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.
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