# Sidereal day

Sidereal day (1 to 2) and
sunny day (1 to 3).

The sidereal day ( Latin sidus ' star ', genitive sideris ) is the period of time between two successive culminations of a fictional, infinitely distant fixed star without its own movement . The sidereal day is the duration of a full revolution of a celestial body around itself in relation to the solid star sky .

The mean sidereal day the Earth takes 23  hours , 56  minutes , 4.099  seconds = 86164.099 s h ≈ 23.9345.

## Comparison with other day definitions

The mean sidereal day on earth is almost 4 minutes shorter than the solar day or its mean value, the civil day , which is based on the apparent orbit of the sun ( synodic period ). This is due to the fact that the earth continues to run on its orbit between two solar culminations and therefore has to turn around 1365 of a turn ≈ 0.986 ° further than for a sidereal day (see illustration). An earth year therefore has - reasonably accurate - one sidereal day more than civil days. This difference, spread over the days of the year, results in the stated value of just under 4 minutes per day.

A sidereal day is also 0.008 seconds longer than the mean sidereal day , which refers to the culmination of the spring point refers to. Because of the nutation and precession of the earth's rotation , the spring equinox shifts against the apparent movement of the sun in the precession cycle by 360 ° every 25,800 years ≈ 0.014 degrees per year (50  arc seconds per year). As a result, the earth has to rotate a little less for a sidereal day than for a sidereal day.

## use

The sidereal day relates - compared to the fundamental system of astrometry - to a coordinate system that is much more stable than the solar day with regard to the annual orbit of the earth around the sun : through a suitable choice of reference stars , it can be applied to a maximum of the inaccuracies caused by the sun's orbit around the galactic Subject to center . It therefore represents - almost - exclusively the rotation of the earth. Therefore, the sidereal day is used to measure astronomical events .

However , the sidereal day is not suitable for defining the length of the day and thus as an astronomical calibration instrument of the second , which should correspond to the calendar calculation: the solar day and tropical period are always used for this purpose ( the sidereal day is also better suited for this in its relation to the vernal equinox than the sidereal day, see ephemeris second , tropical year ).

## Constancy

The rotation speed of the earth is extremely stable compared to the other periods and subject to only minimal fluctuations, both because a top rotates very evenly in principle , and because the moon also stabilizes the earth's axis (see earth-moon system ).

An astronomically measured day like the sidereal day is nevertheless not constant:

• Presumably because of mass shifts in the interior of the earth , the time of rotation of the earth fluctuates by a few milliseconds over a period of several years. Fluctuations of greater amplitude over longer periods of time also appear possible. The water masses of the oceans also have an influence.
• The tidal friction as well as gravitational tidal forces that the masses of the sun and moon exert on the earth's mass increase the duration of the sidereal day by about 1.7 milliseconds per century. The same effect has already led to the moon itself being bound to rotate around the earth.