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How long does Earth take to rotate once on its axis?
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Earth
A composite image of Earth captured by instruments aboard NASA's Suomi National Polar-orbiting Partnership satellite, 2012.
The time it takes for Earth to rotate once on its axis depends on the reference point used:
- Sidereal Day: The time required for Earth to rotate once relative to the background stars is 23 hours, 56 minutes, and 4 seconds. This is considered the true rotation period of the planet.12
- Mean Solar Day: The time between two successive transits of the Sun over the same meridian is approximately 24 hours. This period is slightly longer than the sidereal day because Earth’s orbital motion around the Sun requires the planet to rotate a bit further to bring the Sun back to the same position in the sky.12
It is important to note that Earth's rotation is not perfectly uniform and is gradually slowing down due to tidal friction, which lengthens the day by approximately 1.6 to 2.3 milliseconds per century.342