japan and Norway, where radio telescopes are located, with the help of which scientists hoped for the first time to accurately determine the parameters of the Earth's rotation. “We need regions as far apart as possible to maximize the accuracy of the measurements,” says Walter Aleph from the Max Planck Institute for Radio Astronomy (Bonn).
“This allowed us to simulate a one-of-a-kind virtual giant telescope with dimensions equal to the distance between its individual components,” explains the astronomer.
For one hour a week, all three radio telescopes simultaneously take measurements. The information obtained is then compared with data that was obtained over a longer period. As a result, data regarding the rotation of the Earth for one hour is extrapolated to a day.
Since the area of the celestial sphere under study is constantly changing, this allows scientists to perform calculations in centimeters or degrees in order to calculate the speed of rotation of the planet in the corresponding period based on the results.
As is known, earthly time corresponds to atomic physical time, determined by the atomic vibration frequency of cesium. In 1956, the so-called Coordinated Universal Time, better known as Universal Time, was introduced to coordinate time. This is the universal standard for the Earth. To harmonize Universal Time with astronomical time, one second is added to it every two years.
However, for modern technologies this accuracy is no longer sufficient. It is necessary to determine the speed of rotation of the planet every day with maximum accuracy, which is required, for example, by GPS navigation systems. Even a minimal deviation in such calculations would lead to significant errors in position determination.
Astronomers and surveyors claim that the earth's rotation is currently slowing down by 3-4 centimeters per day, that is, by one millisecond. A network of radio telescopes will further refine this value.
