Glossary term: Sidereal Time

Description: Sidereal time is a measure of time based on the Earth's rotation relative to distant stars in the night sky. At night, we can see the pattern of stars in the night sky rotate around us. If we pick a star and note when it reaches its highest position in the sky, then exactly one sidereal day later, namely 23 hours, 56 minutes, and 4.1 seconds later as measured by our clocks, the star will again reach that highest position.

This is subtly different from a solar day, which is the time between local noon – defined as when the Sun reaches its highest point in the sky – on two consecutive days. The reason is that the Earth orbits the Sun. For an observer on the Earth, this introduces additional changes of the Sun's position in the sky over the course of one year. During the time it takes for the Earth to rotate once with respect to the distant stars, the Sun has moved and the Earth needs to rotate a little bit more to catch up. That is why a solar day is a little bit longer than a sidereal day.

Sidereal time is important for astronomers as it tells them which parts of the sky are overhead at a particular point during the day or night, and thus which objects can be observed. In a standard astronomical coordinate system, the equatorial system, the sidereal time at any place on Earth (excluding the poles) is an angle, namely the right ascension (one of the coordinates used to specify locations in the sky in that system) of the point in the sky directly overhead. In practice, when observing, present-day astronomers make use of the time measured by highly precise atomic clocks, and then use computers to calculate which clock time corresponds to which sidereal time.

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Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

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