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.

What is the Sidereal Time Calculator?

Calculate Greenwich Mean Sidereal Time (GMST) and Local Mean Sidereal Time for any UTC moment and longitude, using the IAU formula based on the Julian Date. Essential for telescope pointing, hour-angle calculations, and observing. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Sidereal Time Calculator

Name: Sidereal Time Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

Get one useful tool a week

Like this tool? Enter your email and we'll send you one genuinely useful Gera tool a week — plus a link to come back to this one. No spam, one-click unsubscribe any time.

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.

Why sidereal time matters for observing

Every star and deep-sky object has a fixed right ascension (RA) on the celestial sphere, analogous to longitude on Earth. A telescope mount in equatorial mode tracks objects by moving at the sidereal rate. When a star’s RA equals the Local Sidereal Time (LST) at your location, the star is exactly on your meridian — the north-south line overhead — where it reaches its highest point and encounters the least atmosphere.

Knowing the LST before a session tells you which part of the sky is currently well-placed for observation. If your LST is 12 hours, right ascension 12h objects are culminating, while objects at RA 18h are rising in the east and objects at RA 6h are setting in the west.

Practical use: hour angle calculation

The hour angle (HA) of an object tells you how far it has passed east or west of your meridian:

HA = LST − RA

A negative HA means the object is still east of the meridian (rising); zero means it is culminating; positive means it has passed the meridian (setting). Most equatorial telescope mounts display HA, but if yours does not, this calculation gives it to you directly.

For example: if your LST is 14.5 hours and you want to observe M42 (Orion Nebula, RA approximately 5h 35m ≈ 5.58h), the hour angle is 14.5 − 5.58 = 8.92h. That means M42 set hours ago and is below the horizon for your current session.

Mean vs apparent sidereal time

This tool returns mean sidereal time, which treats the Earth as rotating smoothly relative to the stars. Apparent sidereal time adds a small correction for the nutation of Earth’s axis (a slight wobble with a period of around 18.6 years). The difference is small — typically a fraction of a second — and is negligible for visual and most astrophotographic work. Precision astrometry and radio observatory pointing use apparent sidereal time; this tool is appropriate for the rest.

Example and tips

For an observer in London (longitude about minus 0.13 degrees) the local sidereal time is essentially the same as GMST. For an observer at longitude minus 90 degrees (near the US Central time zone), the LST is GMST minus 6 hours.

Always feed the tool UTC: if your watch shows 9 PM in a zone that is two hours ahead of UTC, enter 19:00. Time-zone offsets are not automatically removed.

To find a target’s hour angle, subtract its right ascension from the Local Sidereal Time the tool reports; a result near zero means the object is crossing your meridian and is at its best altitude.