so now I want to continue to lecture number two which is titled looking up to the sky goals of this lecture first of all to understand how astronomers describe the sky second to learn about constellations and discuss the major Celestial body that you can see with the naked eye so let's start with the celestial sphere so you can think of the sky as an imaginary sphere which is called the celestial sphere now it's this is not a real sphere like if you go there you can't actually touch the sphere it's just an imaginary way to visualize things although some ancient cultures believed it was an actual sphere so here is Earth and here is the sphere that is supposedly around earth now wherever you are on earth there's always half of that sphere above you and that is called the celestial Dome so here you are the top of the Dome so that is the point that is directly above you is called the Zenit and the place where the Dome meets the ground is called The Horizon so that's basically all around you this very last Circle that intersects with the Dome itself now since the Dome is Round The Horizon is a circle but we almost never see the full circle because maybe it's behind some buildings or mountains that are blocking the view or maybe there's just pull visibility so we just can't see far enough to see the horizon but the Horizon is always there even if we don't see it okay now imagine a line going through the Earth connecting the North and South poles and I'll show you a illustration of that in a second but for now just imagine it in your head so there's the Earth there's the North and South Pole and there's a line connecting kdom this is the axis of rotation of the earth meaning that the Earth rotates around that axis if we extend this line until it meets the celestial sphere the points of intersection are called the north Celestial pole and the South Celestial pole here is the Earth again and here we have the North and South Poles so this is the rotation axis the line that connects the two poles if we extend this line outside of the Earth the point where it intersects with the celestial sphere which is not in the picture where it's kind of like outside the picture that is the north Celestial pole and the point in the south is called the South Celestial pole so you can imagine the Earth rotating around this axis now looking from the north Celestial pole and let's say you were floating up here and looking down the Earth rotates counterclockwise so this is if you're outside of Earth looking down now the Earth complete it's a full rotation approximately every 24 hours this rotation is responsible for the rising and setting of the Sun but not only the Sun Also the moon and the stars rise and set and that's all due to the rotation of the Earth all of them rise in the east and set in the west of course that's because the direction of rotation of the Earth is from west to east the earth rotation is also responsible for the day and night cycle so here is the earth here is the sun obviously they're not that close together uh but just for illustration purposes so the Earth rotates and as it rotates half of the earth is on the side of the sun and the other half is on the opposite side so this half is going to be day we see the sun in the sky and the other half is going to be night because the sun is not in the sky the sun is now behind the Earth it's kind of like under us when it's night on earth the equator is the imaginary Circle halfway between the North and South Poles so here is again the Earth and here is the North Pole and the South Pole remember the Earth rotates around this axis uh and exactly in the middle between the North and South Poles there is this black circle that is the equator now if we extend this circle out towards the celestial sphere we get the celestial Equator so imagine that I'm now extending this circle out until it intersects with a celestial sphere so this this blue line right here this intersection is the celestial equator so let's talk about latitude and longitude first we need to remember about angles so there are 360° in a circle half a circle is 180° which of course is half of 360 and quarter of a circle is 90 which is of course a quarter of 360 the celestial sphere is a full 360 Degrees the celestial doome is half of the sky so it's 180 degrees and then 90 degrees for example would be the angle between the pole and the equator right so if this is the Earth the pole is up here the equator is down here so the angle between them is 90° as also for example the angle between the Zenith which is the point directly above you and the Horizon so now the latitude of a point on the surface of the Earth is the angle it makes with the equator either to the North or to the South which means the latitude of the Equator itself is 0 degrees so latitude of the North Pole is 90° north and latitude of the South Pole is 90° South the longitude of a point on the surface of the Earth is the angle it makes with the prime meridian which is a line stretching from pole to pole so from the North Pole to the South Pole and passing through the Royal Observatory of greenage in England the primary ran itself has a longitude of 0 degrees and of course there is nothing special about the Royal Observatory which was only chosen for historical reason here is an illustration so latitude is north south so you can see the equator is 0 degrees and the North Pole is 90° North South Pole is 90 Dees South at exactly