Sidereal time in short
Our clocks follow the Sun: a solar day of 24 hours brings it back to the same place in the sky. The stars come back about 3 minutes 56 seconds sooner, because Earth also moves along its orbit. One rotation relative to the stars, the sidereal day, lasts 23 h 56 min 4.1 s.
Local sidereal time (LST) is the right ascension currently crossing your meridian. If LST is 5:35, the Orion Nebula (right ascension 5 h 35 min) is due south and at its highest. That is why telescope users and astrophotographers work in sidereal time.
How the calculator works
- Convert your clock time to universal time: UT = local time − UTC offset.
- Count the days D since the J2000.0 epoch, January 1, 2000 at 12:00 UT, including the fraction of the day.
- GMST (hours) = 18.697374558 + 24.06570982441908 × D, reduced to 0–24 hours.
- LST = GMST + longitude ÷ 15, with longitude positive east, reduced to 0–24 hours.
This formula from the US Naval Observatory is accurate to about 0.1 second per century, far better than any manual pointing needs.
Example
At 10:00 PM in New York (UTC −5, longitude 74.0° W) on November 4, 2026, universal time is 3:00 on November 5. GMST is about 5:57 and LST is about 1:01. Objects with a right ascension near 1 h, such as the Triangulum Galaxy (M33), are crossing the meridian.
Tips
- Enter the UTC offset in effect, including daylight saving time: −4 in New York in summer, −5 in winter.
- LST is independent of your latitude; only longitude matters.
- Hour angle = LST − right ascension. A positive hour angle means the object has already crossed the meridian and is moving west.
- Sidereal time gains about 2 hours per month on the clock, which is why the winter constellations rise earlier each evening as the season goes on.
Where it is used
Equatorial mounts with setting circles, planetarium apps, radio telescopes and observatory scheduling all rely on sidereal time. The calculator’s degree output (LST × 15) gives the same value as an angle, useful for spreadsheets and code that work in degrees.