You probably learned about projectile motion in introductory physics class. If you throw something (a baseball, say) then its **horizontal motion** will remain constant, whereas its **vertical motion **will change under the influence of Earth’s gravitational pull. The result is a parabolic arc, right?

Well, no. Saying that projectile motion is parabolic is only an approximation.

In class, I “prove” that the motion of the baseball is a parabola, but in order to do so, I make the (reasonable) assumption that the **effect of gravity is a constant**. That is, I assume that the vector **g** (the acceleration due to gravity) always **points in the same direction** all along the trajectory.

This is actually not quite true, however. I’ve neglected the curvature of the Earth.

Now, this isn’t really a big deal when throwing baseballs. Suppose you toss a ball to your friend 50 m away. The vector **g** for you **does** point in a slightly different direction then **g** for your friend, but the angular difference is miniscule…it’s about 50/637,000,000 radians, or 0.00045 degrees. This is so small that I am comfortable pretending that the two **g**’s are actually parallel, and the derivation thereby leads to a parabolic arc.

But what if you **don’t** make that approximation? What answer do you get?

You get an ellipse. You get an orbit. And here’s the point of my post:

**Every time you throw an object, the object is (temporarily) in orbit until it hits the ground.**

Here’s the orbit of a thrown baseball (not to scale):

Now suppose the Earth had the same mass, but was the size of the Little Prince’s home asteroid B-612, which is as big as a house. The orbit is the same, but this time the baseball **doesn’t** strike the surface:

The takeaway is that **all projectile motion is really orbital motion**. I find this fascinating: you don’t need a fancy rocket to launch something into orbit. Your arm will suffice. It’s just that you need the Earth to not be in the way.