A day, in the context of civil or mean solar time used for clocks and calendars, is defined as exactly 24 hours, or precisely 86,400 seconds.

Types of Days

Earth's rotation defines multiple day lengths. A sidereal day, the time for one full rotation relative to distant stars, measures 23 hours, 56 minutes, and 4.0916 seconds.

A solar day, from noon to noon (apparent Sun position), averages slightly longer at about 24 hours but varies by up to 30 seconds due to Earth's orbital motion and elliptical path.

Variations

Actual day lengths fluctuate by milliseconds daily from tidal friction, earthquakes, and core dynamics, with the Moon gradually lengthening days by about 1.7–2.3 milliseconds per century. Leap seconds adjust atomic clocks to match this drift, keeping them within 0.9 seconds of solar time.