Astronomy education tool
Watch a 24-hour solar clock beside Greenwich sidereal time, the star-based day that lasts 23 hours, 56 minutes, and 4.090 seconds.
Solar clock vs star clock
Compare ordinary local solar time with Greenwich sidereal time, the clock astronomers use to know which right ascension is crossing the meridian.
Your local civil clock
Right ascension on Greenwich meridian
Accumulated since local midnight
Earth must rotate a little farther after one star-relative spin so the Sun appears on the meridian again. That extra angle is why the solar day is longer.
Formula note: Greenwich mean sidereal time uses the simplified Meeus expression also referenced by the U.S. Naval Observatory for approximate sidereal time.
A solar day is the familiar 24 hours from one noon to the next. A sidereal day is one full rotation of Earth measured against the distant stars: 86164.090 seconds, or 23h 56m 4.090s. The gap is almost four minutes because Earth is not only spinning; it is also moving along its orbit around the Sun.
After Earth makes one star-relative rotation, the same stars are back on the meridian, but the Sun appears slightly displaced because Earth has advanced in its orbit. Earth must rotate about one extra degree for the Sun to reach the same apparent position. That small extra turn adds about 236 seconds, so stars rise roughly four minutes earlier each night by an ordinary clock.
Astronomers use sidereal time because sky coordinates are tied to right ascension and declination, not to noon on a civil clock. If the local sidereal time is 10h, objects near right ascension 10h are crossing the local meridian. GPS and other satellite systems need both ideas: solar/civil time for users and Earth-rotation/star-frame math for transforming satellite positions into coordinates on the rotating Earth.
Sources: Jean Meeus, Astronomical Algorithms; U.S. Naval Observatory approximate sidereal time; USNO sidereal time data service.