For early peoples, the only changes that were truly regular, were the motion of the objects in the sky. The obvious of these changes was the alternate daylight and darkness, caused by the rising  and setting of the sun. Each of these cycles of the sun came to be called a day. Another regular change in the sky was the change in the visible shape of the moon . Each cycle of the moon's changing shape takes about 291/2 days, or a month. The cycle of the seasons  gave people and even longer unit of time. There is no longer change in the sky that lasts seven days, to represent the week. The seven-day week jewish custom of observing a sabbath (day of rest) every seventh day. The division of a day into 24 hours , an hour into 60 minutes, and a minute into 60 seconds probably came from ancient babylonians.