if the earth is here we got to figure out where the stars are now ignore this picture right here ignore this model right now which direction are the stars don't overthink this which direction are the stars which way would you have to point to point them out to point them out up somebody said out that's basically the right idea they're up right they're up but remember because the ancient Greeks knew that the earth was round and they knew that no matter where you were standing on it up would point to the Stars that means the stars are out in all directions here okay so there's gonna be stars surrounding the earth now here's the problem though okay let me see if I can I can simulate this okay here we go I got a thing here's my weight here's the thing here we go here's the thing right now based from my point of view this thing is up right this thing is up okay me uh let me kind of get this going on here we go this thing is up all right now if I let go of this thing what's gonna happen to it what's gonna happen to this if I let go of this thing that is up Oh No here it goes Oh No it fell on me and actually it fell over the document can become so things that are up because of gravity come down okay anything that is up comes down and where are the stars they're up so based on that line of logic what should the Stars do what should the stars do if they are up what should happen to those stars yeah those stars should fall right those stars should come down now we know that the strength of Earth's gravity gets less and less as you go away from it if you go just a little ways out from the earth you're not gonna feel much gravity from the earth right but the ancient Greeks they wouldn't have known that they've never been to space right they didn't understand how gravity works so they thought that the Earth's gravity went on forever okay they thought that no matter how far up you are you're gonna fall at the same rate right so yeah they thought that the Stars should fall okay um so they had to come up with a reason that the stars don't fall remember they're trying to build this up using logic right this this all makes sense even though there's parts that are wrong so check it out if I wanted this thing to stay up without me holding it I have to attach it to something right like if I threw this ridiculously hard I could Lodge it into the ceiling of my room right I could I could just jam it up there into the ceiling and if I just jam it into the ceiling and let go of it it would stay I'm pretending right now okay now okay check this out with this logic if I Jam things into the ceiling they don't fall down what did they decide was the situation for the stars why don't the stars fall down okay why don't the stars fall down according to the ancient Greeks let me walk it up remember we said things that are jammed into the ceiling don't fall down why don't the stars fall down yeah they were stuck in the ceiling okay they decided the ancient Greeks decided that the stars were jammed into the ceiling okay they were jammed into the ceiling I know you might be asking yourselves the ceiling what are you talking about well the ancient Greeks decided that surrounding the earth in all directions was a giant ceiling and that the stars were all attached to that ceiling so these little dots are stars just hooked into the ceiling okay and they called that ceiling the the celestial sphere you love this class Thanks I like this class too the celestial sphere and it was held in place by the Earth's gravity pulling it in from every direction because the Earth's gravity pulled in from every direction it could hold this in place up there so yeah the reason that the stars don't fall down is they're attached to this thing okay now here's the thing the Stars rise and set every day like if you watch the constellations they're gonna come up and they're gonna go across the sky and they're gonna set again and we have to come up with a reason us ancient Greeks we have to come up with a reason that happens why what would be one way that you could make the Stars go across the sky every day based on this well one way is to have the earth rotate once every day but there's another way you can figure it out what else could be happening what could make the Stars go across the sky other than the earth spinning once a day what else could happen it'd give you guys a couple of chances here yeah so the other way the other thing that could be happening is that the Stars together are rotating because the celestial sphere is rotating so either the Earth rotates once a day or this rotates once away in the opposite direction it would make the same pattern in the sky or it could be a little of both right it could be one of them turning one way and the other turning the other way but which one based on everything that we've said so far which one did the ancient Greeks probably think was actually rotating did the ancient Greeks think that the earth was rotating once a day or the celestial sphere was rotating once a day which one did they think was happening which one was spinning so remember it's the sphere it's the sphere okay remember why did the ancient Greeks decide to go with the geologic search the geocentric model in the first place because they couldn't feel the motion of the earth right and if the earth they remember they knew the earth was really big so they knew that it would have to spin really fast or it would have to spin it what the surface would have to be moving really fast to get around in one day and they didn't feel that right so they decided that it must be the sky that's spinning okay the celestial sphere is spinning once a day however okay remember the zodiac so this celestial sphere here is just covered in constellations the whole thing has constellations all over it and those constellations