Local Sidereal Time: Live LST Clock

Sidereal time is the clock the sky keeps. It always reads the right ascension crossing your meridian, so a star is at its highest at the moment the clock reads that star's RA. Greenwich sidereal time starts ticking below with no setup at all. Give the calculator a longitude, from your device, a city search, or a typed value, and your local sidereal time runs live to the second.

Sidereal time now

The clock updates every second. Local values need only your longitude; latitude never enters into it.

City search

Longitude in degrees (east positive, west negative)

West longitudes are negative: New York is about -74.01, London -0.13, Tokyo +139.69.

Everything is computed in your browser. Two optional lookups leave your device only when you ask: Use my location sends your coordinates once to a place-name service to show your city, and the city search sends just the text you type once to Open-Meteo to find matching coordinates. Nothing is stored by us. Your longitude is saved only in your own browser, so the clock is ready when you return.

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Greenwich sidereal time

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Local sidereal time

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On your meridian now

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The meridian figure is the right ascension crossing your north-south line this second, in of-date coordinates (a catalog's J2000 right ascensions now differ by about a third of a degree, a bit over a minute of right ascension, and the gap grows every year); objects at that right ascension are as high as they get right now. Find them on the Sky Map.

What this calculator answers: which right ascension is on your meridian right now, and so which objects stand at their highest this moment. It knows nothing about your horizon or your weather. For whether a specific object is up tonight, use Today in the Sky or the Sky Map.

How local sidereal time is worked out

For the models, accuracy and data behind these figures, see the methodology and sources page.

A sidereal clock reads 0:00:00 at the instant the First Point of Aries, the zero point of right ascension, crosses the meridian. From then on it simply tracks the turning sky, advancing through 15 degrees of right ascension per sidereal hour. Whatever right ascension is crossing your meridian, that is your local sidereal time.

The computation starts at Greenwich. The engine returns Greenwich sidereal time for the current instant, and multiplying it by 15 turns hours into degrees. In that form navigators know it as GHA Aries, the Greenwich hour angle of the First Point of Aries, the first column of a nautical almanac's daily pages. Your local value is one addition away:

LST (hours) = ( GHA Aries + east longitude ) / 15

West longitudes count negative, and the sum wraps into the 0 to 360 degree range before the division. That is the whole trick: 15 degrees of longitude equals one sidereal hour, so an observer 75 degrees west of Greenwich reads an LST five hours behind Greenwich's. The almanac data page explains the GHA and SHA system this identity comes from.

Apparent, not mean

The engine's value is apparent sidereal time: it includes nutation, the small periodic nodding of Earth's axis driven mostly by the Moon. Mean sidereal time smooths that nodding out. The difference between the two, the equation of the equinoxes, stays under about 1.2 seconds of time, so nothing you point a telescope at will notice; the label just deserves stating plainly, and this page shows apparent.

The sidereal day

One full lap of this clock takes 23 hours 56 minutes 4.09 seconds, the mean sidereal day, about 4 minutes short of the solar day your wall clock keeps. Those missing minutes accumulate: any given star rises about 4 minutes earlier each night, which is why each season owns its own constellations, and why this clock gains steadily on the one on your wall.

Worked example: from Greenwich sidereal time to your LST

Suppose the Greenwich clock reads 05:30:00 and you observe from longitude 104.99 degrees west, roughly Denver. GHA Aries is 5.5 hours times 15, or 82.5 degrees. Adding the east longitude of -104.99 gives -22.49 degrees, and wrapping into the 0 to 360 range makes 337.51 degrees. Dividing by 15 gives 22.50 hours, so the local sidereal time is about 22:30: right ascension 22h 30m is crossing the Denver meridian.

Worked example: how long until a target culminates

Your LST reads 22:30 and the target sits at right ascension 2h 15m. The wait is 2:15 minus 22:30, plus 24 hours to keep it positive: 3 hours 45 minutes of sidereal time. A sidereal hour is about 10 seconds shorter than a clock hour, so on an ordinary clock the wait comes out about 37 seconds less, near enough 3h 45m for any planning. Had the subtraction come out just barely negative, the target would have crossed the meridian moments ago and be starting down the western side.

Who runs on sidereal time

Sidereal time looks like an astronomer's curiosity until you need to point at something.

Telescope pointing. Classic equatorial mounts carry a right ascension setting circle; set it to the current LST and the circle reads true, and dialing in any object's cataloged RA and declination walks the tube to it. Go-to mounts do the same arithmetic silently.
Planning a culmination. An object is highest, and seen through the least air, when it crosses the meridian, and that happens when the LST equals its right ascension. Subtract the clock from the target's RA and you know the wait.
Celestial navigation. The same quantity in degrees is GHA Aries, the anchor of every star sight; the Nautical Almanac here tabulates it live.
Observatory scheduling. Professional and radio observatories publish schedules in LST, because a target returns to the same spot of sky at the same LST on every night of the year, no seasonal correction needed.

For what all of this looks like overhead, open the Sky Map: the line running north to south through your zenith is the meridian this clock is timing.

Frequently asked questions

What is local sidereal time?

Local sidereal time is the right ascension crossing your meridian, the north-south line through the point straight overhead, at this moment. It is time kept against the stars instead of the Sun: a sidereal clock gains about 4 minutes a day on an ordinary clock, and when it reads the same value as a star's right ascension, that star is as high in your sky as it ever gets.

Why is a sidereal day about 4 minutes shorter than 24 hours?

Because Earth moves along its orbit while it spins. One full turn against the stars takes 23 hours 56 minutes 4.09 seconds, but in that time Earth has traveled about 1 degree around the Sun, so it must keep turning a little longer to bring the Sun back over the same meridian. Clocks follow that longer solar day, which is why the stars rise about 4 minutes earlier each night.

How do I use sidereal time at the telescope?

A target crosses the meridian, its highest and steadiest placement, when your local sidereal time equals its right ascension. Subtract the current LST from the target's RA, adding 24 hours if the result goes negative, and you have the wait until it culminates in sidereal hours. If the LST has already passed the RA, the object is west of the meridian and on its way down. Owners of older equatorial mounts also use LST to set the right ascension circle.

What is GHA Aries, and how does it relate to sidereal time?

GHA Aries is the Greenwich hour angle of the First Point of Aries, and it is Greenwich sidereal time expressed in degrees: multiply the sidereal hours by 15. Navigators read GHA Aries from an almanac's daily pages and add a star's SHA to find that star's own hour angle; astronomers add east longitude and divide by 15 to get local sidereal time. It is one quantity in two units.

Is this mean or apparent sidereal time?

Apparent. The clock uses the engine's Greenwich apparent sidereal time, which includes nutation, the small periodic wobble of Earth's axis. Mean sidereal time smooths that wobble out. The two differ by the equation of the equinoxes, which stays under about 1.2 seconds of time, far below anything that matters for pointing a telescope or planning a night.

Why does the clock need my longitude but not my latitude?

Sidereal time measures how far Earth has turned, and everyone on the same north-south meridian has turned by the same amount, so only east-west position matters. Each 15 degrees of longitude east of Greenwich puts your sidereal clock one hour ahead. Latitude decides how high objects climb once they reach your meridian, not when they cross it, so the clock never asks for it.

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