[Physics] Relative size of the 2 tidal bulges

forcesnewtonian-gravitytidal-effect

I am interested in making a simplified model that represents only the effect that the moon has on Earths tides.
In this model I am going to assume that the earth is completely spherical (with no continents or surface variation) and covered entirely by a single ocean of water.

In simple terms I want to know if the tidal bulges on the earth are different sizes or if they are of equal size?

In many diagrams it appears that the bulges are symmetrical, but if the side of the earth facing the Moon is experiencing a greater gravitational force, and the far side a weaker gravitational force, would that not result in the side facing the Moon having a proportionally larger tidal bulge?

I do appreciate that most diagrams are purely illustrative and that the actual difference in size would be too small to practically depict on a visual diagram.

I have attached a diagram, that while exaggerated, I am wondering if it is more accurate that showing the bulges as equal in size?

Tidal bulges

Best Answer

Yes, to some degree, but not for exactly the reason you're imagining.

Even in the idealized model you're using, the size of the tides is not controlled by the strength of the moon's gravitational pull at $A$ and $C$, but by the difference in the moon's gravitation between $A$ and $B$, or between $C$ and $B$.

This difference in gravitation between two neighboring points fall off proportionally to the inverse cube of distance rather than inverse square of the graviational field itself.

Since the moon is about 30 earth diameters away, we would expect the tidal effect to be about one part in 90 less on the far side of the earth.


In reality the "tidal bulge" model is very inaccurate for explaining how the oceans rise and fall. It is more accurate to consider the oceans as a very complex driven oscillator, where the water is slowly sloshing around. The periodic minor variations in gravity caused by the moon contribute energy to the sloshing, but the actual shape and timing of the sloshes is much more influenced by coastlines than than by the moon itself.

Indeed tides in the earth's oceans do have a component with a period of 25 hours (the approximate time from moonrise to moonrise) as well as the more well-known component with a period of 12½ hours. In some places (such as the Gulf of Mexico) the 12½-hour mode almost vanishes, such that even though it is driven at 90 times the strength globally, the amplitude of the 25-hour oscillation dominates locally. This leads to diurnal tides.

Related Question