When I was little I learned that The earth rotates. My grandfather once told me about sundials and what their principle was. It's so common to watch sunrise and sunset Sun, but what will happen if The earth will stop?

Which direction does the Earth rotate?

It all depends on how you look at it. Relatively South Pole, the globe will rotate in the direction clockwise, and quite the opposite on North Pole. It is logical that the rotation occurs in the direction of the east - after all, the Sun appears from the east and disappears in the west. Scientists have found that the planet is gradually slows down by thousandths of a second per year. Most of the planets in our system have the same direction of rotation, the only exceptions being Uranus And Venus. If you look at the Earth from space, you can notice two types of movement: around its axis, and around the star - the Sun.

Few people didn't notice whirlpool water in the bathroom. This phenomenon, despite its commonality, is quite a mystery for the scientific world. Indeed, in Northern Hemisphere the whirlpool is directed counterclock-wise, and in the opposite - everything is the other way around. Most scientists consider this a show of power Coriolis(inertia caused by rotation Earth). Some other manifestations of this force can be cited in favor of this theory:

• V northern hemisphere winds of the central part cyclone they blow counterclockwise, in the south - vice versa;
• the left rail of the railway wears out the most in Southern Hemisphere, whereas in the opposite - right;

• by the rivers in Northern Hemisphere pronounced right steep bank, in Yuzhny it’s the other way around.

What if she stops

It's interesting to imagine what would happen if our planet stops rotating. For an ordinary person, this would be equivalent to driving cars at 2000 km/h and then sudden braking. I think there is no need to explain the consequences of such an event, but this will not be the worst thing. If you are at this moment equator, the human body will continue to “fly” at a speed of almost 500 meters per second, but those who are lucky enough to be closer to poles, you will be able to survive, but not for long. The wind will become so strong that the force of its action will be comparable to the force nuclear bomb explosion, and wind friction will cause fires all over the planet.

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Rotation of the Earth around its axis

The Earth rotates around an axis from west to east, that is, counterclockwise when looking at the Earth from the North Star (North Pole). In this case, the angular velocity of rotation, i.e. the angle through which any point on the Earth’s surface rotates, is the same and amounts to 15° per hour. Linear speed depends on latitude: at the equator it is highest - 464 m/s, and the geographic poles are stationary.

The main physical proof of the Earth's rotation around its axis is the experiment with Foucault's swinging pendulum. After the French physicist J. Foucault c. In the Parisian Pantheon he carried out his famous experiment, the rotation of the Earth around its axis became an immutable truth.

Physical evidence of the Earth’s axial rotation is also provided by measurements of the arc of the 1° meridian, which is at the equator and at the poles. These measurements prove the compression of the Earth at the poles, and this is characteristic only of rotating bodies. And finally, the third proof is the deviation of falling bodies from the plumb line at all latitudes except the poles. The reason for this deviation is due to their inertia maintaining a higher linear velocity of point A (at altitude) compared to point B (near the earth's surface). When falling, objects are deflected to the east on the Earth because it rotates from west to east. The magnitude of the deviation is maximum at the equator. At the poles, bodies fall vertically, without deviating from the direction of the earth's axis.

The geographic significance of the Earth's axial rotation is extremely large. First of all, it affects the figure of the Earth. The compression of the Earth at the poles is the result of its axial rotation. Previously, when the Earth rotated at a higher angular velocity, the polar compression was greater. The lengthening of the day and, as a consequence, a decrease in the equatorial radius and an increase in the polar one is accompanied by tectonic deformations of the earth's crust (faults, folds) and a restructuring of the Earth's macrorelief.

An important consequence of the axial rotation of the Earth is the deviation of bodies moving in the horizontal plane (winds, rivers, sea currents, etc.) from their original direction: in the northern hemisphere - to the right, in the southern - to the left (this is one of the forces of inertia, called the Coriolis acceleration in honor to the French scientist who first explained this phenomenon).

According to the law of inertia, every moving body strives to maintain unchanged the direction and speed of its movement in world space.

