Definition

1. Adjective:
   • Relating to or measured by the apparent motion of the stars: Used to describe time periods or astronomical measurements that are based on the Earth's rotation relative to distant stars, rather than the Sun.
   • Of or relating to the stars or constellations: Pertaining to celestial bodies, especially stars.

Usage and Examples

• Adjective:
  • A sidereal day is about four minutes shorter than a solar day. (This describes a time measurement based on stars.)
  • Astronomers use sidereal time to track the positions of celestial objects. (This shows its use in the context of astronomical observation.)
  • The study focused on sidereal bodies like distant stars and nebulae. (This illustrates its meaning of relating to stars.)

Advanced Usage

• "Sidereal period": The orbital period of a planet or moon relative to the background of stars, not relative to the Sun.
  • The Moon's sidereal period is approximately 27.3 days.
• "Sidereal month": The time it takes the Moon to complete one orbit around the Earth relative to the fixed stars.
  • A sidereal month is different from a synodic month, which is based on the Moon's phases.

Variants and Related Words

• Sidereally (adverb): In a sidereal manner; with reference to the stars.
  • The telescope was aligned sidereally to track the star's motion.
• Sidereal day (noun phrase): The time it takes for the Earth to complete one rotation relative to the vernal equinox, approximately 23 hours, 56 minutes, and 4 seconds.
• Sidereal year (noun phrase): The time it takes for the Earth to complete one orbit around the Sun relative to the fixed stars, about 20 minutes longer than a tropical year.

Synonyms

• Stellar: Of or relating to stars.
• Astronomical: Relating to astronomy or celestial objects. (Note: This is broader, often including planets and other phenomena.)

Antonyms

• Solar: Relating to or measured by the Sun (e.g., solar day, solar time).
• Terrestrial: Relating to the Earth.

Adjective

1. (of divisions of time) determined by daily motion of the stars
   • sidereal time
2. of or relating to the stars or constellations
   • sidereal bodies
   • the sidereal system