8/9/2019 Gravity and the Earth

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Gravity and the Earth's Tides

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. riction! the

Earth's rotation! the tilt of its a"is 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. #t the point on the Earth directly beneath the Moon!

the ocean is tugged into a bulge of high water. #t 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 a"is once every $% 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 e&uator. This creates slight differences between the two tide each

day and adds a daily set of local variations to this natural rhythm.

# 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. #t the new and full moons! when the two

 bodies are in line! they combine to create e"tra high spring tides. hen the Moon is in its first

and last &uarters! the Sun is at right angles to it! and their gravitational pulls work against eachother to create e"tra low neap tides.

The story continues as the tidal waves are weakened by friction between the ocean and the

seabed to the point where the twice daily tidal waves get slightly left behind the orbiting

Moon. #t the same time! the Earth spinning on its a"is causes the tidal waves to oscillate

around the world's ocean basins like water in a bath. This means that high tides do not

necessarily occur when the Moon is overhead! but when the oscillations accumulate to their

greatest height. each ocean basin is a different shape and so has its own pattern of oscillations.

(n the )orth #tlantic the sweep in a counterclockwise direction. *ntil recently! the sheer

comple"ity of the tidal forces acting on the earth meant that the only way to predict tides was

 by years of patient observation. now! computer programs do the +ob, a development for which

oceanographers are e"tremely grateful.

-uring the /01s! rench astronomer Edouard 2oche tried to work out what would happen if

the Moon were nearer the Earth. #ccording to his calculations if the Moon were +ust one third

of its present distance! the force would be $3 times greater. This implies that ocean tides would

 be so enormous that they would regularly swamp many of the world's lowland regions at every

high tide. The tidal force would also be strong enough to cause parts of the earth's crust to fle"

up and down with the tides resulting in catastrophic earth&uakes.