Deflection is the result of the body participating in both translational and rotational movements simultaneously. At the equator, where the meridians are parallel to each other, their direction in world space does not change during rotation and the deviation is zero. Toward the poles, the deviation increases and becomes greatest at the poles, since there each meridian changes its direction in space by 360° per day. The Coriolis force is calculated by the formula F=m*2w*v*sinj, Where F– Coriolis force, m– mass of a moving body, w– angular velocity, v– speed of a moving body, j– geographical latitude. The manifestation of the Coriolis force in natural processes is very diverse. It is because of it that vortices of different scales arise in the atmosphere, including cyclones and anticyclones, winds and sea currents deviate from the gradient direction, influencing the climate and through it the natural zonality and regionality; The asymmetry of large river valleys is associated with it: in the northern hemisphere, many rivers (Dnieper, Volga, etc.) for this reason have steep right banks, left banks are flat, and in the southern hemisphere it’s the other way around.

The rotation of the Earth is associated with a natural unit of time - the day - and there is a change between day and night. There are sidereal and sunny days. Sidereal day is the time interval between two successive upper culminations of a star through the meridian of the observation point. During a sidereal day, the Earth makes a complete rotation around its axis. They are equal to 23 hours 56 minutes 4 seconds. Sidereal days are used for astronomical observations. A true solar day is the time interval between two successive upper culminations of the center of the Sun through the meridian of the observation point. The length of the true solar day varies throughout the year, primarily due to the uneven movement of the Earth along its elliptical orbit. Therefore, they are also inconvenient for measuring time. For practical purposes, the average solar day is used. Mean solar time is measured by the so-called mean Sun - an imaginary point that moves evenly along the ecliptic and makes a full revolution per year, like the true Sun. The average solar day is 24 hours long. They are longer than sidereal days, since the Earth rotates around its axis in the same direction in which it moves in its orbit around the Sun with an angular velocity of about 1° per day. Because of this, the Sun moves against the background of the stars, and the Earth still needs to “turn” by about 1° for the Sun to “come” to the same meridian. Thus, during a solar day, the Earth rotates approximately 361°. To convert true solar time to mean solar time, a correction is introduced - the so-called equation of time.

Its maximum positive value was +14 min on February 11, its greatest negative value was -16 min on November 3. The beginning of the average solar day is taken to be the moment of the lowest culmination of the average Sun - midnight. This counting of time is called civil time.

More articles about extraterrestrial space

More articles about the Earth as a planet

When viewed from the North Pole, the Earth rotates counterclockwise, and when viewed from the South Pole, it rotates clockwise. And the Earth (like all the planets of the solar system, except Venus) rotates around its axis counterclockwise. The snail's house spins clockwise from the center (that is, the rotation occurs in a counterclockwise direction). What else is spinning and spinning? One cat’s tail spins clockwise when it sees sparrows (these are her favorite birds), and if they are not sparrows, but other birds, then it spins counterclockwise.

Therefore, experimental evidence of the rotation of the Earth comes down to the proof of the existence of these two inertial forces in the reference frame associated with it. This effect should be most clearly expressed at the poles, where the period of complete rotation of the pendulum plane is equal to the period of rotation of the Earth around its axis (sidereal day).

There are a number of other experiments with pendulums used to prove the rotation of the Earth. The first such experiment was carried out by Hagen in 1910: two weights on a smooth crossbar were installed motionless relative to the surface of the Earth. Then the distance between the loads was reduced.

There are a number of other experimental demonstrations of the Earth's daily rotation. In general, the reason for the precession and nutation of the Earth is its non-sphericity and the mismatch of the equator and ecliptic planes.

As a result of the gravitational attraction of the Moon and the Sun at the equatorial thickening of the Earth, a moment of force arises that tends to combine the planes of the equator and ecliptic.

The explanation of the daily rotation of the sky by the rotation of the Earth around its axis was first proposed by representatives of the Pythagorean school, the Syracusans Hicetus and Ecphantus. About a century later, the assumption of the rotation of the Earth became part of the first heliocentric system of the world, proposed by the great astronomer Aristarchus of Samos (3rd century BC).

The fact that the idea of ​​the daily rotation of the Earth had its supporters back in the 1st century AD. e., evidenced by some statements of the philosophers Seneca, Dercyllidas, and the astronomer Claudius Ptolemy.

Clockwise or counterclockwise?

One of Ptolemy's arguments in favor of the immobility of the Earth is the verticality of the trajectories of falling bodies, just like Aristotle. From the work of Ptolemy it follows that supporters of the hypothesis of the rotation of the Earth responded to these arguments that both air and all earthly objects move together with the Earth.

