What is sidereal time and why does it differ from clock time?

Sidereal time tracks the rotation of the Earth relative to the distant stars rather than the Sun. A sidereal day is about 23 hours 56 minutes 4 seconds, roughly four minutes shorter than a solar day, so a given star rises about four minutes earlier each night.

What is the difference between GMST and local sidereal time?

Greenwich Mean Sidereal Time is the sidereal time at the prime meridian. Local Mean Sidereal Time adds your longitude, at the rate of one hour per 15 degrees east of Greenwich, giving the sidereal time at your own location.

Why must I enter UTC and not local time?

The formula is anchored to Coordinated Universal Time. If you enter your local civil time the result will be off by your time-zone offset. Convert your wall-clock time to UTC before entering it.

How accurate is this calculator?

It uses the IAU mean sidereal time expression based on the Julian Date relative to J2000.0, accurate to roughly a tenth of a second. It returns mean sidereal time and does not apply the small nutation correction that distinguishes apparent sidereal time.

How do I use sidereal time when observing?

A star is on your meridian, the highest and best-placed point in its path, when the Local Sidereal Time equals the stars right ascension. Subtracting right ascension from local sidereal time gives the hour angle a telescope mount needs.

Sidereal Time Calculator

Sidereal time is the clock astronomers use to point telescopes, because it follows the stars rather than the Sun. This tool computes Greenwich Mean Sidereal Time and your Local Mean Sidereal Time from any UTC instant and your longitude.

How it works

The calculation starts from the Julian Date, then measures the elapsed days since the J2000.0 epoch and applies the standard mean sidereal time expression:

JD   = unix_ms / 86400000 + 2440587.5
D    = JD − 2451545.0
GMST = 280.46061837 + 360.98564736629 × D   (degrees, then mod 360)
hours = GMST / 15
LST  = (GMST/15 + longitude/15) mod 24

The coefficient 360.98564736629 is slightly more than 360 because the Earth must turn a little extra each solar day to face the Sun again; relative to the stars it completes just under one full turn per solar day.

Example and tips

For an observer in London (longitude about minus 0.13 degrees) the local sidereal time is essentially the same as GMST. Always feed the tool UTC: if your watch says 9 PM in a zone that is two hours ahead of UTC, enter 19:00. To find a target’s hour angle, subtract its right ascension from the Local Sidereal Time the tool reports; a result of zero means the object is crossing your meridian.