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Sidereal and Solar Time
Sidereal vs solar day
After 365 solar days, Earth has spun ~366 times relative to the stars — one extra sidereal day per year.
Orbit diagram
Before you start
Why is a solar day longer than a sidereal day? Because Earth moves along its orbit while it spins.
Key idea
Earth must rotate slightly more than 360° for the Sun to return to the same sky position. Stars return 4 minutes earlier each night.
What you see
- Orbit diagram with Earth moving around the Sun
- Optional equinox and solstice markers
- Day slider advancing the year
Try this
- Enable equinox/solstice points and identify each on the orbit
- Advance days and watch Earth orbit
- Count: 365 solar days ≈ 366 sidereal days per year
Common mistakes
- Thinking all days are exactly 24 h by definition (solar day varies slightly)
- Confusing sidereal month with synodic month
Glossary
- Solar day
- Sun returns to same position — 24 h on average.
- Sidereal day
- Stars return to same position — 23 h 56 min.