The Moon's gravity decreases with distance, so it pulls unequally at the Earth and its oceans (arrows in the diagram). This tends to produce two tidal bulges: The Moon pulls harder on the closer ocean than it pulls on the solid Earth. And it pulls harder on the solid Earth than it pulls on the more distant ocean. The Moon is about twice as effective as the Sun at producing tides. When Moon and Sun act in concert during New and Full Moons, the tidal range is maximized.

If the oceans were deep and continuous, the Earth's rotation under two tidal bulges would produce two tidal cycles each day. However, the presence of continents so complicates the process that it is better to consider tides, confined to each ocean basin, as akin to wine sloshing in an oscillating oyster shell. The slosh is maximized when the natural period of sloshing matches the 12-hour, 25-minute period of lunar forcing, or the 12-hour period of solar forcing. Extreme 50-foot tides in the Bay of Fundy are a consequence of such resonances. The tidal range on one side of Panama is 22 feet, on the other side it is 1 foot. Despite such complications, tides are predictable on the basis of extensive past observations. The pattern is dependent upon orbital variations and repeats every 18.6 years.

The map depicts the distribution of lunar tides around North America as derived from satellite radar ranging of sea level. The shading shows the magnitude of the tidal range. Curved lines radiating from each node of minimal tidal range are drawn along the crest of the tide at hourly intervals as the tide circulates one way or the other, depending chiefly upon which hemisphere is involved.

The Moon stabilizes the direction of the Earth's spin axis. Without it the Earth would have experienced extreme seasons and life would not have thrived. Rent the DVD If We Had No Moon, and be very thankful that we do have one.