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Sidereal Time & Hour Angle
Sidereal time explained
Sidereal time is based on the stars, not the Sun. One sidereal day ≈ 23 h 56 min — slightly shorter than a solar day because Earth moves along its orbit.
Hour angle (HA) tells how far west a star has traveled since crossing the meridian. HA = Local Sidereal Time − Right Ascension.
Animated diagram (inspired by sidereal time references)
Before you start
Sidereal time and hour angle help astronomers point telescopes. They're based on the stars, not the Sun.
Key idea
A sidereal day is about 4 minutes shorter than a solar day. Hour angle tells how far west a star has moved since crossing the meridian.
What you see
- Animated diagram of Earth rotating with a star and meridian
- LST and hour angle sliders
Try this
- Increase hour angle and watch the star move west
- Adjust LST to change the vernal equinox direction
- Compare with the sidereal vs solar time tool
Common mistakes
- Confusing sidereal and solar time
- Forgetting HA = LST − RA for a star
Glossary
- Sidereal time
- Time based on stars returning to the same position.
- Hour angle
- Westward angle from the meridian to a star.