The Truth About the Length of a Day

A day isn't the time required for a planet-wide 360° rotation. The Earth, moving in its orbit around the Sun and spinning on its axis, always defines “noon” and “midnight” in the same fashion: where the Sun’s height above or below the horizon is maximized. We rotate 360° each 23 hours, 56 minutes and 4.09 seconds, leaving us 00:03:55.91 short. A full rotation, astronomically, is a sidereal day: different from a solar (calendar) day. Earth's rotation is affected by its motion around the Sun, its axial tilt, and its elliptical orbit. This results in most days being either longer or shorter than 24 hours, with only four days a year having precisely 24 hours.