At the same time, he, however, rejected one of Varahamihira’s arguments: in his opinion, even if the Earth rotated, objects could not come off it due to their gravity. The possibility of rotation of the Earth was considered by many scientists of the Muslim East. However, the role of air was no longer considered fundamental: not only air, but also all objects are transported by the rotating Earth.

A special position in these disputes was taken by the third director of the Samarkand Observatory, Alauddin Ali al-Kushchi (XV century), who rejected the philosophy of Aristotle and considered the rotation of the Earth physically possible.

In his opinion, astronomers and philosophers have not provided sufficient evidence to refute the rotation of the Earth. Buridan and Oresme rightly disagreed with this, according to whom celestial phenomena should occur in the same way regardless of whether the rotation is made by the Earth or the Cosmos. If the Earth rotates, then the arrow flies vertically upward and at the same time moves east, being captured by the air rotating with the Earth.

Basic movements of the Earth in space.

However, Oresme's final verdict on the possibility of the Earth's rotation was negative. Thus, the main role in the unobservability of the Earth’s rotation is played by the entrainment of air by its rotation. When refuting the arguments of opponents of the hypothesis about the rotation of the Earth, Bruno also used the theory of impetus. He also predicted that due to the action of centrifugal force, the Earth should be flattened at the poles. A number of objections to the rotation of the Earth were associated with its contradictions with the text of Holy Scripture.

I became interested in the topic of what rotates clockwise and what rotates counterclockwise, and this is what I discovered.

In this case, the axial rotation of the Earth was affected, since the movement of the Sun from east to west is part of the daily rotation of the sky. Since the command to stop was given to the Sun, and not to the Earth, it was concluded that it was the Sun that performed the daily movement. You have set the earth on firm foundations: it will not be shaken forever and ever. Proponents of the rotation of the Earth (notably Giordano Bruno, Johannes Kepler, and especially Galileo Galilei) advocated on several fronts.

See what “EARTH ROTATION” is in other dictionaries:

What kind of news is this? In the end they would consider him a fool, and he would indeed be a fool. These arguments were considered unconvincing by the Catholic Church, and in 1616 the doctrine of the rotation of the Earth was prohibited, and in 1631

Galileo was convicted by the Inquisition for his defense. It must be added that religious arguments against the movement of the Earth were given not only by church leaders, but also by scientists (for example, Tycho Brahe).

Annual movement of the Earth.

According to the law of right-hand traffic adopted in our country, circular traffic goes counterclockwise. That is, in some countries helicopters are made with a rotor rotating clockwise, and in others - counterclockwise.

Flocks of bats, flying out of caves, usually form a “right-handed” vortex. But in the caves near Karlovy Vary (Czech Republic), for some reason they spin in a spiral, twisted counterclockwise... But the dog, before going on business, will definitely spin counterclockwise. The spiral staircases in the castles were twisted clockwise (if viewed from below, and counterclockwise if viewed from above) - so that it would be inconvenient for attackers to attack when ascending.

The mass of Uranus, sandwiched between the mass of Saturn and the mass of Neptune, under the influence of the rotational moment of Saturn's mass, received a clockwise rotation. The DNA molecule is twisted into a right-handed double helix. This is because the backbone of the DNA double helix is ​​made entirely of right-handed deoxyribose sugar molecules.

In the Solar System, counterclockwise rotation (as viewed from the North Pole of the ecliptic) is predominant and therefore more likely. In this sense of the word, non-inertial motion, including the rotation of the Earth around its axis, can be called absolute. The idea of ​​the Earth's rotation forced us to reconsider not only mechanics, but also cosmology.

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Category: NonverbalTags: Earth

The Earth is constantly in motion, rotating around the Sun and around its own axis. This movement and the constant tilt of the Earth's axis (23.5°) determines many of the effects that we observe as normal phenomena: night and day (due to the rotation of the Earth on its axis), the change of seasons (due to the tilt of the Earth's axis), and different climate in different areas. Globes can be rotated and their axis is tilted like the Earth’s axis (23.5°), so with the help of a globe you can trace the movement of the Earth around its axis quite accurately, and with the help of the Earth-Sun system you can trace the movement of the Earth around the Sun.

