Is Gravity Lower at the Earth’s Equator and What Does This Have to Do With the Moon’s Orbit?

Photo: Rhett Allain. The moon.

I saw the question posted on Reddit. It goes “something” like this:

“I know the Earth’s gravity is weakest at the equator. Then why does the moon orbit above the equator where gravity is the weakest?”

It’s a fun question. I’m going to give a more detailed answer, but let me start off with a very short version.

1. The Earth’s gravitational field is not weaker at the equator but instead the apparent weight is lower due to the rotation of the Earth.
2. The moon doesn’t actually orbit right over the equator, but the path of the moon’s orbit is caused by an early impact earlier in the solar system. Most of the solar system moves in a flat plane.

So, we have two things to talk about. Let’s get started.

Earth’s Gravitational Field and Apparent Weight

The magnitude gravitational field due to a point mass looks like this:

If you are outside an object with spherical symmetry then the direction of this gravitational field points towards the center of the Earth and decreases in magnitude as you get farther away. It’s possible to make a visual representation of this with python (full code here).