How Does the Moon Stay in Orbit?
Douglas Adams famously described flying as a knack: “The knack lies in learning how to throw yourself at the ground and miss.” (Life, the Universe, and Everything, the third of five books in the Hitchhiker’s Guide to the Galaxy trilogy - affiliate link)
Oddly enough, this is a fair description of how the moon stays in orbit around the earth, and how the earth stays in orbit around the sun.
First, some definitions.
Gravity, or gravitational force, is a force that pulls all objects with mass towards one another. It depends on the mass of the objects and how close they are together. Since the closest, biggest thing to us is the earth, most of the gravity we experience on a daily basis is the earth pulling objects towards its center.
Inertia is a property of all objects to keep doing what they’re doing. When you’re sitting on the couch, it’s hard to get up. When your bicycle is rolling down the hill, it’s pretty hard to stop - and if you try to do so too quickly, your body will keep on going due to its inertia. Inertia depends on an object’s mass and velocity (how fast it’s going), so a car will have more inertia than a bicycle.
The Moon is enormous, so it has a lot of inertia. It just wants to keep going and going. However, the Earth is also enormous, and even though it’s a bit far away from the moon, the gravitational force between the earth and the moon is quite strong. At all times, the earth and moon are being pulled towards one another, although since the moon is much smaller than the earth, the moon ends up being pulled towards the earth. Also, the moon’s inertia wants to keep it going in a straight line.
The moon is constantly falling towards the earth while also moving in a straight line, so as a result, its path curves around the earth. Basically, the moon is throwing itself at the ground and missing.