4/9/2023 0 Comments Tide graph images![]() ![]() period, the moon is roughly stationary with respect to our simple diagram. There are some simplification here that we need to be clear about, but before we do that, let's follow through and see the logical conclusions of our model. The tidal bulge on the opposite side of Earth from the moon is produced by this inertial effect, referred to as centrifugal force. ![]() Both the moon and Earth move during the roughly 28 day period it takes for the orbit, and because of this, water in the ocean is thrown to the outside, the same as the water in your bucket. The moon orbits Earth, but the rotation axis for this orbit is not at Earth's center. The same thing happens during a lunar month as the Earth-moon system rotates. The rotation produces a 'centrifugal force' that holds the water in the bucket, even when it's overhead and gravity is trying to make the water fall out of the bucket. One relevant common experience is that of swinging a bucket of water around with your arm. Now, what about the other bulge? Well, you have to think about inertia and what happens when masses rotate around a point. So, in our simple picture, the tidal bulge under the moon is produced by gravitational attraction between water molecules in the ocean and the moon. Tidal forcing is proportional to the ratio of mass over distance cubed. He also showed that a spherical body can be represented as a point mass at its center. The one under the moon is thought of as 'gravitational.' Newton told us that every point mass in the universe is attracted to every other mass, via gravity, with resulting force proportional to the product of the masses divided by the square of the distance between the masses. One is under the moon and the other is opposite to that. In this view, there are two ocean tidal bulges on opposite sides of Earth. The first involves the assumptions we make for the equilibrium theory, when trying to show the simplest way to understand things. There are a few simple things to keep straight with tides. Also, the concepts we develop for the moon apply equally to the Sun-Earth system. The moon accounts for about 2/3rd of the Ocean tides anyway, so we'll have almost everything. Let's start with the equilibrium theory of tides, and let's ignore the Sun for a moment. Le Mont-Saint-Michel, an island commune in Normandy, France. ![]()
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