Tides

Introduction

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.(Huang,14)

Discussion

The biggest waves in our oceans are the tides. These are caused by the gravitational forces between the earth and the sun and the moon. The moon has the biggest influence because it is close. It essentially pulls up a bulge in the ocean on the side of the earth closest to it. It actually pulls up the land too, but not as much. There is also a bulge on the side opposite the moon. This one is tougher to understand. I've heard it explained two ways that seem to help:(Hitchcock,291)

Because of centrifugal force (more an effect of the earth and moon revolving together than an actual force), the ocean on the side of the earth opposite the moon is sort of thrown outward, like you are when you go around a bend in your car.

Imagine a race car, minivan, and bicycle starting a race. All three accelerate, and from the point of view of the minivan, the race car shoots out in front and the bicycle gets left behind. The way they spread out depends on the differences in rate of acceleration. Similarly, the side of the earth nearest the moon gets pulled out harder than the side away from the moon relative to the earth itself. The nearside shoots out ahead, and the backside gets left behind.

Since it takes 24 hours for the earth to complete a rotation, plus we have to catch up a little because while the earth was rotating, the moon was revolving around the earth, we are directly under a bulge, or experiencing high tide, about every 6 1/2 hours.(Huang,616)

Twice daily tides like this are called semidiurnal tides. It is also possible to have only one high and one low tide per day. That would be a diurnal tide. Partly this depends on your latitude, but it turns out that some 400 variables go into predicting the tide at any one place, so it isn't nearly this simple.(Hitchcock,291)

The sun tugs on the oceans too, but since it's so far away, it has less influence than the moon. You can see the influence when the moon and sun and earth are all lined up. This would be during a full moon and a new moon. With both the sun and moon pulling the same direction, we get extra high high tides and extra low low tides (a big tidal range). These happen twice a month and are ...