Work out sidereal time instantly with clear inputs, formula shown and shareable results.
Sidereal time measures Earth's rotation against the stars rather than the Sun, so a sidereal day is 23h 56m 4s - about four minutes short. Local sidereal time equals the right ascension currently on the meridian, which is exactly what an observer needs to know which objects are highest.
Greenwich sidereal time
GST = 18.697374558 + 24.06570982441908 x days since J2000
Local sidereal time
LST = GST + longitude / 15
Because Earth also advances along its orbit, so it must turn slightly more than 360 degrees to face the Sun again - about four extra minutes.
Pointing telescopes. An object culminates when the local sidereal time equals its right ascension.