I recently read that the ancient Greeks, Romans, and Egyptians divided the day into 24 hours, but their hours were not all the same length. They varied depending on the season they were in. But what I am wondering is why was 24 segments (hours) chosen to divide the day? Why not 12 or 10?
You start with a 4-way division of the earth’s rotation (or journey of the Sun God’s barque, whatever), marked out by three obvious points: sunrise in the east, high noon in the south, and sunset in the west (the fourth point, the sun unseen in the north at midnight, is inferred).
Each of these quadrants is subdivided into six segments just by eyeballing the sky, so you can get a rough idea of the time of day from the sun’s position. Thus from sunrise to sunset you get twelve hours. From sunset to sunrise twelve more hours, which you can track by observing the rising and setting of stars or planets.
Twelve is a handy number to work with, because it has so many whole-number factors: 1, 2, 3, 4, 6, and 12. More than any other number under 20. It is a factor of 60, which fit in well with the base-60 Babylonian system (which is, I believe, the source of the 12-hour divisions of day and night). Sixty has more whole-number factors than any other number under 100, so the Babylonians used it to make their computations easier.