# What is sidereal

1. Jul 23, 2014

### Greg Bernhardt

Definition/Summary

Dealing with the rotational or orbital motion of a heavenly body with respect to the fixed stars.

Equations

Extended explanation

A sidereal period is the time it takes for an orbiting, or spinning, astronomical body to return to the same position relative to a line from the fixed stars to the body it is orbiting, or to its own centre, respectively.

Examples include the sidereal day (the time it takes for the Earth to complete one rotation with respect to the vernal equinox), which is about 4 minute shy of 24 hrs, and the sidereal month (the time it takes the Moon to orbit the Earth once with respect to the stars), which is 27.32 days long, or about 2.2 days shorter than the synodic month (full moon to full moon).

The Earth's sidereal day has nearly the same duration throughout the year. The Earth's solar day (noon to noon) is shorter when the Earth is closer to the Sun. This is why sundials need a correction, known as the Equation of Time, for each day of the year.

Sidereal time is technically defined as the length of time since the vernal equinox has crossed the local celestial meridian. This is very close to, but not identical to, the length of time measured with respect to the fixed stars.

Sundials follow the actual sun, and show solar time. Ordinary clocks follow the "mean sun", an imaginary body which the Earth orbits in a circle once a year, and show mean solar time. Sidereal clocks follow the vernal equinox, and show sidereal time.

The number of sidereal days in a planet's "year" is always one more than the number of ordinary days (noon to noon). For example, there are (about) 3661/4 sidereal days in the Earth's year, because the Earth must turn 3651/4 times relative to the line joining it to the Sun, and that line must turn once round the Sun, for the Earth to return to the same position.

Generally when the rotational or orbital period of a body is given, it is given as the sidereal period.

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