halfway between we have 45 degrees south and here we have 45 degrees north but we also have all the values like you know it was 10 degrees 20 degrees 30 degrees and so on um the longitude is the angle to the west or east so again we have zero at the greenage and then we can go west or east the maximum we can go is half of a circle which is 180 degrees so it can go all the way up to 180 West or 180 East to remember the difference between latitude and longitude because a lot of people get confused notice that if you switch the first two letters of latitude ude you get altitude which is another word for height and maps of the earth always have the north on top so the latitude tells you the height along this map so if I look here the latitude is kind of like the altitude as I go up or down this map of the Earth let's say that you are right now at a latitude of 90° North so you are at the North Pole because that is where 90 Dees North is and you can see this in this illustration so you're standing here at the North Pole and now if you look up you see your Zenit which is always the point that's directly above you no matter where you are so if you're standing at the North Pole the Zenit is the north Celestial pole which is like I said if you extend the line from the axis of rotation of the Earth all the way up to the celestial sphere then that point of intersection is the north Celestial pole and the celestial equator is a till Horizon I guess the Horizon would be here on Earth and then the celestial equator will be at a 90° angle to the Zenit so it's the Horizon now when you are at that point the Stars don't rise all set they just circle around the pole you are standing on the axis of rotation so you are not rotating only things around you are rotating you can only see half of the sky at that point and you always see the same stars because again you're standing here you only see this half of the sky you're never going to see any other half because you're never going to be at any other direction you're always going to be at exactly the same point because you're not rotating you're on the axis of rotation here is a photo where they used very long exposure to capture the paths of the stars in the sky so basically just kept the exposure open for a few hours and then each star instead of just a single point it traces a line that CHS the direction that star Moves In The Sky of course remember the Stars don't actually move in the sky the sky rotates now the center of the these circles is in this case the South Celestial pole so what I said before about the North Pole also applies if you're in the South Pole the same thing here at exactly the center if you imagine that the sky is the celestial sphere or the celestial Dome then that point exactly the center of the circles that is the celestial Pole it's basically the point that the celestial sphere rotates around or seems to rotate around because again the Earth is the thing that actually rotates now because the Earth rotates around the pole the pole itself doesn't change its position in the sky let's say that you are standing at Latitude zero de somewhere on the equator by the way the 90° North is exactly one point it's just the North Pole but 0 degrees is it can be anywhere along the Equator when you're at this point standing on the equator then the celestial equator passes through your Zenit right so here's the Zenit uh the celestial equator would pass through here the celestial poles are On Your Horizon so here is the North Pole and here is the north Celestial pole that is on your horizon which is this red circle and similarly the South Pole you can extend it over here and that's going to be the South Celestial pole that's also On Your Horizon when you're standing here on the equator over a 24hour period you will see all the stars in the sky rising in the East and setting in the west or at least it would see them if the sun wasn't in the way during daytime so imagine there's no sun you're standing here on the equator now the Earth rotates around the North and South Pole axis so you just rotate all around the earth which means that there's no point of the sky that you don't see because you are making a full circle so any point in the sky is going to be above you at some point during the day again if the sun wasn't in the way you could have actually seen all the stars okay now let's say that you are not EX exactly at the North Pole and not exactly on the equator you are in agra falls which is 43 degrees north so basically almost halfway between the Equator and the North Pole in that case the north Celestial pole will be 43° above the northern Horizon okay so you are standing here and now the the North and South Pole over here so this is now the axis of rotation you're going to have a northern Horizon and a southern Horizon right so here to the north this is your Northern Horizon and here is your Southern Horizon and the north Celestial pole is over here is the point where if I extend this axis it touches the celestial sphere this is the celestial pole that's going to be 43° above the northern Horizon right right so the angle between kind of here and here that is going to be 43° the South Celestial pole is going to be 43° below the South Horizon so it will never be visible so again continue this line from the South Pole to the celestial sphere this is now below your horizon something is below your horiz Horizon you never see it so you will never see the