jay@npois.UUCP (Anton Winteroak) (04/10/85)
Charlie, tides are due to differential gravitation, which involves the force of gravity being different at different distances from the body causing tides. Try imagining that at the center of the Earth, the force of gravity is exactly balanced by centripidal acceleration. Nearer to the Moon, fluids are pulled towards the moon, while on the opposite side, they are flung from the moon. In the middle, the tide is low. The sun also has a noticable tidal effect, but since the scale of the Earth-sun system is much (400 times) larger, the differential gravitation is smaller. (differential gravitaion goes down as the third power of radius). Because of these effects, tides are most extreem when the sun, moon, and Earth line up, in any order. More extreem still when the moon is at perigee, or the Earth at Perihelion. Conversely tides are at there lowest when the Earth-moon radius is at a right angle to the Earth-sun radius, especially if the Earth is at perihelion, and the moon is at apogee. Now, these tides are only really generated where the oceans completely circle the globe, just north of antarctica, and just north of Greenland. The tides from these two places come up to the rest of the world in waves. So for most places, the tides are a few days behind the sun-moon-Earth configuration. Also there is some interference from the much smaller tidal waves coming down from the north, but these interference patterns are standing. Anton Winteroak