How to Use Sidereal Time Calculator
- Enter the UTC date and UTC time — Switch to UTC first if your local clock differs.
- Enter the observer’s longitude — East positive, west negative, in decimal degrees.
- Read the result — The Sidereal Time Calculator shows GMST in HH:MM:SS and decimal hours, plus LST.
Formula & Theory - Sidereal Time Calculator
The IAU 1982 polynomial expresses GMST in seconds from the Julian Date JD at 0h UT (JD0) and the elapsed UT hours H:
T = (JD0 - 2451545.0) / 36525
GMST0 = 24110.54841 + 8640184.812866 * T + 0.093104 * T^2 - 6.2e-6 * T^3
GMST_sec = GMST0 + 1.00273790935 * H * 3600
GMST = (GMST_sec / 3600) mod 24 // in hours
LST = (GMST + longitude / 15) mod 24The conversion to HH:MM:SS is straightforward:
H = floor(LST)
M = floor((LST - H) * 60)
S = round(((LST - H) * 60 - M) * 60, 2)Use Cases for Sidereal Time Calculator
The Sidereal Time Calculator is essential for:
- Telescope alignment — Equatorial mounts use LST to point at celestial objects.
- Astrophotography — Plan exposures around an object’s hour angle.
- Astronomy education — Show students why stars rise ~4 minutes earlier each day.
- Ham radio — VHF/UHF work using moonbounce relies on sidereal references.
- Satellite passes — Some software inputs require LST or GMST.
- Historical research — Reconstruct sky positions for old observations.
The Sidereal Time Calculator is implemented in pure JavaScript and runs entirely in your browser, so your queries remain private and instant.