I know the length of a day is determined by the earth's rotation around the sun and that it really doesn't matter if there are 24 hours in a day, or 12 hours that are twice as long (following?) but who decided on 24 as the number of hours? Apparently, or so the theory goes, it was the ancient Egyptians who began the process (clever people huh?). They divided their nights into twelve segments based upon different star patterns that appeared throughout the night. Later in time the Greeks formalised the system by dividing daylight hours into twelve segments also. The 24 segments became known as equinoctial hours (based upon there being 12 hours of daylight and 12 hours of darkness on days of equinoxes).