Siderea

Meet Siderea, an Earth who finishes one turn against the stars before she finishes one turn against the Sun.

A solar day is the time from one noon to the next, 24 hours on average. A sidereal day is one rotation measured against the distant stars. They differ because Earth also travels along its orbit. After one star-measured rotation, Earth has moved about a degree around the Sun, so it must turn a little more to bring the Sun back to the same place in the sky.

Over a year Earth makes one more turn against the stars than against the Sun, so 1/T_sid = 1/T_sol + 1/T_yr. With T_sol = 86,400 s and T_yr ≈ 365.24 solar days: T_sid = 86,400 × 365.24 / 366.24 ≈ 86,164 s = 23 h 56 min 4 s, nearly 4 minutes shorter.

That is why stars rise about 4 minutes earlier each night, and why the night sky changes with the seasons. Observatories set telescopes by sidereal time.

The figures are averages. Earth's spin slows very slightly over time, and the length of a solar day varies through the year because the orbit is elliptical and tilted.