Sidereal Day

A sidereal day is one true rotation of the planet, measured against the stars. It is shorter than the solar day because Earth also advances about a degree along its orbit each day, so it must turn that extra degree — very nearly four minutes' worth — to bring the Sun back to the meridian. Over a year the difference accumulates to exactly one extra rotation: 366.24 sidereal days to 365.24 solar ones.

Observatories run on sidereal time because a star returns to the same position after exactly one sidereal day, and satellite operators care because a geostationary orbit must have this period, not 24 hours, to hang motionless over a point on the equator. Earth's rotation is not perfectly steady either: tidal friction lengthens the day by about 1.8 milliseconds per century, and seasonal winds and core motions shift it by a millisecond or two, which is why leap seconds exist.