Sidereal time

How to read it

Measured against the stars rather than the Sun, the Earth turns once in about 23 hours 56 minutes 4 seconds.¹ A sidereal clock counts 24 sidereal hours in that time, so it gains about four minutes on an ordinary clock every day.

Sidereal time is the angle from your meridian to the March equinox, expressed in hours. A star crosses your meridian at the same sidereal time every night, so astronomers use it to know what is overhead.¹ Enter your longitude, east positive, to get local sidereal time; it stays in your browser.

Where it came from

The solar day is longer because the Earth also moves along its orbit, and has to turn a little further each day before the Sun is back at noon.¹ Over a year the difference adds up to one whole extra turn: the stars rise one more time than the Sun does.

This clock uses the U.S. Naval Observatory’s approximate formula for Greenwich mean sidereal time, and treats your computer’s clock as Universal Time.²

Sources

1. Sidereal time (Wikipedia, CC BY-SA 4.0)
2. Computing Greenwich Apparent Sidereal Time (U.S. Naval Observatory, U.S. government work)