- adjective referring to the stars or determined with reference to their apparent motion
- Relating to the stars. As well as being used in a general fashion, the term is applied to definitions of the day, month or year measured in terms of the stars. Thus a Sidereal Day is the period needed for the stars to return to the same position in the sky as they were the day before, about 23 hours 56 minutes. The Sidereal Month (27.32 days) is the time taken for the Moon to reappear at the same point relative to the stars, and the Sidereal Year is the time taken for the Sun to reach the same point in the sky relative to the stars, 365.26 days.