What is the length of a sidereal day on Earth?

A sidereal day on Earth lasts about 23 hours 56 minutes. More precisely, one rotation relative to the distant stars takes approximately 23 hours, 56 minutes, and 4.09 seconds.

The familiar 24-hour day is a mean solar day: the time Earth takes to bring the Sun back to the same apparent position in the sky. During one rotation, Earth also moves along its orbit around the Sun, so it must turn a little farther before the Sun reaches the same position. That extra rotation takes nearly four minutes.

Astronomers use sidereal time because distant stars provide a nearly fixed reference frame. A star therefore returns to roughly the same position in the sky about four minutes earlier each night, accumulating to about one additional sidereal day over a year.

The term comes from the Latin sidus, meaning “star.” Sidereal and stellar day are often treated as synonyms, although technical astronomy distinguishes the equinox-based sidereal rotation period from the rotation period measured against truly fixed distant stars by a tiny fraction of a second.