Rotation of the Earth around its axis

The Earth rotates on its own axis from west to east (counterclockwise when viewed from the North Pole). It takes the Earth 23 hours, 56 minutes, and 4.09 seconds to complete one full rotation on its own axis. Day and night are caused by the rotation of the Earth. The angular velocity of the Earth's rotation around its axis, or the angle through which any point on the Earth's surface rotates, is the same. It is 15 degrees in one hour. But the linear speed of rotation anywhere at the equator is approximately 1,669 kilometers per hour (464 m/s), decreasing to zero at the poles. For example, the rotation speed at latitude 30° is 1445 km/h (400 m/s).
We do not notice the rotation of the Earth for the simple reason that in parallel and simultaneously with us all objects around us move at the same speed and there are no “relative” movements of objects around us. If, for example, a ship moves uniformly, without acceleration or braking, through the sea in calm weather without waves on the surface of the water, we will not feel at all how such a ship is moving if we are in a cabin without a porthole, since all objects inside the cabin will be move parallel with us and the ship.

Movement of the Earth around the Sun

While the Earth rotates on its own axis, it also rotates around the Sun from west to east counterclockwise when viewed from the north pole. It takes the Earth one sidereal year (about 365.2564 days) to complete one full revolution around the Sun. The path of the Earth around the Sun is called the Earth's orbit and this orbit is not perfectly round. The average distance from the Earth to the Sun is approximately 150 million kilometers, and this distance varies up to 5 million kilometers, forming a small oval orbit (ellipse). The point in the Earth's orbit closest to the Sun is called Perihelion. The earth passes this point in early January. The point of the Earth's orbit farthest from the Sun is called Aphelion. The earth passes this point in early July.
Since our Earth moves around the Sun along an elliptical path, the speed along the orbit changes. In July, the speed is minimal (29.27 km/sec) and after passing aphelion (upper red dot in the animation) it begins to accelerate, and in January the speed is maximum (30.27 km/sec) and begins to slow down after passing perihelion (lower red dot ).
While the Earth makes one revolution around the Sun, it covers a distance equal to 942 million kilometers in 365 days, 6 hours, 9 minutes and 9.5 seconds, that is, we rush along with the Earth around the Sun at an average speed of 30 km per second (or 107,460 km per hour), and at the same time the Earth rotates around its own axis once every 24 hours (365 times per year).
In fact, if we consider the movement of the Earth more scrupulously, it is much more complex, since the Earth is influenced by various factors: the rotation of the Moon around the Earth, the attraction of other planets and stars.

For billions of years, day after day, the Earth rotates around its axis. This makes sunrises and sunsets commonplace for life on our planet. The Earth has been doing this since it formed 4.6 billion years ago. And will continue to do this until it ceases to exist. This will probably happen when the Sun turns into a red giant and swallows our planet. But why Earth?

Why does the Earth rotate?

The Earth was formed from a disk of gas and dust that revolved around the newborn Sun. Thanks to this spatial disk, dust and rock particles fell together to form the Earth. As the Earth grew, space rocks continued to collide with the planet. And they had an effect on it that made our planet rotate. And since all the debris in the early Solar System orbited the Sun in roughly the same direction, the collisions that caused the Earth (and most other bodies in the Solar System) to spin spun it in that same direction.

Gas and dust disk

A reasonable question arises: why did the gas-dust disk itself rotate? The Sun and the Solar System were formed at the moment when a cloud of dust and gas began to become denser under the influence of its own weight. Most of the gas came together to become the Sun, and the remaining material created the planetary disk surrounding it. Before it took shape, gas molecules and dust particles moved within its boundaries evenly in all directions. But at some point, randomly, some molecules of gas and dust combined their energy in one direction. This established the direction of rotation of the disk. As the gas cloud began to compress, its rotation accelerated. The same process occurs when skaters begin to spin faster if they press their arms closer to their body.

There are not many factors in space that can cause planets to rotate. Therefore, as soon as they begin to rotate, this process does not stop. The rotating young solar system has high angular momentum. This characteristic describes the tendency of an object to continue spinning. It can be assumed that all exoplanets probably also begin to rotate in the same direction around their stars when their planetary system is formed.

And we are spinning in reverse!

