This concept originated in ancient times.Length of day is not in doubt that even found expression in the proverb: "The day yes night - a day off."The time it takes for the beginning of the day varies from nation to nation and from age to age.Now the end of the previous day and the beginning of the next is considered midnight.In ancient Egypt was considered day from dawn to dawn, the ancient Jews - from evening to evening (at present such an account preserved in the Orthodox Church).

Day on Earth

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

In astronomy day counted from the meridian crossing luminary.This intersection is called the upper culmination,
as a starting point has traditionally adopted the Greenwich meridian.What matters is the intersection of the meridian of the center of the visible solar disk (this is called true sun), average sun (an imaginary point which is within the tropical year makes a complete revolution around the vernal equinox, moving uniformly along the equator) and the vernal equinox or a specific star.In the first case we speak of the true solar day, the second - on the mean solar, in the third - on the star.

duration of the sidereal day is different from the length of the sun.Earth not only rotates around its axis, it also moves around the sun.The sun appeared in the sky, the Earth has to do a little more than a complete revolution around its axis.Therefore, the length of the solar day, used in everyday life is 24 hours, and the star - 23 hours 56 minutes 4 seconds.This period of time is taken into account in solving astronomical problems.

duration of the true solar day is constantly fluctuating due to uneven motion of the Earth in its orbit, so for convenience based time accounts put the mean solar day, the duration of which is 24 hours.

Day in other solar system objects

Even more startling phenomena related to the length of day can be observed on other planets and moons.Regarding the latter, it is not only the rotation around its axis and movement around the Sun, but also the treatment of the planet and around its axial tilt.For example, on the Moon mean solar day on Earth lasts 29 days account 44 minutes 2.82 seconds, and the deviation of the true solar day on this indicator can reach 13 hours.

Besides the Moon, Phobos, Deimos and Charon all satellites in the solar system orbit the giant planet.The gravity of these huge planets slows down the rotation of the satellite, so most of them turn out to be a day orbital period of the planet.But there is a celestial body that is out of the general picture - Hyperion, one of Saturn's moons.Because of the orbital resonance with another satellite - Titan - its rotational speed is constantly changing.One day on Hyperion could differ from the others for a few tens of percent!

Among the planets for the duration of the day is closest to Earth Mars: Martian day lasts 24 hours 39 minutes 35.244 seconds.
Ā«record-holders" for the duration of the day can be considered as Venus and Jupiter.On Venus, the longest day - 116 Earth days, and on Jupiter - the shortest, a little less than 10 hours.However, in relation to Jupiter and the other gas giants on the length of the day speak only as average.Agent comprising gas ball is rotated at different speeds at different latitudes.For example, the exact length of a day on the equator Jupiter - 9 hours 50 minutes 30 seconds, and at the poles - at least one second.