So what you see is that Io, the closest in-planet,

has the shortest period. Europa, whose period,

as we talked about earlier, is double Io, its

distance, we can plug that in, is going to be 2 to

the 3, to the 2 3rds power times Io's distance.

This 2 to the 2 3rds will give us back the right factor.

Ganymede is at 4 times the distance, so Gam, 4 times

the period, so its distance is 4 to the 2 3rds power.

So now, let's take these laws and apply them to Eris.

And that's what I'd like you to do next.

And then we'll move on and talk about

Newton's laws and get an understanding of where

those Kepler's laws, come from, and how we

can derive them from our understanding of gravity.