When the Days were Shorter
Exceptionally high tides occur when the earth, moon and sun are all aligned, but overall, it is the moon that dominates. In fact, lunar tidal forces have affected life on earth much more than we might assume from watching the mere coming and going of the daily tides. To a large extent, the moon has been a major factor in determining the length of our day, and the day hasn't always been as long as it is now. Further, it will never again be as short as it is now
The constant circling of the tidal bulges acts on the earth like the friction of brake drums on a rotating wheel, gradually slowing it down. In fact, the earth's tidal forces stopped the moon from rotating relative to the earth a long time ago, with the result that it always presents the same face to us now. (The planet Mercury always presents the same face to the sun for the same reason). The earth, being larger than the moon, is taking a longer time slowing down, but it is happening nonetheless.
These same braking forces have affected not only the length of the day, but the length of the lunar month (the period of time it takes for the moon to orbit the earth) as well. The moon used to be a lot closer to the earth. Back then, like a tether ball on a rope wrapped around the pole or keys on the end of a chain swinging around your finger, it "swung" a lot faster because it was "wound up" closer to the pivot.
Lunar cycles affect life on earth (and possibly even human emotions) in a myriad of ways that we don't entirely understand. But there is little doubt that many life forms exhibit behavioral and structural patterns that can be related to phases of the moon. One form which illustrates this beautifully is the chambered nautilus (a large sea-snail).
Present-day nautilus shells almost invariably show thirty daily growth lines (give or take a couple) between the major partitions, or septa, in their shells. The length of the present lunar month is 29.53 days.
Looking back through the fossil record, however, paleontologists find fewer and fewer growth lines between septa in progressively older fossils. The earlier animals lived when the lunar month was shorter, in terms of earth days, than it is now. In other words, the moon was closer to the earth and revolved about it faster. At the same time, the earth itself was rotating faster on its axis than it is now.
The very first nautiloids, 420 million years old, have only nine growth lines between septa. Paleontologist Peter Kahn of Princeton and physicist Stephen Pompea of Colorado State University have used findings such as these to point to a time, 420 million years ago, when the moon circled the earth once every nine days. How strange it would seem to us -- the day had only twenty-one hours, and the moon loomed enormous in the sky at less than half its present distance from earth.
