What causes high tide and low tide and why?

Asked by: Jen Kinsey


The high and low tide are caused by the gravitational forces between the earth and the moon. However, the source of the real effect takes some explaining to grasp. The incorrect way of thinking is that the moon attracts all the water to itself, therefore causing a high tide on the side of earth close to the moon, and a low tide on the side far from the moon. However, if this was the case, there would be a high tide once per day. But there are TWO high tides a day. The reason is, the part of the earth both AWAY from the moon, and CLOSE to the moon BOTH get high tides...

Now we need to explain why this is so. The earth and moon rotate around each other, each pulling the other towards itself. The moon attracts every piece of matter on earth. Since gravity is inversely proportional to the square of the distance, this force is greater on the side of the earth closer to the moon, and lesser on the side of the earth further from the moon. Since the earth is quite a rigid object, this difference in forces fails to deform the earth (much). However, it succeeds quite well in deforming the oceans -- which are not as rigid. Since the waters on the moon side are attracted more strongly than average, they tend to bulge TOWARDS the moon, hence causing a high tide. The waters on the opposite side of the moon, since they are attracted less strongly than average, tend to 'lag behind' the rigid earth, and bulge AWAY from the moon, which in this case, is also AWAY from the earth, again, causing a high tide. Low tide occurs at about right angles to the moon, where the force on the waters match the average pull of the moon on the earth closely.

The question that usually follows this is, why doesn't the sun cause any tides (comparable to that of the moon) although its pull on the earth is larger? The answer is, although the gravitational pull of the sun on the earth is larger than that of the moon, due to the much greater distance, the force changes very little from one end of the earth to the other. Since it is the difference in the force than the average magnitude of the force that matters for creating tides, the net effect is much less than that for the moon.
Answered by: Yasar Safkan, Ph.D. M.I.T., Software Engineer, Istanbul, Turkey