Everyday & Conversions
Sidereal Time Calculator
Work out Greenwich and local mean sidereal time for a date, hour and longitude — the clock astronomers use to point a telescope.
Your inputs
Change any value to update the result instantly.
Local mean sidereal time
17:02:54
Greenwich mean sidereal time
11:54:06
GMST in hours
11.9018 h
Longitude correction
5.1467 h
This is mean sidereal time from the standard low-precision expression. It ignores nutation (the equation of the equinoxes), which shifts apparent sidereal time by at most about 1 second.
What this calculator does
Computes Greenwich and local mean sidereal time for a date, UT hour and longitude.
What your result means
Local mean sidereal time — the right ascension currently on the meridian.
Formula and working
GMST (hours) = 18.697374558 + 24.06570982441908 × D, where D = Julian Date − 2451545.0; LMST = GMST + longitude ÷ 15
- 1
Collect the inputs
Date (UT) = 2026-03-21; Universal Time = 0 h UT; Longitude (east positive) = 77.2 °
- 2
Apply the formula
GMST (hours) = 18.697374558 + 24.06570982441908 × D, where D = Julian Date − 2451545.0; LMST = GMST + longitude ÷ 15
- 3
Result
Local mean sidereal time = 17:02:54 h
Frequently asked questions
Symbols and assumptions
- D
- Days from Julian Date 2451545.0 (J2000.0)
- λ
- Longitude in degrees, east positive
- • Low-precision GMST expression: GMST = 18.697374558 + 24.06570982441908 × D.
- • Longitude east of Greenwich is positive; 15° of longitude equals one hour.
Sources and review
Reviewed 2026-09-22 · Next review 2027-09-22
- • Mean, not apparent, sidereal time: the equation of the equinoxes (up to about 1 second) is not applied.
Important limitation
This is an educational estimate based only on the values and formula shown. Verify current rates, rules, units, and professional standards before relying on the result. It is not personalized financial or investment advice.
Report a mistake or request an improvement
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