Paritian

Science

Sidereal Time Calculator (Local Sidereal Time)

Local and Greenwich sidereal time for any date, time and longitude — the right ascension crossing your meridian, for pointing a telescope.

Results

Local sidereal time — hours 21 h
Local sidereal time — minutes 41 min
Local sidereal time — seconds 21 s
Local sidereal time in decimal hours 21.68906 h
Greenwich mean sidereal time 22.29839 h

What this tool does

Sidereal time follows the stars instead of the Sun. Because the Earth also moves around the Sun, a sidereal day is about 3 min 56 s shorter than a solar day, so the stars rise four minutes earlier each night. Local sidereal time equals the right ascension of the stars on your meridian right now: a star with RA 5h 35m is highest in the sky when the LST reads 5:35. Enter the time in UTC, not local clock time; the formula is the one in Meeus's Astronomical Algorithms and is good to a fraction of a second.

Formula

GMST = 280.46061837° + 360.98564736629° × (JD − 2451545.0) + 0.000387933 T² − T³ ÷ 38 710 000 · LST = GMST + longitude (Meeus)

Variables

SymbolMeaningUnit
yrYear—
moMonth (1–12)—
dyDay of the month—
huTime (UTC, decimal hours)h
loLongitude (east positive)°
LHLocal sidereal time — hoursh
LMLocal sidereal time — minutesmin
LSLocal sidereal time — secondss
LDLocal sidereal time in decimal hoursh
GMGreenwich mean sidereal timeh

Worked example

  • Year2026
  • Month (1–12)10
  • Day of the month10
  • Time (UTC, decimal hours)21 h
  • Longitude (east positive)-9.14 °
  • Local sidereal time — hours21 h
  • Local sidereal time — minutes41 min
  • Local sidereal time — seconds21 s
  • Local sidereal time in decimal hours21.68906 h
  • Greenwich mean sidereal time22.29839 h

Limitations

  • The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.
  • The calculation runs at full precision and only the display is rounded. If you copy an intermediate value and retype it, small differences can appear.