Sidereal Time
Local and Greenwich sidereal time for a UTC instant and longitude.
About this tool
Sidereal time identifies the right ascension on the upper meridian. Compare an object’s RA in the matching mean/apparent equinox system with local sidereal time to estimate its transit.
Enter the date and time in UTC, then longitude: east positive, west negative, within ±180°. Calculate gives local mean and approximate apparent sidereal time. Now calculates for the current UTC instant. Copy copies the two displayed times. Greenwich values, angular values and the Julian date are available in Details.
Example: 2000-01-01 12:00 UTC, longitude 0° → mean sidereal time 18:41:51 (rounded). At 30° east it is 20:41:51.
Uses the IAU 1982 mean-time polynomial and a four-term nutation approximation. UTC substitutes for UT1; no Earth-rotation data, TT conversion or precision apparent astrometry. Outputs are rounded to seconds, not guaranteed to be accurate to one second. Intended for an overview in 1900–2100.