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Local Sidereal Time Calculator (LST, GMST & When an Object Crosses the Meridian)

Calculate local sidereal time (LST) and Greenwich mean sidereal time for any date, clock time and longitude, plus the hour angle of a star or deep-sky object and the local time it transits the meridian.

Date

Hour (local clock, 0–23)

Minute

Time zone (UTC offset incl. daylight saving)

Longitude (+E, −W)

Object right ascension (optional)

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How it works

JD from the date and UT (= local − offset) · GMST = 280.46061837° + 360.98564736629° × (JD − 2451545.0) + 0.000387933·T² − T³ ÷ 38,710,000, T = (JD − 2451545) ÷ 36525 · LST = GMST + east longitude · hour angle HA = LST − RA; transit when HA = 0 (sidereal hours run 1.0027379× faster than clock hours)

Sidereal time is time measured by the stars rather than the Sun: a sidereal day (23 h 56 m 4 s) is one rotation of Earth relative to the distant stars. Local sidereal time equals the right ascension currently on your meridian, so an object is highest in the sky when LST equals its right ascension — which is why telescope users and astrophotographers need it to plan observations and align equatorial mounts. The calculator uses the IAU expression for Greenwich mean sidereal time from Meeus's Astronomical Algorithms.

Worked example

In New York (74.006° W) at 22:00 EDT (UTC−4) on October 4, 2026 — 02:00 UT on October 5 — Greenwich mean sidereal time is 02:55:04 and local sidereal time is 21:59:03 (329.7613°). The Orion Nebula (RA 5.588 h) has an hour angle of −7.604 h, so it is still rising in the east and will cross the meridian at 05:35 local time on October 5, 2026.

Sources: Meeus, Astronomical Algorithms (2nd ed.), ch. 12 Sidereal Time · US Naval Observatory — Sidereal time

Frequently asked questions

What is local sidereal time used for?

To know which part of the sky is on your meridian: any object whose right ascension equals the LST is due south (north in the Southern Hemisphere) and at its highest. Planetarium software, star charts and setting circles on equatorial mounts all rely on it.

Why does sidereal time gain about 4 minutes a day?

Because Earth moves along its orbit, the Sun shifts about 1° east against the stars each day, so a solar day is about 3 min 56 s longer than a sidereal day. Stars therefore rise four minutes earlier every night.

Is the longitude east or west positive?

Enter east longitudes as positive and west as negative (New York −74.0, Madrid −3.7, Tokyo +139.7). Each 15° of longitude shifts LST by one hour.

What is the difference between mean and apparent sidereal time?

Apparent sidereal time includes the nutation of Earth's axis (the equation of the equinoxes, at most about ±1.2 seconds). Mean sidereal time is good enough for observing and pointing telescopes.

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This calculator is for educational purposes. Double-check important results before acting on them.