MadSci Network: Astronomy |
The Moon's gravitational effect on the Earth is greatest for the part of the oceans closest to the Moon, and it decreases across the Earth, so that it is least on the ocean on the far side opposite the Moon. Relative to the Earth's center, this difference in force across the body of the Earth, called a tidal force, causes the ocean on the side nearest to the Moon to shift slightly closer to the Moon while the ocean on the opposite side is not pulled as strongly. That is, the Moon's gravitational pull on the oceans produces two tidal bulges (one bulge on the near side, one bulge on the far side because it is "left behind" as the Earth is pulled toward the Moon) on opposite sides of the Earth in line with the Moon. Consequently, water piles up in the form of an ellipsoid whose long axis is directed toward the Moon. Midway between the high tides are the low tides.
The Earth's rotation underneath the tidal bulges results in alternating high and low tides in the oceans twice each day. Because there is a slight lag before the oceans fully adjust to the Moon's tidal force, the tidal bulges are dragged by the rotating Earth somewhat ahead of the line joining the centers of the Moon and the Earth. When you consider putting continents in the way and the effects of changing earth-moon distance, land topography, ocean currents, prevailing winds, etc., tides can be complex. At any given location, though, the times of the tides can be worked out (often quite easily); these are published in tide tables.
The Sun also contributes to the tides, but only about one-third as much as the Moon does because of its much greater distance, despite its larger mass. When the Sun and the Moon are roughly along a straight line, such as at new or full moon phase, their combined gravitational pull is greatest, producing the largest tides (spring) and at 1st or 3rd quarter phase, produce the weakest tides (neap). Some help in exlaning what causes earth's tides can be found in my lecture notes or at Windows to the Universe site.
[The friction generated between the oceans and the land is also responsible for a gradual slowing of the Earth's rotation and the Moon moving away from the Earth. Tides generated within the Moon itself (within the solid rock, as the Moon has never had oceans) are also responsible for the Moon becoming tidally locked to the Earth so that it shows only one face to us.]
As for the Moon's effect on humans, the word lunatic is derived from the Latin word for the Moon, luna. People used to believe that the full moon, its phases, or even its gravity has an effect on human behavior (e.g., werewolves). Today, even though there is no scientific basis for any of these claims, though many people continue to believe in them.
Try the links in the MadSci Library for more information on Astronomy.