South Celestial pole when you are at this latitude now stars that are within 43° of the North Pole or the North Celestial Pole to be more precise can never set they're always above the Horizon and they circle around the north Celestial Pole that part of the sky the part where the the Stars never set is called the north circum poar Zone let's continue to talk about asterisms on a perfect night when there are no clouds and there's no artificial light it is possible to see about 3,000 Stars with the naked eye not in the middle of the city but just go somewhere far from the city when there's no artificial light or what we call light pollution then you have a perfectly dark sky and let's say that the weather is also perfect so there's no clouds anywhere the sky is completely clear and the area is completely dark with your naked eye you can see about 3,000 Stars the positions of these stars are random but humans love to find patterns in things so ancient cultures gave names to specific geometric patterns that were particularly noticeable these patterns of stars are called asterisms now they are not called constellations a constellation is something else as we will see shortly identifying these patterns is a bit like solving a Celestial connect the dot puzzle so you have stars and you connect them with lines in a way that kind of makes a shape that looks interesting now historically these patterns were helpful in navigation because before we had GPS they allowed people to orient themselves at night so it could look up at the sky and you would see a familiar pattern and you would use that pattern for example to find where the North Star is and that will tell you where North is and then you can Orient yourself and know where the North South West and East are located here are some examples of asterisms you may already be familiar with or maybe you just saw them in the sky and you thought oh that looks interesting but you didn't know they had names I'm sure everyone is familiar with this the big Depot which consists of seven bright stars and they look a bit like a bowl with a handle so connect the dots there's also the little deeper also called also minor or the little Bear is similar in shape but smaller in size so it also happens to have the same shape it's just smaller and also oriented differently in the sky and the very bright star at the end of the handle over here that is Polaris also known as the North Star which is very close to the north Celestial Pole if you're ever lost and your GPS isn't working and you want to know where North is here's how you can find it you can find the Big Dipper and you go to the end of the bowl and then you go along that line about five times the length of the line You'll Find Polaris and you can verify that it is Polaris because it will be the end of the the handle of the little Deo let us now move on to constellations so in modern times astronomers divide the celestial sphere into 88 regions and these regions are called constellations these regions cover the entire sphere or in other words the entire sky so any point in the sky is located in exactly one of these constellations it's kind of like country so you split the land on Earth into countries it's pretty much an arbitrary split uh but it's just some splits some people decided on and now any point on Earth belongs in exactly one country of course unless it's disputed so maybe this analogy isn't perfect but uh the idea is you have these 8 region and any point on the sky you can say okay that is in the Virgo constellation or whatever constellation it is now each constellation contains one or more prominent asterisms so patterns of stars and they get their names from various animals objects and mythological characters or creatures now many of these constellations originated as early as 5,000 years ago in ancient Mesopotamia by the way I'm just talking about the constellations defined by the uh International astronomical Union of course different cultures had defined different constellations but this is what we agreed to use all over the world sometime last century now this uh Mesopotamia is the historical region where Iraq is located today and the civilizations of that area that we're going to learn a lot more about later include the Sumerians and the Babylonians the ancient Greeks adopted the Babylonian constellations around 400 BC the ancient astronomer toy uh who was an ancient Greek described 48 of the 88 modern constellations in his influential book almagest around the year 150 now the Greeks of course could not see the entire Sky since they never saw any stars from the south circumpolar Zone okay so remember if you are on the equator you will see all the stars but if you are to the north of the equator you want to see some of the stars and there is a region of the celestial sphere that you're never going to see so when European explorers began traveling to the southern hemisphere in the 15 Century they gradually added new constellations that can only be seen in the southern Sky the modern list of 88 constellations along with their boundaries because remember constellation is a region so this region has boundaries kind of like countries have borders which cover the entire cesal sphere was adopted by the international astronomical Union in 19 28 okay so what is the difference between an asterism and a constellation a constellation is a region in the sky and asterism is a specific pattern of stars however each