It is interesting that in the solar system some planets have a direction of rotation opposite to their movement around the Sun. Venus rotates in the opposite direction relative to the Earth. And the axis of rotation of Uranus is tilted by 90 degrees. Scientists do not fully understand the processes that caused these planets to acquire such rotation directions. But they have some guesses. Venus may have received this rotation as a result of a collision with another cosmic body at an early stage of its formation. Or perhaps Venus began to rotate in the same way as the other planets. But over time, the Sun's gravity began to slow down its rotation due to its dense clouds. Which, combined with friction between the planet's core and its mantle, caused the planet to spin in the other direction.

In the case of Uranus, scientists suggested that the planet collided with a huge rocky debris. Or perhaps with several different objects that changed its axis of rotation.

Despite such anomalies, it is clear that all objects in space rotate in one direction or another.

Everything is spinning

Asteroids rotate. The stars are spinning. According to NASA, galaxies also rotate. It takes the solar system 230 million years to complete one revolution around the center of the Milky Way. Some of the fastest spinning objects in the Universe are dense, round objects called pulsars. They are the remnants of massive stars. Some city-sized pulsars can rotate around their axis hundreds of times per second. The fastest and most famous of them, discovered in 2006 and called Terzan 5ad, rotates 716 times per second.

Black holes can do this even faster. One of them, called GRS 1915+105, is believed to be capable of spinning between 920 and 1,150 times per second.

However, the laws of physics are inexorable. All rotations eventually slow down. When, it rotated around its axis at a rate of one revolution every four days. Today, our star takes about 25 days to complete one revolution. Scientists believe that the reason for this is that the Sun's magnetic field interacts with the solar wind. This is what slows down its rotation.

The Earth's rotation is also slowing down. The Moon's gravity affects the Earth in such a way that it slowly slows down its rotation. Scientists have calculated that the Earth's rotation has slowed down by a total of about 6 hours over the past 2,740 years. This amounts to just 1.78 milliseconds over the course of a century.

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Basic movements of the Earth in space

© Vladimir Kalanov,
website"Knowledge is power".

Our planet rotates around its own axis from west to east, that is, counterclockwise (when viewed from the North Pole). An axis is a conventional straight line crossing the globe in the region of the North and South Poles, that is, the poles have a fixed position and “do not participate” in rotational motion, while all other location points on the earth’s surface rotate, and the linear speed of rotation is surface of the globe depends on the position relative to the equator - the closer to the equator, the higher the linear speed of rotation (let us explain that the angular speed of rotation of any ball is the same at its various points and is measured in rad/sec, we are discussing the speed of movement of an object located on surface of the Earth and the higher it is, the further the object is removed from the axis of rotation).

For example, at the mid-latitudes of Italy the rotation speed is approximately 1200 km/h, at the equator it is maximum and amounts to 1670 km/h, while at the poles it is zero. The consequences of the Earth's rotation around its axis are the change of day and night and the apparent movement of the celestial sphere.

Indeed, it seems that the stars and other celestial bodies of the night sky are moving in the opposite direction to our movement with the planet (that is, from east to west). It seems that the stars are around the North Star, which is located on an imaginary line - a continuation of the earth's axis in a northerly direction. The movement of the stars is not proof that the Earth rotates around its axis, because this movement could be a consequence of the rotation of the celestial sphere, if we assume that the planet occupies a fixed, motionless position in space, as was previously thought.

Day. What are sidereal and solar days?

A day is the length of time during which the Earth makes a complete revolution around its own axis. There are two definitions of the concept “day”. A “solar day” is a period of time for the Earth’s rotation, in which the Sun is taken as the starting point. Another concept is “sidereal day” (from lat. sidus- Genitive sideris- star, celestial body) - implies another starting point - a “fixed” star, the distance to which tends to infinity, and therefore we assume that its rays are mutually parallel. The length of the two types of days differs from each other. A sidereal day is 23 hours 56 minutes 4 seconds, while the duration of a solar day is slightly longer and is equal to 24 hours. The difference is due to the fact that the Earth, rotating around its own axis, also performs an orbital rotation around the Sun. It's easier to figure this out with the help of a drawing.

Solar and sidereal days. Explanation.

