How time works
Why is a day 24 hours long?
Earth’s rotation gives us the day; ancient counting systems gave it 24 named hours.
Nature gives us the cycle, not the number
A day comes from Earth rotating relative to the Sun. The familiar 24-part division is a human convention layered onto that cycle, not a physical rule that days must contain 24 units.
Twelve daylight hours and twelve night hours
Ancient Egyptian timekeepers divided daylight into twelve parts and the night into another twelve. The lengths of these seasonal hours originally changed through the year; later, mechanical clocks encouraged equal hours and the two sets became a 24-hour day.
Solar days are not perfectly identical
The apparent solar day varies slightly because Earth’s orbit is elliptical and its axis is tilted. Civil clocks therefore use a mean solar day, smoothing those variations into equal 24-hour days.