constellation has one or more defining patterns within this region and sometimes they are referred to colloquially as the constellation so when someone tells you the constellation but they're referring to a specific pattern meaning to a specific asterism what they actually mean is the asterism that is most prominent in that particular constellation the international astronomical Union only defined the regions of each constellation they didn't defin how to connect the Stu that is basically just a matter of convention so it's of scientific interest to have this region so that you can say okay well here is a nebula and that is in this constellation so we know which region of the sky it's in but how to connect the Styles that's not really of scientific interest so here's an example so before I showed you the big deeper asterism which is located inside the constellation Elsa Major or great B I think maybe it's a bit hard to see the difference in colors but uh the boundary of the constellation is around here so there it's kind of like this darker part is the constellation and then outside there's this gray part these are other constellations so also major has the boundaries would go like this and over here and around here and down here and back here so that is the constellation meaning the region of the sky and you can see that there are some prominent Stars within that region uh and you can also see that whoever made this illustration connected some of the stars and can imagine that this is a great Bear right so here is the body of the bear and here is the front leg and here are the back legs and here is the very long tail and the Big Dipper is this tail basically so you can see here the shape of the Big Dipper in this tail now it's very hard to see this shape in this illustration because there's a lot going on so here is a more artistic illustration of this particular asterism in that constellation ancient people saw it as looking kind of like a giant bear and here you can see the big deeper is made of these particular Stars constell Orion is easy to find due to its very distinctive asterism which is Orion's Belt so here you can see a similar illustration of the constellation so there is a specific cion that you may or may not be able to see uh but it's basically this is the boundary of the constellation and Beyond it are other constellations now within this constellation there is this very distinctive pattern of three very bright stars this is oran's belt and here is Orion so again if you have a very strong imagination you can kind of imagine that this is indeed an actual belt of this Warrior and uh there also some Stars here that are the sword and these are the legs and that's the head and here there's a bow so you see that all of these have a specific interpretation now what's actually going on inside this constellation here is kind of a deep look into what's actually in the sky there if you could see everything so you don't see this with the naked eye but with a telescope you can see this so uh here is the belt the belt is made out of three Styles alitak Alam and minaka these are the three Bright Stars over here and now here I have the leg which is Regal and the other leg is scythe and here I have B juice and by the way in the video in the beginning uh I believe we saw Regal and beet juice as a size comparison and B juice is much much larger than most other stars you can also say that it's in a different color so it's kind of orange while the other styles are more kind of blue or white and here is another star called Bellatrix now here you can see see the oran nebula so I showed you a photo of that I think in the first lecture so this nebula is actually located over here it's called the oran nebula because it is in the Orion constellation and another thing you can see here you can see this kind of a half circle of red that is a huge structure of gas that's called Bernard's Loop okay so now I want to explore some constellations and asterisms using this very very cool software called stellarium which is free so anyone can install it if you're interested let's just St on some stuff over here so basically this shows you how the sky looks like at this moment in time basically uh I can tell the atmosphere on or off so if I turn the atmosphere off now you can see that I can see a lot better because the atmosphere is not in the way of course you can't do that in real life so that's why you have computers and I can also turn the ground off so it doesn't block my Horizon and now I can look in the sky and see all of these uh different constellations Let's uh see if we can find okay so here is also major you can see this Bell over here I can turn on Constellation out and now you can see the Bell and here is also minor also minor is also a bear it's just a smaller Bel that you can see kind of upside down over here and here is Polaris the North Star I can also see planet so here is Jupiter now if you look closely maybe I'll turn off everything if you look closely you can see these lines I don't know if you can see them because the projector uh doesn't have very good contrast but these lines over here these are the constellation regions here we can see the Milky Way this is the Sun and you can also see Mercury and Venus here's the moon here's Mars now here is ble JW which means which um constellation would this be or right so let's uh here you can see this distinctive belt of three stars and if you zoom