Lunar Tides

A second benefit of Earth's large moon is tides, which are due to gravitational effects of both the sun and the Moon. The pull of these two bodies produces bulges in the ocean pointing both toward and away from the Moon and the sun. The complexity of Earth's present tidal effects is well illustrated by the tidal charts cherished by clam diggers, anglers, and sailors. The daily variations seen in the charts are caused by the interplay of both lunar and solar tides. Both the Moon and the sun cause ocean bulges on their respective near and far sides of the planet. As Earth spins under the bulges, the sea rises and falls at any particular location. When the Moon lines up with the sun every 2 weeks, the tidal ranges are at a maximum, and when they are 90 degrees apart in the sky (the quarter moon is overhead at sunrise or sunset), the range of tidal change is minimized. With a smaller or more distant moon, the lunar tides would be lesser and would have a different annual variation.

Soon after the Moon formed, it was perhaps 15,000 miles from Earth. Instead of being a few meters high, as they are today, it is possible that lunar tides rose hundreds of meters or higher. The extreme effects of such a close moon could have strongly heated Earth's surface. The ocean tides (and land tides) from a nearby Moon would have been enormous, and the flexing of Earth's crust, along with frictional heating, may have actually melted the rocky surface. However severe their effects, the enormous tidal variations would have been short-lived because the forces responsible for tides also cause the Moon to move outward, thus diminishing the effect. Early land tides may have been a kilometer high, but they dropped to moderate levels in less than a million years.

The retreat of the Moon is a natural consequence of gravitational pull between the Moon and the tidal bulges. The Earth's lunar tidal bulges don't actually correspond with a line from Earth to the Moon but, rather, lead ahead as the Moon orbits the planet (see Figure 10.2). This offset produces a torque that causes Earth's spin rate to decrease slowly and the distance be-

Figure 10.2 Earth's leading tidal bulge produces a constant forward force on the Moon that is not totally balanced by the backward pull of the more distant trailing bulge. The net forward force on the Moon causes it to spiral outward from its place of origin close to Earth. If the Moon had formed orbiting in the opposite direction, this effect would have caused it to spiral inward to a catastrophic collision. Such a dramatic fate awaits Triton, Neptune's large and backward-orbiting moon.

Figure 10.2 Earth's leading tidal bulge produces a constant forward force on the Moon that is not totally balanced by the backward pull of the more distant trailing bulge. The net forward force on the Moon causes it to spiral outward from its place of origin close to Earth. If the Moon had formed orbiting in the opposite direction, this effect would have caused it to spiral inward to a catastrophic collision. Such a dramatic fate awaits Triton, Neptune's large and backward-orbiting moon.

tween Earth and Moon to increase. Besides measurement by laser, the recession of the Moon can also be detected in the fossil record. Daily and annual layers in Devonian horn corals show that about 400 million years ago there were 400 days in each year, the Moon was closer, and Earth was spinning faster. The coupling of these two effects is due to conservation of angular momentum, the same physical law that allows ice skaters to spin faster by pulling their arms against their bodies. The outward movement of the Moon would be reversed if the Moon happened to be orbiting in the opposite direction. Instead of retreating, it would approach Earth and would eventually collide with it. Although we have nothing to fear from Earth's moon in this respect,

Triton, the large moon of Neptune, is in a retrograde orbit and will collide with Neptune within a few hundred million years.

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