Sidereal Time Calculator

About this calculator

Sidereal time measures the rotation of the Earth relative to the distant stars rather than the Sun, which is why a sidereal day (about 23h 56m 4s) is slightly shorter than a solar day. Greenwich Mean Sidereal Time (GMST) is derived from the Julian date using a polynomial approximation, and Local Mean Sidereal Time (LMST) is GMST adjusted by the observer's longitude — the value that directly equals the right ascension of whatever celestial object is currently crossing the observer's meridian.

This calculator converts a UTC date, time, and longitude into GMST and LMST. Amateur and professional astronomers use LMST to point telescopes using equatorial mounts and to work out which right-ascension objects will be well-placed for observation on a given night, while planetarium software and astronomical almanacs rely on the same GMST calculation as their timekeeping foundation.