in close enough it will tell you the names of these Stars which we talked about before and again I can tell on the constellation lines um and even the art and you can see Orion over here with his belt and his legs and everything now if I zoom in let's turn this off if I zoom in very closely in on this region I might be able to see yeah here is the or nebula okay so this is just a lot of fun to explore you can download it for free works on any operating system uh it also works on Android or iPhone so you know if he wants to get familiar with the constellations I highly recommend downloading it and playing around with it now let's move on to the next topic which is the stars in three dimensions so the sky looks like a two-dimensional sphere the celestial sphere and the stars look like they're located on the sphere itself and indeed some ancient civilizations actually thought this was a sphere that was rotating around the earth madees of some Divine material or other um but in reality of course the Earth is the thing that's rotating more importantly the stars are not located on a two-dimensional sphere they are distributed in three dimensions all over the Galaxy and I find that this is something that a lot of students get really confused about so when you look in the sky of course you see a two-dimensional pattern but in reality you're just seeing a projection of styles in different places in three dimensions on this two-dimensional sky so our sky with the specific patterns like Orion or the big deeper is unique to us imagine aliens on a distant planet many Li years away what do they see when they look up at the sky they don't see Orion and the big deeper and all of these patterns because they are in a different place so they see a different orientation of all of these Stars they see the same three-dimensional distribution of stars but just from a different point of view stars that seem to be close together in our two-dimensional sky might actually be very far apart in the actual three-dimensional space so if two stars happen to be in the same direction as seen from Earth they will appear close together even if in reality they are many lighter away from each other so for example if I forget that this is a three-dimensional space and I just look in One Direction all of the students sitting on this column they all look like they're right next to each other well in fact there is also a depth that I don't see if I interpret it as a two-dimensional image in our's belt for example remember we have these three stars alac Alam and Mina so alak and Mina the two outer stars are both approximately 1,200 light years away from Earth but the star in the middle Alam is 2,000 light years away from us meaning that it is 800 light years away from the other two stars so these two stars are actually close to each other this one if you just imagine there's also depth to the picture this one is just much much farther away way behind these two but when we look at the sky we only see the two-dimensional projection of this three-dimensional space so it seems to us like these three stars are right next to each other okay so um let's finish with this video in this video you can see the Orion asterism so here they're just kind of showing you the belt and ble juice and regle and belrix so all of these Styles and now we're just kind of rotating around so I'm moving to a different place in the galaxy and you'll notice that everything looks completely different from from this other Viewpoint so the belt is no longer a belt and these tars just completely move away from everything else and now this here is the Middle star so here you can see remember I told you that um the two stars at the sides of the belt are closed together but the one in the middle is very far away now I got to an angle a point of view where I can actually see that distance okay I'll keep rotating so around now I am actually 180 degrees away so seeing it from the other side this is what aliens living on the other side of the Galaxy would see okay I keep rotating and again you can see this huge distance between the Middle star and the two outer stars of the belt now slowly we go back to Earth so basically we just took a 360 degree path around the entire galaxy and we saw what aliens in other planets would see when they look at the same Styles now finally we got back to Earth so again we see this a in shape okay so remember this so another thing that we need to understand about constellations is the stars are not fixed in place so again this is something that the ancient people uh some of them thought that the stars are actually fixed onto this celestial sphere that rotates around the Earth but of course in reality we have a huge Galaxy and there are just stars all over this galaxy in three Di iions and all of these stars move around so like I said before the sun for example orbits around the center of the Galaxy and all other stars do the same thing so each star has its own independent Motion in three dimensions and different Stars even in the same constellation move in different speeds and in different directions so over tens of thousands of years the patterns that we recognize now in the sky like the Big Dipper and Orion and all of those things I showed you last time may not exist anymore for example one star in a constellation might move north and another one moves East whatever pattern they created is not going to be there anymore now prehistoric