Let's consider two positions (see figure) that the Earth occupies when moving along its orbit around the Sun, “ A" - the observer's place on the earth's surface. 1 - the position that the Earth occupies (at the beginning of the countdown of the day) either from the Sun or from any star, which we define as the reference point. 2 - the position of our planet after completing a revolution around its own axis relative to this star: the light of this star, and it is located at a great distance, will reach us parallel to the direction 1 . When the Earth takes its position 2 , we can talk about “sidereal days”, because The Earth has made a full revolution around its axis relative to the distant star, but not yet relative to the Sun. The direction of observing the Sun has changed somewhat due to the rotation of the Earth. In order for the Earth to make a full revolution around its own axis relative to the Sun (“solar day”), you need to wait until it “turns” about 1° more (equivalent to the daily movement of the Earth at an angle - it travels 360° in 365 days), this It will take just about four minutes.

In principle, the length of a solar day (although it is taken to be 24 hours) is not a constant value. This is due to the fact that the Earth's orbital movement actually occurs at a variable speed. When the Earth is closer to the Sun, its orbital speed is higher; as it moves away from the sun, the speed decreases. In this regard, a concept such as "average solar day", precisely their duration is twenty-four hours.

In addition, it has now been reliably established that the period of rotation of the Earth increases under the influence of the changing tides caused by the Moon. The slowdown is approximately 0.002 s per century. The accumulation of such, at first glance, imperceptible deviations means, however, that from the beginning of our era to the present day, the total slowdown is already about 3.5 hours.

Revolution around the Sun is the second main movement of our planet. The Earth moves in an elliptical orbit, i.e. the orbit has the shape of an ellipse. When the Moon is in close proximity to the Earth and falls into its shadow, eclipses occur. The average distance between the Earth and the Sun is approximately 149.6 million kilometers. Astronomy uses a unit to measure distances within the solar system; they call her "astronomical unit" (a.e.). The speed at which the Earth moves in orbit is approximately 107,000 km/h. The angle formed by the earth's axis and the plane of the ellipse is approximately 66°33", and is maintained throughout the entire orbit.

From the point of view of an observer on Earth, the revolution results in the apparent movement of the Sun along the ecliptic through the stars and constellations represented in the Zodiac. In fact, the Sun also passes through the constellation Ophiuchus, but it does not belong to the Zodiac circle.

Seasons

The change of seasons is a consequence of the Earth's revolution around the Sun. The reason for seasonal changes is the inclination of the Earth's rotation axis to the plane of its orbit. Moving along an elliptical orbit, the Earth in January is at the point closest to the Sun (perihelion), and in July at the point farthest from it - aphelion. The reason for the change of seasons is the inclination of the orbit, as a result of which the Earth tilts towards the Sun with one hemisphere and then the other and, accordingly, receives a different amount of sunlight. In summer, the Sun reaches the highest point of the ecliptic. This means that the Sun makes its longest movement over the horizon during the day, and the length of the day is maximum. In winter, on the contrary, the Sun is low above the horizon, the sun's rays fall on the Earth not directly, but obliquely. The day length is short.

Depending on the time of year, different parts of the planet are exposed to the sun's rays. The rays are perpendicular to the tropics during the solstice.

Seasons in the Northern Hemisphere

Annual movement of the Earth

Determining the year, the basic calendar unit of time, is not as simple as it seems at first glance, and depends on the chosen reference system.

The time interval during which our planet completes its orbit around the Sun is called a year. However, the length of the year varies depending on whether the starting point is taken to measure it infinitely distant star or Sun.

In the first case we mean “sidereal year” (“sidereal year”) . It is equal 365 days 6 hours 9 minutes and 10 seconds and represents the time required for the Earth to completely revolve around the Sun.

But if we measure the time required for the Sun to return to the same point in the celestial coordinate system, for example, at the vernal equinox, then we get the duration "solar year" 365 days 5 hours 48 minutes 46 seconds. The difference between the sidereal and solar years occurs due to the precession of the equinoxes; every year the equinoxes (and, accordingly, the sun stations) come “earlier” by approximately 20 minutes. compared to the previous year. Thus, the Earth moves around its orbit a little faster than the Sun, in its apparent movement through the stars, returns to the vernal equinox.

Considering that the duration of the seasons is in close connection with the Sun, when compiling calendars, it is taken as a basis "solar year" .

Also in astronomy, instead of the usual astronomical time, determined by the period of rotation of the Earth relative to the stars, a new uniformly flowing time, not related to the rotation of the Earth and called ephemeris time, was introduced.

Read more about ephemeris time in the section: Theories of the Moon's motion. Ephemeris time.

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