Day refers to the period of time required for one rotation of the earth on its axis. In ancient times, the Babylonian day began with sunrise and with sunset among other peoples. The earliest method of measuring parts of the day used the sun's shadow. Sundials, for example, divided the length of daylight into the same number of parts regardless of season. Later, an improvement was made when days were divided into equal parts by using the rate at which water or sand ran out of a container.