Sidereal Time Calculator
Calculate Greenwich Mean Sidereal Time (GMST), Greenwich Apparent Sidereal Time (GAST), and Local Sidereal Time (LST). Sidereal time is essential for astronomical observations.
Sidereal Time Results
GMST (Greenwich Mean Sidereal Time)
22h 51m 34.65s
342.894379 degrees
GAST (Greenwich Apparent Sidereal Time)
22h 51m 34.68s
342.894481 degrees
LMST (Local Mean Sidereal Time)
22h 51m 34.65s
342.894379 degrees
LAST (Local Apparent Sidereal Time)
22h 51m 34.68s
342.894481 degrees
Additional Parameters
Julian Date
2461137.908912
UT (hours)
9.8139
Equation of Equinoxes
0.3680s
Nutation in Longitude
6.02"
True Obliquity
23.4384 deg
Julian Century (T)
0.262639532
About Sidereal Time
Sidereal time is the hour angle of the vernal equinox. A sidereal day is about 23 hours, 56 minutes, 4 seconds - shorter than a solar day because Earth moves in its orbit. Mean sidereal time uses a fictitious mean equinox, while apparent sidereal time accounts for nutation. Local sidereal time equals GMST plus the observer's longitude (in hours).