are locked in place on the sphere the whole sphere would rotate but the constellations are locked in place the zodiac that's the constellations that the Sun passes through okay so even the ancient Greeks knew that the Sun moved through the constellations okay now if the Sun moves through with the constellations can the Sun be locked in place on the celestial sphere if the Sun has to go through the zodiac can the Sun be locked in place in a certain place on the sphere no that's right no the Sun can't be locked in place but does the Sun fall on us no so the Sun can't be attached to the celestial sphere but it also doesn't fall on us so can you guys figure out how they addressed that where is the Sun gonna be what did they have to add to this to make the Sun work see if you guys can figure that one out we need the Sun to not fall on us but it can't be attached to this what do we got another another sphere yeah some people say another ring or another surface yeah they added another sphere inside the celestial sphere and attached the Sun to that this sphere was not called the celestial sphere I'll just call it the Sun sphere and it's spun at a different speed than the celestial sphere did right and because of that the Sun was able to go through constellations as it went through the sky right and this sphere though it couldn't be the same color as the celestial sphere because check this out what color is the celestial sphere what color is the celestial sphere what color is it between the stars see if you could figure that one out what color is the celestial sphere not the Stars but the space between them yes it's black the celestial sphere is black okay so why can't the sun's sphere be all black so why can't the Sunspear be all black why would that not match with what we see and by the way while you guys are answering that remember that all of this is not true this is just how the ancient Greeks thought things worked we now know that all of this that we're drawing is not right so yeah the celeste the Sun sphere can't be black because if it was it would block our view of the stars okay the Sun sphere can't block our view of the stars so that means that the sun's sphere has to be clear it has to be cleared and you have to be able to see through it in other words to be able to see the stars and so they decided that the sun's sphere was probably made out of something kind of like glass okay or quartz or are some kind of crystal right that said they the ancient Greeks because they were good scientists they did not say that this sphere was actually made out of crystal okay because they knew in order to prove that it was made out of crystal they'd have to go up and touch it and they had never been there so they just said based on how it looks I think it's made out of crystal so that was a hypothesis okay now that said the Sun was not the only thing that had to have its own sphere because there was another thing that moved at its own pace anything that moves at its own pace has to be attached to another sphere in this system what else had to get its own sphere yeah the moon so they attached another sphere in here and had the moon on that sphere alright and the celestial sphere and the Sun sphere and the moon sphere all spun at slightly different rates and based on this system you can actually get pretty good at predicting where the sun's gonna be and where the month moon is gonna be on different days of the year so even though it's all wrong you could actually make uh some accurate predictions from it okay if you map it out pretty well but here's the thing over time over time they got better and better and better at mapping out where exactly the stars and the Sun and the moon were on different days of the year okay and when they did that they realized with their more accurate maps that there were a few stars maybe just a handful of stars just like four or five stars that moved at their own speed they didn't stay in a constellation they moved across the constellations and so they came up with a new word for these special stars they used an ancient Greek word for wandering star and that ancient Greek word was Planeta so can anybody guess what these wandering stars actually are these special stars that moved on their own that they called planeta what are these things really yes the ancient Greeks discovered the planets here and essentially they thought that they were stars because what do they look like to the naked eye they look like stars okay they look like stars because they're so far away the only way you can tell any difference is that they because they are actually closer to us we can see their motion right but what that means is that they gave special spheres to the planets too so every one of the planets had to have another sphere in here sorry for my drawing here it's just gonna get worse and worse each one of the planets needed another sphere inside there okay there's a picture from your book it's a little bit better about this let me see if I can bring that up here you go so right here we've got the earth in the center if you look somewhere out one of these is for the Sun one of them is for the moon and the rest are for the planets and all of these spun at different speeds around the Earth okay now check this out adding that in they could pretty accurately predict where the planets were and where the Sun was and where the moon was gonna be on different days of the year but over time they got better at mapping the Stars again over time as maps of the Stars got more accurate they would realize that their old system wasn't matching up as well right every time they got better at mapping the Stars this old system didn't match up very well so what they had to do is every time they got better at mapping the Stars they had to tweak the system to make it work okay