Sidereal Time Calculator
Positive for East, Negative for West
Local Sidereal Time (LST)
--:--:--
Greenwich Mean Sidereal Time (GMST)
--:--:--
Note: This calculator uses standard astronomical approximations (IAU 1982/2000 reduced) suitable for general
observation.
What is Sidereal Time?
Sidereal time is a "star time" scale based on Earth's rotation relative to fixed stars rather than the Sun. It runs about 4 minutes fast per day compared to solar time.
$$ \text{LST} = \text{GMST} + \frac{\text{Longitude}}{15} $$