The sidereal time is at one moment and in place given the time Angle of the vernal Point.

Definition

In Astronomie the position of a star on the celestial Sphère is located by two coordinates, the Right ascension and the variation: at any moment the sum of the Right ascension of a star and time sound Angle is equal to sidereal time. To some extent the sidereal time measurement the displacement of the vault of heaven in a place given compared to the Meridian local.

In spite of its name sidereal time is not a Temps with the usual direction, and is not used as tel. time that we use the every day is based, with some nuances close, on the movement of the Sun. It is the solar Temps: the sun seems to turn around the Ground of 24 hours and the every day, when it is midday, the sun is with the highest in the Ciel.

Sidereal day

The apparent sidereal day is defined as being time passing between two successive culminations of the vernal Point. The average sidereal time is him calculated, not according to the real vernal point whose position varies according to the season, but according to an average vernal point. It is shorter 4 minutes than the solar day: if, a certain night, with 23:25, you see a star above a Colline, the following night will see it to you at the same place with 23:21. This difference in approximately 4 minutes is explained as follows: in one year, the Earth carries out approximately 365 turns on itself compared to the Sun, but carries out also a full rotation around the Sun. Therefore, compared to stars, 365 days solar are equivalent to 366 days sidereal (364 if the terrestrial Orbite were Rétrograde). The sidereal days are thus a little shorter than the solar days. The real period of the year being 365,2422 days solar, the exact duration of the sidereal day is of: 365,2422/(365,2422+1) = 0,9972696 solar day, is 23:56 m 4,09s.

Sidereal clock

A sidereal clock is a clock regulated according to sidereal time. It indicates to a moment given the meridian line of right ascension which culminates with the place where it is established. Such a clock thus advances four minutes per day compared to a usual clock; thus if a star is visible with the meridian line with 23:22, the following night it passes there to 23:18.

Precession of the equinoxes

It will be noted that the stars do not return exactly to the same place after one day sidereal in particular because of the Précession of the equinoxes and other disturbances of the rotation of the Earth, the such Nutation.

Internal bonds

• Table of the astrophysical constants
• celestial Sphere
• Right ascension
• variation

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