Sidereal Time Calculator
Enter a UTC date, time, and longitude to calculate the Local Sidereal Time for astronomical observations.
What sidereal time is
Sidereal time tracks the rotation of the Earth relative to the distant stars rather than the Sun, which is why a sidereal day (about 23 hours, 56 minutes) is slightly shorter than a solar day. Astronomers use Local Sidereal Time (LST) to know which part of the sky is currently overhead at a given longitude.
How this calculator works
The calculator converts your UTC date and time into a Julian Date, then applies the standard Greenwich Mean Sidereal Time (GMST) polynomial formula used in astronomical almanacs. It then adjusts GMST by your longitude (added for east, subtracted for west) to get Local Sidereal Time.
This is the standard formula used for general astronomy and stargazing planning; extremely high-precision applications (e.g. professional telescope pointing) may also apply small nutation and polar motion corrections not included here.