TIDES

Key Concepts

(1) Tides are caused by the difference between the Moon's gravitational pull on the two sides of the Earth.

We think of tides in association with the seashore.

Approximately twice a day, the level of the sea rises; when the level is highest, it is referred to as high tide. When the sea level is lowest, midway between high tides, it is referred to as low tide.

The fact that the time between high tides is equal to half the interval between moonrises was noted very early by people living near the sea. In fact, the Earth's tides are caused primarily by the gravitational pull of the Moon.

The Earth's tides can be defined as the distortion of the ocean (and also of the solid Earth) by the difference in the Moon's gravitational pull on the two sides of the Earth - the side closer to the Moon and the side farther away. Gravity is universal - every two objects in the universe exert a gravitational attraction on each other. The Earth pulls on the Moon; conversely, then, the Moon exerts a gravitational pull on the Earth.

The gravitational force exerted by the Moon (or by any other object) decreases as you get farther away from it, as one over the square of the distance. The Moon's gravitational pull on the side of the Earth that is closer to it is greater than its pull on the side of the Earth that is farther away. (The pull on the closer side is about 7% greater.)

If the gravitational pull of the Moon on the Earth were uniform throughout the Earth, then the Earth would be undistorted. As it is, however, once you subtract the average gravitational force, you find that the side of the Earth closest to the Moon moves toward the Moon (the gravitational force is greater than average there); the side of the Earth farthest from the Moon moves away from the Moon (the gravitational force is less than average there). The net result is that the Earth has not one, but TWO tidal bulges, one on the side closest to the Moon, and the other on the side farthest from the Moon.

Since the Moon's gravity pulls on rock and water equally, why are tides something we associate with the ocean? Rock is stiff, and resists being deformed by gravitational forces. As a result, the tidal bulges in in rock are only 0.3 meters (about a foot) high. Water, by contrast, is fluid, and flows readily in response to gravitational forces. As a result, the tidal bulges in the water of the ocean are 1 meter high. About twice a day, as the Earth rotates, the seashore is carried into a tidal bulge of the ocean. When the seashore is beneath the highest part of the bulge, it is high tide.

Since gravity is a universal force, every object creates tides on every other object. In particular, the Sun creates tides on Earth. The Sun is more massive than the Moon, but it is also farther away than the Moon. Calculations reveal that the Sun-created tides are only half as high as the Moon-created tides. When the Sun, Moon, and Earth are all aligned (that is, at full Moon or new Moon), the Sun-created tidal bulges is added on top of the Moon-created tidal bulges to create high tides that are exceptionally high. The extreme high tides that occur at new Moon and full Moon are called spring tides; the less extreme tides that occur during first quarter and last quarter Moon (when the Sun, Earth, and Moon are at right angles) are called neap tides.

Complicating factors exist. For instance, the Earth is not entirely covered with water, so complex flow patterns can be set up when a tidal bulge in the ocean hits land. For instance, high tides can be funneled into narrow bays, thereby increasing their height. In the Bay of Fundy, for instance, between Nova Scotia and New Brunswick, the difference between high and low tide can be 12 meters (40 feet) or more.

Another complicating factor is friction between the ocean's tidal bulges and the ocean floor. Since the Earth rotates on its axis in a shorter time than the Moon revolves around the Earth, the frictional forces between the rapidly rotating Earth and the ocean waters resting on its surface tend to drag the ocean's tidal bulges in the direction of rotation. The end result is that the Earth's tidal bulges are slightly eastward of where they would be if friction were not present.

(2) Tidal forces are gradually slowing down the Earth's rotation.

Friction heats things up (as you know if you've ever tried to warm your hands by rubbing them together). Thus, the friction between the ocean's tidal bulges and the solid earth beneath heats up the ocean. It's a small effect, but it's there. Energy can't be created out of nothing. Where did the energy to heat up the ocean come from? It came from the kinetic energy of the Earth's rotation on its axis. Thus, as the oceans gain energy, in the form of heat, the Earth loses rotation energy, and the rotation slows down. This process - the slowing of rotation due to the presence of tides - is called tidal braking.

Tidal braking doesn't work very rapidly. Currently, the length of the day here on Earth is increasing by roughly 2 milliseconds every century. During the Triassic period, about 240 million years ago, the Earth's day was only 23 hours long, and consequently there were 381 days per year. (Fossilized marine animals, such as coral, which show both daily growth rings and annual growth rings, confirm this result.)

Just as the Moon creates tidal bulges in the solid Earth, so the Earth creates tidal bulges in the solid Moon. Tidal braking, resulting from the big tidal bulges raised by the massive Earth, has ALREADY slowed the Moon to the point where it is ``locked in'' with the same side always facing the Earth. The fact that the Moon's period of rotation is equal to its period of rotation is not a coincidence, but is due to tidal braking.

(3) Tidal forces are gradually increasing the size of the Moon's orbit.

Consider the tidal bulge of the Earth which is closest to the Moon. Friction, as noted above, drags the waters of the bulge in the direction of the Earth's rotation, so that the bulge is always leading the Moon in its orbit, like a carrot held in front of a donkey. The extra little gravitational force exerted by the tidal bulge upon the Moon gives the Moon an extra little acceleration. The Moon, as a result of this extra acceleration, spirals slowly outward.

Currently, the distance between the Earth and Moon is increasing by 3.8 meters (about 4 yards) per century. This is not very rapid (it's about the rate at which your fingernails grow) but over billions of years, it adds up. In roughly 10 billion years, the Earth-Moon system will look significantly different from its current appearance:

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