Sidereal day

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A sidereal day is the time between two upper culminations of the vernal equinox , caused by the rotation of the earth. The sidereal day forms the basis of the so-called sidereal time , a time scale used in astronomy and geodesy .

A mean sidereal day lasts about 86,164.0905 seconds , measured in SI units , or 23.93446958  h or 23 hours 56 minutes 4.0905 seconds and is thus around  1365 shorter than a mean solar day (on which the civil day and the usual calendar day is also based on this).

In precise astronomical yearbooks , the star locations are usually tabulated for the time of the culmination in Greenwich . Thus the table interval is an integer number of sidereal days. In order to prevent errors in the interpolation , the argument is often not given in hours but in fractions of a day .


Despite its name, sidereal day does not refer precisely to the stars, but to the vernal equinox , which gradually shifts from the fixed stars due to the precession of the earth .

In general, in astronomy, the word “tropical” is used for “related to the vernal equinox”; therefore the annual period corresponding to sidereal day , caused by the orbit of the earth around the sun, is called a tropical year . However, while one speaks of a tropical year and a tropical month , the designation tropical day instead of sidereal day is not in use, although it would be logical and consistent.

Difference to the sidereal day

In contrast to sidereal day , a sidereal day actually refers to the fixed star sky as a background and describes the time span between two upper culminations of a distant star (free from its own movement ) (see also sidereal period ). A sidereal day is about 0.008 seconds longer than a sidereal day.

The designation in English-language literature occasionally leads to confusion, since the sidereal day there is called sidereal day , and the sidereal day is called stellar day as the period not influenced by precession .

Sidereal day and earth rotation

In astronomical practice - in a simplified calculation of the coordinates of observed celestial bodies - the duration of the mean sidereal day is often referred to as the (approximate) rotation duration of the earth's rotation , although the duration of the mean sidereal day would be measured closer to the fundamental system of astronomy. However, since the speed of the earth's rotation is not constant, the current time spans of both a sidereal day and a sidereal day are subject to slight fluctuations in the range of a few milliseconds.

Individual evidence

  1. Compare Useful constants , Earth Orientation Center , International Service for Earth Rotation and Reference Systems ( IERS ); accessed on July 1, 2020.
  2. ^ "The term for one rotation with respect to the ICRS is a" stellar day. " The IERS lists a stellar day as 86164.098903691 s, in comparison to sidereal day duration of 86164.09053083288 s and a conventional solar day (without leap seconds) of precisely 86,400 s. "(David Crouse: An Overview of Major Terrestrial, Celestial, and Temporal Coordinate Systems for Target Tracking. Naval Research Laboratory, August 2016, p. 24 ).
  3. Michel Capderou: Satellites: Orbits and Missions. Springer Science & Business Media 2006, ISBN 978-2-287-27469-5 , p. 144. google books
  4. Cesare Barbieri: Fundamentals of Astronomy. CRC Press 2017, ISBN 978-1-351-99217-6 , Chapter 10 The Astronomical Times. google books