Sidereal Time Calculator
Calculate the Local Mean Sidereal Time (LMST), used by astronomers to locate celestial objects.
East is positive, West is negative.
What is Sidereal Time?
Sidereal time is a timekeeping system that astronomers use to locate celestial objects. Unlike solar time, which is based on the Earth's rotation relative to the Sun, sidereal time is based on the Earth's rotation relative to the distant stars.
Solar vs. Sidereal Day
A solar day is exactly 24 hours. However, because the Earth orbits the Sun, it must rotate slightly more than 360 degrees to bring the Sun back to the same position in the sky. A sidereal day is the time it takes for the Earth to rotate exactly 360 degrees, which is about 23 hours, 56 minutes, and 4 seconds.
Frequently Asked Questions (FAQ)
Why do astronomers use sidereal time?
A star will always rise and set at the same sidereal time every day, making it easy to predict when an object will be visible.
What is "Right Ascension"?
Right Ascension (RA) is the celestial equivalent of longitude. An object's RA is equal to the sidereal time when it crosses your local meridian.