sidereale

Sidereal, or sidereale in Italian, is an astronomical term describing phenomena and measurements referenced to the positions of stars rather than the Sun. It is commonly used to describe sidereal time, a time scale that tracks Earth's rotation relative to distant stars and is essential for precise celestial observations.

A sidereal day is the time required for Earth to rotate 360 degrees relative to the background stars, which is 23 hours 56 minutes 4.0905 seconds. This is about four minutes shorter than the mean solar day, the 24-hour day used for daily life. The difference arises because Earth moves along its orbit around the Sun during each solar day, changing the Sun’s apparent position against the stars.

In addition to days, sidereal measurements exist for other orbital cycles. The sidereal year is the time for Earth to complete one orbit around the Sun relative to fixed stars, approximately 365.25636 days. The Moon’s sidereal month, its orbital period relative to the stars, is about 27.321661 days. In astronomy, coordinates are often expressed in sidereal units, for example right ascension in hours, minutes, and seconds of sidereal time.

The term emphasizes a star-based frame of reference and is widely used in telescope pointing, star catalogs, celestial navigation, and ephemerides. Its etymology traces to Latin sidus, meaning star, with the suffix -arius. Sidereal concepts are fundamental for accurately mapping and predicting celestial motions independent of the Sun’s position.