Gravity and the Earth's Tides
Uploaded by hawk18 on May 06, 2004
Gravity is the key to the Earth's rising and falling tides. The combined gravitational effects of the Sun and the Moon constantly pull the world's oceans in different directions and create tidal effects. But there are several other factors that complicate this basic process. Friction, the Earth's rotation, the tilt of its axis and the gravitational pull given off by the Sun and Moon that affects Earth's atmosphere. These forces together conspire to make our planet's oceans into a battleground. These forces tug the oceans this way and that way around the globe, thus creating high tides and low tides.
The Moon's gravity stretches the earth into an oval. The effect is so tiny that the solid parts of the planet are distorted by little more than eight inches. But because of of water's fluidity, the effect on the oceans is more noticeable. At the point on the Earth directly beneath the Moon, the ocean is tugged into a bulge of high water. At the same time, a second tidal bulge forms on the opposite side of the planet. This is partly a result of the centrifugal force created by the Moon and Earth's combined rotation around their common center of mass, a theoretical point called the barycenter.
Because the Earth spins on its axis once every 24 hours, the two bulges sweep around the planet in waves, creating two high tides per day at every point on the globe. But the twice daily cycle is complicated by he fact that the Earth is tilted, which puts the Moon alternately to the north and south of the equator. This creates slight differences between the two tide each day and adds a daily set of local variations to this natural rhythm.
A further complication is added by the Sun, whose gavational pull on the earth also affects the tides. The tidal force of the Sun and Moon together is almost a third more than that of the Moon alone, with the Sun imposing a solar rhythm. At the new and full moons, when the two bodies are in line, they combine to create extra high spring tides. When the Moon is in its first and last quarters, the Sun is at right angles to it, and their gravitational pulls work against each other to create extra low neap tides.
The story continues as the tidal waves are weakened by friction between the ocean and the...