Dr. Katherine Volk

So if you were a stationary observer, like here we're looking down on the solar system, we would see the planets moving and all the asteroids here moving.

I'm always really jealous of the asteroid people because they have so many more data points to work with than I do So some features here that if you're really close to the Sun like mercury here You're orbiting faster at a higher velocity than when you're further away from the Sun So that's one thing to keep in mind that we're going to come back to But of course the other component of the motion is

is because we are observing on Earth and of course the Earth is also orbiting the sun.

So we're not just some stationary observer looking at the solar system, we're on a moving observing platform.

So part of the motion we see for these solar system objects against the background stars is due to parallax in our motion.

So here's an example, a lot of people have heard of planets being in retrograde

All that means is we're seeing a shift in the direction of motion as we are orbiting the Sun.

So here we've got the Earth's orbit here, we've got Mars, and this is tracking over time, these marked time positions, the motion of Mars on the sky as seen from the Earth.

So when we're over here, we're catching up with Mars, but Mars is still moving in this direction.

But because we are closer to the sun than Mars, we're going to go past Mars, and it's going to cause a backwards apparent motion of Mars as we're passing by.

And then once we've passed Mars, Mars will keep going in the same direction.

So that's what's illustrated here with real images for Jupiter and Saturn doing their little prograde motion, and then their retrograde loop, and then back to their prograde motion.

So this is, of course, just parallax.

So when we are looking at something and we're having our vantage point moving, it will appear to move against the background.

So if we're really close to the thing we're observing and the Earth is moving, this parallax effect will be big.

If it's really far away, the parallax effect will be small.

The stars that are very, very distant from us don't appear to move to our eyes at all.

So that's why they're kind of the stationary background stars, even though, of course, everything in the galaxy is in motion.

But the things in the solar system being so much closer to us move pretty quickly due to both parallax and their inherent motion.

So for nearby things, inner solar system things like near Earth asteroids, main belt asteroids, they have fast proper motion because they're close to the sun and they're orbiting quickly.