SciFri 3.4.05

Time and Tides

By Deane Morrison

Published on March 4, 2005

The Earth and the moon are partners in a perpetual dance, but it was not always so. The Earth formed as a lone planet, but early in its history it suffered a catastrophic collision with an unknown body that gave birth to the moon. The reverberations of this event live on in the form of moon-driven tides, the gradual slowing of Earth's rotation, and the lunar cycle beloved of scientists and poets alike.

It all began when Earth and the rest of the solar system were still in their infancy, says U of M physics professor Bob Pepin, who researches the evolution of the solar system. Earth endured numerous collisions with other bodies, but sometime--probably between 50 million and 100 million years after the solar system formed--a Mars-sized body came along and whacked our young planet.

"The Earth was probably not molten at impact, but there would have been substantial melting" as a result, says Pepin. The hit splashed up a tremendous amount of material, which coalesced to form the moon. Much of the projectile ended up in the moon, but the projectile's core fused with Earth's.

Growth rates in coral during the Devonian period (417 million to 354 million years ago) showed 400 daily growth rings within each annual growth cycle, indicating that Earth's year had 400 days back then. Even today, we feel the slowing of the Earth's rotation, and so we add a leap second every few years to keep our sunrises and sunsets coming at the same time of day.

When the moon formed, it was much closer to Earth than it is now. It was also moving a lot faster, and Earth was spinning faster. All these factors added up to a situation where Earth's surface was subjected to strong tides of molten rock, thanks to the tremendous gravitational forces between the two bodies. (There is a gravitational force between any two bodies, including people--slight force--and the sun, Earth, or planets--large force.)

The interaction between Earth and moon is similar to two skaters coming together, says astronomy professor Terry Jones. Suppose a linebacker-sized skater is spinning fast and a kid skates over and grabs his hand. The kid will start to spin around the big guy, and the big guy will be slowed down. The kid will be "spun out"; that is, he or she will move out from the big guy. As long as the skaters can keep extending their arms and staying in contact, the kid will spin in a larger and larger radius and the big guy will keep slowing down. If the big guy is the Earth and the kid is the moon, that's what has happened over time, except that the contact between the two bodies is gravity, which can extend over much larger distances than human arms. The moon is constantly moving out into a larger orbit at the expense of the Earth's spinning, and is now receding from us at about an inch-and-a-half per year.

How much has Earth slowed down from its interaction with the moon? According to Pepin, an ingenious study of growth rates in coral during the Devonian period (417 million to 354 million years ago) showed 400 daily growth rings within each annual growth cycle, indicating that Earth's year had 400 days back then. Even today, we feel the slowing, and so we add a leap second every few years to keep our sunrises and sunsets coming at the same time of day. Unchecked, the moon's pull would eventually slow the Earth's rotation to the point where a day and a month are equal. At that point, the Earth would always present the same face to the moon, just as the moon always presents the same face to Earth. But this won't happen for many billions of years, says Jones. By that time, the sun will have reached the end of its life, after having expanded to swallow Earth and the moon.

The most obvious effect of the moon's pull is, of course, the tides. The sun also exerts such a force, but the moon, being closer, has the upper hand. The moon appears to move over the Earth from east to west as a result of Earth's rotation, dragging the seas with it. This creates a tidal bulge, not only in oceans but also in Earth's crust. (The movement of solid rock is imperceptible, only an inch or so, according to Jones, but it's there.) There are two tidal bulges (high tides) at any given moment: one on the side of the Earth facing the moon and one on the other side; thus there are two high tides (and two low tides) every day.

At full moon and new moon, the sun, moon, and Earth are in a straight line, and so the sun's gravity reinforces the moons and we experience big "spring" tides. When the moon is at first or last quarter, it is at a right angle to the sun, and we get small "neap" tides.

Tides don't come on a 24-hour cycle, though, because the moon also moves eastward in its monthly orbit. Earth's rotation takes about 50 minutes to catch up to the moon's movement, and this adds about 50 minutes to the tidal cycle each day. The tidal bulge lags behind the motion of the moon. Consider a mass of water in the middle of the ocean. Earth's rotation carries the water east. But Earth's rotation also makes the moon appear to move westward over the ocean during the course of a day, just as the sun does. Therefore, the moon's pull on the water is toward the west at the same time the Earth is pulling it toward the east. The result of these opposing forces is that the tidal bulge moves westward, but it lags behind the moon.

Predicting tides based on anything but past experience is essentially impossible, because the Earth's geography is so irregular and there's plenty of friction between the floors of ocean basins and seawater. Certain places experience huge tides, thanks to quirks of geography, as Pepin knows only too well. He grew up in a village by the Bay of Fundy in Nova Scotia, where tides regularly run 30 feet. It seems the tides are amplified first by the Gulf of Maine, then again by the long, narrow bay, he says.

During winter when Pepin was eight, he and a friend were exploring a cave in a huge ice block that had been thrown up by the sea--not an unusual occurrence. When they emerged, the tide had risen and floated their ice block. The tide had then begun to ebb, and they found themselves 50 yards from shore, en route to being swept out to sea.

"We had to hop on a little ice floe, then another, and paddle back to shore," recalls Pepin. The boys' frantic parents watched from shore, coaching them as they paddled their ice rafts. After they made it back, the parents expressed their displeasure with the boys in no uncertain terms.

The Bay of Fundy is famous for its tides, but sometimes mariners came around and docked their boats, unaware of the dangers.

"When we were kids, we always waited for a mariner to come in at high tide and tie up with a rope that was too short," says Pepin. "The guy would come back hours later, and if the rope was strong, his boat would be dangling. Otherwise, the boat would be sticking out of the mud below the pier."