siderische

The German adjective siderisch, meaning "sidereal," describes phenomena tied to the stars rather than to the Sun. The term derives from Latin sidus, sidereus, and appears in various inflected forms depending on gender and case. It is commonly used in astronomy, geodesy, and navigation.

Common phrases include siderische Zeit (sidereal time) and siderischer Tag (sidereal day). A sidereal day is about 23 hours 56 minutes 4 seconds, roughly 4 minutes shorter than the solar day. Sidereal time is used to determine when a celestial object crosses the local meridian, aiding telescope pointing.

Related concepts include the sidereal year (time for Earth to complete one orbit relative to the fixed stars, about 365.25636 days) and the sidereal orbital period of the Moon (about 27.321661 days). In celestial coordinates, the term siderisch describes positions tied to the equatorial system, with right ascension and declination as standard measures.

Compared with tropical contexts, siderisch emphasizes star-based references. The term remains common in German-language astronomical literature and in discussions of timekeeping, celestial navigation, and ephemerides.