A sidereal day – one turn of Earth relative to the stars – is about 23 h 56 min 4 s, so sidereal time gains about 3 minutes 56 seconds a day on clock time. Local sidereal time equals the right ascension of stars on your meridian, which tells you what is best placed to observe.
How to use the Sidereal Time Calculator
- Date
- Time (UTC)
- Longitude (east +, west −)
Results update as you type, and every field accepts the units shown in its menu.
How it works
Pick the date and time in UTC and enter your longitude (east positive, west negative).
Tips
- Check whether the end date should be included – event durations usually include it, elapsed time does not.
- Months have 28–31 days, so “one month” is counted by calendar date, not as 30 days.
- Use 24-hour time or set AM/PM carefully when entering clock times.
- Time zones and daylight-saving changes are not applied unless the calculator says so.
Common mistakes to avoid
- Counting a 30-day month when a calendar month is meant (or the reverse).
- Forgetting leap years when counting days across February.
- Mixing decimal hours (7.5 h) with clock time (7:30).
Inputs explained
| Field | What to enter |
|---|---|
| Date | A date |
| Time (UTC) | A clock time (hours and minutes) |
| Longitude (east +, west −) | In ° |
What each result means
- Julian date = [Date] + 2440587.5 + [Time (UTC)]÷24
- GMST (decimal hours) = mod(18.697374558 + 24.06570982441908×([Julian date] – 2451545), 24)
- LST (decimal hours) = mod([GMST (decimal hours)] + [Longitude (east +, west −)]÷15, 24)
- Greenwich Mean Sidereal Time = [GMST (decimal hours)]
- Local Sidereal Time = [LST (decimal hours)]
- Local Sidereal Time (degrees) = [LST (decimal hours)]×15
Formula
D = Julian date − 2,451,545.0GMST (hours) = 18.697374558 + 24.06570982441908 × D (mod 24)LST = GMST + longitude (east positive) ÷ 15Worked example
Inputs — Date: 2026-10-25; Time (UTC): 22:00; Longitude (east +, west −): -0.1276 °.
Results — Julian date = 2,461,339.41667; GMST (decimal hours) = 0.2868; LST (decimal hours) = 0.2783; Greenwich Mean Sidereal Time = 00:17:12; Local Sidereal Time = 00:16:42; Local Sidereal Time (degrees) = 4.174 °.
Frequently asked questions
What is sidereal time?
Time measured by Earth's rotation relative to the stars instead of the Sun.
How long is a sidereal day?
23 h 56 min 4.09 s.
Why does LST matter?
A star is highest in the sky when the local sidereal time equals its right ascension.
What formula does the Sidereal Time Calculator use?
D = Julian date − 2,451,545.0.