sidereal

Sidereal refers to the stars. In astronomy, sidereal describes measurements and references that are based on the positions of fixed stars rather than the Sun.

A sidereal day is 23 hours 56 minutes 4 seconds, about four minutes shorter than the solar day. This difference arises because Earth must rotate a full 360 degrees relative to the distant stars, while the Sun’s apparent position slowly shifts due to orbital motion around the Sun.

Local sidereal time measures the right ascension currently on the local meridian. Astronomers use it to locate objects and to relate celestial coordinates to clock time, facilitating telescope pointing and timekeeping during observations.

A sidereal month is the Moon’s orbital period relative to the stars, about 27.321661 days; the synodic month (about 29.53 days) measures the Moon’s phases relative to the Sun.

A sidereal year is the time for Earth to return to the same position against the fixed stars, about 365.25636 days, slightly longer than the tropical (solar) year because of the precession of the equinoxes.

In coordinates, sidereal references use the equatorial coordinate system (right ascension and declination) tied to standard epochs such as J2000.0.

In astrology, sidereal astrology uses fixed-star references; the sidereal zodiac is offset from the tropical zodiac by roughly 23.4–24 degrees due to precession, so signs shift over time relative to their tropical counterparts.