Is Mercury in Retrograde? What does that mean? Learn the astrophysics behind retrograde motion of the planets and how planetary motion helped Copernicus discover that the Earth orbits the Sun! Dr Brad is an astronomer at the Mt Stromlo Observatory in Australia.
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00:00 Hey, it's Brad and I'm here at Mount Shrum Observatory where I spend lots of time working
00:03 studying space with my beloved dog Luna. We're going to talk about retrograde motion. Now,
00:08 retrograde motion, sometimes we hear Mercury or Mars is in retrograde, but what actually is that?
00:13 Well, ultimately, it's just an optical illusion. All the planets move around the Sun in essentially
00:18 the same direction at a constant speed, but at certain times it can appear that the planets
00:25 or a planet compared to the background stars, distant stars and galaxies, is actually looking
00:30 like it's moving backwards even though it hasn't changed direction or speed. So how does this
00:35 optical illusion work? Let's go do a demo to find out. So we're going to do a little demo of this.
00:40 Now I'm going to be Mars or Mercury and all these planets can have retrograde motion compared to the
00:45 Earth. You, the camera, is Earth and this is my dog Luna and she will be my Moon. Now we're all
00:50 going to move about the same speed in the same direction around the Sun, but we're going to be
00:55 moving differently, apparent motion, compared to our background trees and playing the role of kind
01:00 of the background stars. And as we start to move, we will be moving forward, but at a certain time
01:07 when you've caught up, it will look like you overtake me and that's when I start to look like
01:11 I'm moving backwards. So we're going to start moving. Come on, Moon. And as we start to go,
01:16 it looks like I'm moving forward, but you're going to start catching up pretty quickly.
01:21 And as you start to catch up, again compared to the background stars, the trees,
01:26 it's now going to start looking like I'm going to be moving backwards, but I haven't changed my
01:31 speed. I haven't changed my speed. You haven't changed your speed. We're still moving in the
01:35 same direction and I am now moving backwards to you, but now that you're starting to sufficiently
01:41 catch up and pass me, it's going to look like I'm now going backwards and moving forwards again,
01:47 that it's now reversed, but I haven't changed my motion and you haven't changed your motion.
01:52 And so that small period where you've overtaken me and I look backwards is called a retrograde
01:58 motion. So that's how retrograde motion works. Now you can actually do this and see this quite
02:03 often if you're ever in a car and you're driving. Look how other cars on the road either passing you
02:10 or you passing them look compared to the distant background. It doesn't work for something really
02:15 nearby. You need the horizon or distant trees. And you will notice that anything moving at two
02:20 different speeds in the same direction can have retrograde motion. This isn't a special thing to
02:25 planets. This is just a thing that happens when things move at different speeds in the same
02:30 direction. So retrograde motion, a regular thing that happens and not that special. Now, in fact,
02:36 retrograde motion is important to our understanding of the solar system in the universe.
02:40 Thousands of years ago, the model of the universe was the earth was at the center, not the sun. But
02:46 the problem was how do you reconcile these planets or these objects that move forwards and backwards?
02:52 In fact, that's where the name comes from. Planetis is wandering star. These objects were wanderers.
02:57 But this problem was kind of solved when someone called Copernicus came along and said, "Hey,
03:04 what if we put the sun at the center, not the earth? If we put the sun at the center,
03:08 this explains how we can get planets that move forward and occasionally through an optical
03:12 illusion move backwards." So the retrograde motion of planets is important to our understanding of
03:18 the universe and its history and discovery, but it's something that happens all the time.
03:22 I hope you like this video and make sure to like and subscribe to help me make more videos
03:27 about the awesomeness of space. And sometimes my dog will tag along too.