# Would the Earth still need a moon if it had rings?

If the Earth had Saturn-like rings going around the equator would there still be a need for a moon? Would the rings be able to take care of the tides because of their gravity? Would the moon become just another light in the sky?

• Of course we would need a moon. How else would the werewolves be able to do their thing? With ringlight every night they would be exhausted. – WhatRoughBeast Jun 23 '17 at 2:09

## 1 Answer

No, the rings would not be able to cause tides. The tides are caused because of what we call a "gravitational differential". Basically, what this means is that the Moon pulls on the near side of the Earth more than it pulls on the far side. This is a result of how gravity falls of with distance: $$F_g = \frac{GMm}{r^2}$$ The force of gravity falls off with the square of the distance. Because of this, material on the near side of the Earth is strongly pulled towards the Moon. This creates a tidal bulge on that side. On the opposite side of the Earth, the material is not pulled as strongly as the middle is pulled, creating a tidal bulge on that side. Now, all this means that rings will not cause tides. They are symmetric about the Earth, so there is no differential gravitational field.

HOWEVER, THE SUN WILL STILL HAVE A TIDAL FORCE ON THE EARTH. The Sun produces a differential gravitational field just like the Moon does. However, due to its distance relative to the Earth's radius, the effect is much smaller. But it still produces a small effect. This is what causes "spring" and "neap" tides. So we would still have tides, they would just be really small.