Sidereal Time & Julian Date Calculator

What is sidereal time?

Sidereal time measures the Earth’s rotation relative to the distant stars rather than the sun. A sidereal day is about 23 hours 56 minutes 4 seconds – nearly four minutes shorter than a solar day – so the stars rise four minutes earlier each night. Astronomers use local sidereal time (LST) to point telescopes: a star is highest in the sky when the LST equals its right ascension.

The Julian Date is a continuous count of days since noon UTC on 1 January 4713 B.C., used to avoid calendar complications in astronomy. The Modified Julian Date (MJD) is JD − 2,400,000.5 and starts at midnight.