Full transcript
0:00so we've discussed this Lester sphere
0:03and how it looked like all the stars
0:05were just rotating around us and we've
0:08also talked how it looked like the Sun
0:09was rotating around us and the moon we
0:12had this again very Earth centered idea
0:14earth was the center of universe great
0:17everything rotating around us and for
0:19the most part everything behaved in a
0:21very nice simple way it went through
0:24what we call pro-grade motion the idea
0:26of the normal eastward motion of objects
0:29across the sky
0:30most things did that and it was easy to
0:32describe and say hey things are orbiting
0:35around us in a circle but the problem
0:39was there was weird objects some
0:42wandering star as they called them and I
0:45mean we now know they're planets but
0:47these behave strangely every once in a
0:49while they'd have what is called
0:51retrograde motion they would go the
0:55opposite direction they'd all be going
0:57eastward but every once in a while that
0:59weird little dot would actually seem to
1:01go westward and then back eastward again
1:05kind of like this image right here show
1:06so normal motion pro-grade and then all
1:10of a sudden for a little bit it would go
1:11opposite retrograde motion and it was
1:15just weird I mean it's a hard thing to
1:18reconcile and understand because why was
1:22it all of a sudden shifting directions
1:23what would cause that how can we
1:25describe this if the earth is the center
1:27now it's worth mentioning what it
1:31actually is now we now know this is an
1:33actually retrograde motion this is an
1:35apparent retrograde motion what's
1:38happening is let's say we are looking at
1:40I don't know
1:42Mars Mars orbits further out than we do
1:46and since we are rotating around the Sun
1:48just like Mars what happens is that we
1:52can actually pass by it all right we're
1:55on the inner loop yeah we're on a closer
1:58loop to the Sun so we're actually
2:00traveling faster around the Sun than
2:02Mars or whatever our mysterious planet
2:05is in this case but whatever and from
2:07our perspective we look and see this
2:10other planet you can see at Point a it
2:12looks like it's over here at Point
2:14be it's moved here and it looks to be
2:16there but by point see we're starting to
2:19pass this planet and because we're
2:22passing it
2:23we actually would make it would look
2:25from our perspective like it's moving
2:28backwards it's not it's still going in
2:30the same direction but since it's going
2:31slower and we're passing it it looks
2:34like it's going backwards and I'm
2:37actually gonna bring in a quick
2:38simulation of this from NASA your book
2:42references this I think it's a helpful
2:44one if you're struggling at all to
2:46picture this so let's take a look
2:49this is gonna be Earth and Mars orbit
2:51and watch what happens
2:53this shows the actual well we would see
2:56in the night sky and this is what's
2:58actually happening so again you can see
3:01as we pass it it looks like in loops
3:03backward it's not it's still going the
3:05same direction but because we're looking
3:07at it relative to our own motion it
3:10seems to go backward and again this may
3:13be a hard idea to really understand but
3:15think about when you're on the road if
3:18you've been a passenger in a car at any
3:20point in your life this will hopefully
3:22be a more familiar idea than what you're
3:24you thinking like let's say you're on
3:26the expressway and you're driving along
3:28well when you're going by another car
3:31you're both going the same direction but
3:34as you pass that car from your
3:38perspective it actually looks like it's
3:40gonna start moving backwards so it's not
3:44we both know that it's because you are
3:47passing it and relative to that from
3:50your perspective it looks like it's
3:52moving backwards this same idea here
3:54it's not actually its own motion it's an
3:57apparent retrograde just like that car
4:00is apparently looking like it's moving
4:02backwards but it's not you're just
4:04passing it so we know the explanation as
4:07to what was going on these are planets
4:09and it not actually going through
4:11retrograde motion it just so happens
4:13because of us orbiting the Sun and them
4:15orbiting the Sun we can make this
4:17apparent retrograde motion but remember
4:20back early on they thought earth was the
4:23center so they had to somehow try and
4:26reconcile
4:27this knowledge or I mean ideally they
4:30would have taken this and from that
4:32started to draw new conclusions and new
4:34models but they were very set on this
4:37idea of an earth-centered model so
4:40people started working on trying to
4:41figure out ways to describe how this
4:43could happen how it could be that
4:46certain times some of these objects
4:48would seem to go backwards so what they
4:52ended up adding to the earth-centered
4:54model was the idea of an epi cycle and a
4:57deferent epi cycle and a defferent more
5:00circles however at this time circles
5:02were thought to be a perfect shape so
5:04everything was generally made as circles
5:08so what's going on to try and describe
5:11this motion what they started saying was
5:14that these objects these were objects
5:17are orbiting around Earth
5:18we're actually going on little mini epi
5:21cycles little circles epicycle circle
5:26little circle right here but the center
5:29of that circle was itself going around
5:32in a circle which we refer to as the
5:35different the deferent is this big
5:37circle the overall path around Earth
5:40supposedly and there's an epi cycle
5:44which is the object is going on a small
5:47circle which the center of that circle
5:50is going around on the defferent so
5:52adding quite a bit of complexity to the
5:54model in general in science we look for
5:56the simplest solutions but this was back
5:59before those times these this was very
6:02informed by look the earthís Center huh
6:04how are we gonna make this work how do
6:06we describe this data how are we gonna
6:09actually be able to make up a model that
6:10will show us the kind of movement we
6:14expect cuz remember star is going
6:16backwards well why would that happen
6:18because of this epi cycle in this outer
6:22part of the EPI cycle it's moving the
6:25same direction as the defferent and so
6:27the star would reason going in the
6:28normal pro-grade motion but within the
6:32inner part of the loop you can see it's
6:35actually going in the opposite direction
6:36as the different so at that point we
6:39could observe that retrograde
6:41motion this was their explanation
6:43circles on circles so I mean obviously a
6:48very complicated model in fact Ptolemy
6:50is the one who's credited with coming up
6:52the most robust the best one of these
6:56models and they have having 80 different
6:58circles on it circles within circles
7:01remember epicycles on deference a happy
7:04cycle where the center of that goes
7:06around on a different so it required 80
7:08circles on credibly complicated model
7:11but one that would stick around for a
7:13good almost 1500 years or so well to me
7:17one of the best parts about this is that
7:19again this was so we could have our
7:22earth-centered universe by having this
7:25complicated model it allowed us to still
7:27have that earth-centered bias the best
7:30part though is look this is the center
7:33it was an earth the center of our
7:36so-called best earth centered model
7:38wasn't earth it was off a little bit
7:40because if we had centered at an earth
7:42we couldn't make a complicated enough
7:44model to describe all the motion so even
7:46this model was still not quite centered
7:49at earth so I always kind of got a kick
7:51out of that it was a bias we had and one
7:55that we tried to make as complicated
7:58models as we could to stick with that
7:59model rather than thinking maybe there's
8:02a better explanation