Comprehensive Calculator Guide
This calculator works out local sidereal time from the date, time and longitude: time measured by Earth’s rotation relative to the stars rather than the Sun. In Riyadh (46.72° E) at 9 pm on 27 October 2026, local sidereal time is 23:31, so stars with a right ascension near 23.5 h are crossing the meridian. Amateur astronomers use it to point telescopes and know what’s up.
What sidereal time is
Earth turns once relative to the stars in about 23 h 56 min 4 s, the sidereal day; the solar day is about 4 minutes longer because Earth also moves along its orbit. Local sidereal time equals the right ascension of stars on your meridian.
The calculator finds Greenwich sidereal time from the days since noon UT on 1 January 2000 using the US Naval Observatory’s approximation, then adds longitude ÷ 15. Each degree of longitude error shifts the result by 4 minutes.
How to Use
- Enter the date and local time in 24-hour format.
- Enter your UTC offset (3 for Saudi Arabia) and longitude.
- Press “Calculate” to see local and Greenwich sidereal time and what’s crossing overhead.
Formula Used
GMST (h) = 18.697374558 + 24.06570982441908 × days since 2000-01-01 12:00 UT | LST = GMST + longitude ÷ 15
Practical Examples
- Example 1: Riyadh
- 21:00 on 27 October 2026 (18:00 UT): GMST 20:24; LST = 20.41 + 46.72 ÷ 15 = 23.52 h, or 23:31.
- Example 2: Jeddah
- Midnight on 1 January 2027 at 39.17° E: GMST 03:41, LST 06:18, with Betelgeuse and Sirius near the meridian.
Important Tips
- Take an exact longitude from a map app.
- Use it with a star chart.
- Hour angle = LST − right ascension; positive means past the meridian.
Common Mistakes to Avoid
- Entering west longitude as positive.
- Forgetting the UTC offset.
- Confusing sidereal time with clock time.
Frequently Asked Questions
- Why is the sidereal day shorter?
- Earth’s orbital motion means it must turn about 4 minutes more each day to bring the Sun back; the stars return sooner.
- What is it used for?
- Knowing which part of the sky is overhead and pointing telescopes by hour angle.
- Does it differ within a time zone?
- Yes, it depends on longitude: Riyadh is about 30 minutes ahead of Jeddah.