# Celestial Mechanics You look up into the sky. A meteor streaks through the air. Which way is it going? Or you see a comet. What is its orbit?

What did you actually see? Well, you saw something moving against the background of stars. From this scant data, how can astronomers calculate the orbits of these celestial objects?

This field is called celestial mechanics.

Challenge: Let’s see if you can grasp the main ideas in 15 minutes or less. (Hey, it usually takes a semester in a graduate-level celestial mechanics course, but you are pretty smart, right?)

Before you start, you should download the Mathematica Player so that you can see and manipulate these objects (and make the asteroids crash).

We shall attempt to explain a bit about celestial mechanics. Specifically, we will begin with the very basics—

• orbits as ellipses
• the main parameters that define an ellipse
• Kepler’s laws, and how they relate planetary motion to elliptical orbits
• how the mathematician Gauss predicted the location of the asteroid Ceres in 1801 (thereby becoming world famous), and the techniques that he used
• how astrophysicists can predict the orbits of bodies in space, based upon only a few measurements (2-3 observations of position at specific times)

We will use a few “hands-on” mathematical models that you can manipulate to see at the least the basics of how this works in practice.

If you are the math kind of person, and you actually want to understand how all this stuff works, then start here with the science of celestial mechanics.

Okay, you really would rather play first?

We can then understand how this works in a minute.

Meteor video source: Astronomy Picture of the Day, found at: http://tinyurl.com/prhtl52