How do Astronomers Know Where Planets Are?

Dominic - We know the laws of gravity that the planets are orbiting under and we can see how long it takes those planets to orbit around the Sun. There's actually a very simple numerical relationship according to Newton's Law of Gravity as to how long a planet should take to circle the Sun as a function of how far away from the Sun it is. And so, that's how we've historically made estimates of how far away the planets are. In fact, in the last 20 years or so, we started firing radar at Venus and Mars, which are the nearest planets to us, and we can time how long it takes the radar pulses to come back. And that tells us within a matter of hundreds of metres where those planets are.