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)
09h 31m 39.90s
142.916237 degrees
GAST (Greenwich Apparent Sidereal Time)
09h 31m 39.92s
142.916347 degrees
LMST (Local Mean Sidereal Time)
09h 31m 39.90s
142.916237 degrees
LAST (Local Apparent Sidereal Time)
09h 31m 39.92s
142.916347 degrees
Additional Parameters
Julian Date
2461189.212951
UT (hours)
17.1108
Equation of Equinoxes
0.3960s
Nutation in Longitude
6.47"
True Obliquity
23.4381 deg
Julian Century (T)
0.264044160
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).
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Editorial Note
MyCalcBuddy Editorial Team
This page is maintained as an educational calculator reference.
Formula Source: Standard Mathematical References
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