Tides

As the Earth spins on its axis, completing one turn every 24 hours, the Moon orbits around Earth, completing one orbit every 28 days. Consequently, during the time Earth rotates once (24 hours), the Moon has moved across about 1/28th of the sky. Earth, conseqently, must rotate one day plus 1/28th of a day (about 50 minutes) to bring the Moon into the same position overhead. This time period—24 hours, 50 minutes—is termed a lunar day. Since tides are caused by the Moon, they recur on a lunar-daily schedule, rather than a 24-hour schedule, and consequently shift their times of occurrence with respect to the 24-hour clock. As a result, on a coast with diurnal tides, each day the high tide (or low tide) will occur 50 minutes later than the day before; on a semidiurnal coast, each high tide (or low tide) will occur 12 hours, 25 minutes later than the previous high tide.