Sidereal Time

- overview of Sidereal time and what it is.

Sidereal time is a star based time scale.

Sidereal time is used by astronomers to determine where in the sky telescopes should be orientated to see a given star.

It is also used for many time calculations for satellites, where a form of universe time is required rather than one that is purely Earth / Sun based.

The term sidereal day is derived from the Latin sidus meaning "star", i.e. the meaning for sidereal time could be interpreted as "star time".

Sidereal time basics

A sidereal day is the time taken for the Earth to make one rotation relative to the vernal equinox.

This means that a mean sidereal day is approximately 23 hours, 56 minutes, 4.0916 seconds, i.e. 23.9344699 hours or 0.99726958 mean solar days.

Sidereal time arises from the fact that the Earth moves around the Sun. As the period of a day is governed by the relationship of the Earth to the Sun, and the Earth orbits the Sun, the Earth is not in the same direction to stars in the universe for a defined time such as midday, for example.

Basic sidereal time concept
Basic sidereal time concept

This is a simplified explanation of the way in which sidereal time arises and relates to the more widely used solar based time.

Other effects

The simplified view of the Earth's orbit and rotation provides the main reason for the requirement for sidereal time. However as might be expected, the Earth's rotation is not quite as simple as the straightforward rotation of the earth around the Sun in a parallel axis.

It is found that the Earth's rotational axis also rotates around a second axis that is orthogonal to the Earth's normal solar orbit and the period of this is about 25 800 years. This is called the precession of the equinoxes. Because of this precession, the stars appear to move around the Earth in a manner more complicated than a simple constant rotation.

As a result, the precise definition of a sidereal day is the time taken for one rotation of the Earth in this pre-cessing reference frame.

Sidereal time facts and figures

Aspect Details
Mean sidereal day
hours mins secs
23 hours, 56 minutes, 4.0916 seconds
Mean sidereal day
23.9344699 hours
Mean sidereal day
proportion of solar day
0.99726958 mean solar day

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