humans who lived hundreds of thousands of years ago actually saw a different Sky than we do when they looked at the sky they just didn't see the same patterns that we see so now I'm going to show you a video uh again of the Orion constellation or I guess the Orion asterism to be more precise how it looks like from Earth at different points in time if you look here at the bottom left you'll see the year so right now it's at minus 2.6 million years and uh it doesn't really look like Orion here you can see B juice so remember B juice is the top left star that is going to be somewhere over here right now it's actually moving it hasn't reached its destination yet and here is belrix which is the star at the top right of Orion and again it is not there yet it's just moving slowly towards where we see it today so now we are at minus 1.6 million years some stars are passing by these are not stars that we'll see in Orion uh but these Stars you can see here minaka and alitak these are stars that we see today as the belt and they exist more or less in the same place um over millions of years as far as we can see so okay slowly Beetle Juice and belrix are arriving at their positions we are now at minus 700,000 years here also Regal and Scythe which remember these are the two legs of Rion and here now we can see the the full belt as we see it today more or less now we're at minus 250,000 years so humans have already existed at this time mod humans I mean and B juice and belrix now arrive at their correct positions oops B juice just gone super NOA it exploded and it doesn't exist anymore well too bad but um you can see that the stars are still um kind of in the same place now we are at half a million years in the future okay so if you want to watch the full video you can do it on YouTube the point is stars move around some of them explode and just don't exist anymore so patterns uh in the sky depend on what time you view them now the pades also known as the Seven Sisters are a star cluster around 424 light years away now this Stars unlike the stars that make up Orion they are actually close to each other in threedimensional space so not just in the two-dimensional Sky which is just a projection of this three-dimensional space on two Dimensions which hides the depth difference between the stars but in this case the stars are actually next to each other in three-dimensional space because they are a star cloud fter so that means they will be seen together from different places and times they're not going to move the way that for example we saw be juice just move from offscreen into the pattern of stars now of course they're not always going to have the same shape that we see them right now uh but we always will see them together next time you you are far away from city lights at night take a look at the sky and try to find all the constellations that I mention today Sky Map is a great Android app that will show you the location of the constellations when you point your phone Towards the Sky uh there is also an iPhone app so check the course website for a link to the iPhone app this is is really useful when you just go out you know camping or something like that and you want to uh identify constellations you can just point this up and it will tell you the name of each constellation that you see in the sky okay so now let's talk about the ecliptic so there is of course uh one particular star that is closest to Earth and that is the sun other stars are many light years away but the sun is only about 8.3 light minutes away so remember a light year is the distance that light flows in a year that is a huge distance of 9 and a half trilli kilom now light minute is the distance light travels in one minute that is of course a much shorter distance but still very very large in human terms so we call the distance of 8.3 light minutes which is the average distance from Earth to the Sun an astronomical unit and we denote it by AU and it's equal to around 150 million kilomet so in this calls every time you see Au or astronomical unit you should know I'm talking about this distance from the Earth to the sun which is 8.3 light minutes or 150 million kilom since the sun is so much closer to us than the other stars it's also appears much larger and brighter however this doesn't mean it's actually larger or brighter than the other stars since those stars are so far away we only see them as Tiny dots in the sky but if we put one of these stars in real size next to the Sun the other star could turn out to be much larger and or brighter so in the beginning of the previous lecture I showed you this video that uh showed you the sizes of different celestial objects and in that video you saw that if you put stars next to each other there are indeed some stars that are much much much larger than the Sun and also stars that are smaller than the Sun so the size in the sky and the brightness in the sky depends on on how far the star is from us and how bright the star actually is and the star that is brighter in the sky doesn't necessarily mean it is brighter if you actually go there and compare the two stars side by side so here is an example the sun in this image is just one pixel over here and here I have different Stars so here is Regal that is the star at the bottom right of Orion and some other stars that are larger and of course this continues there are even larger stars there just not enough space on the screen to show them you can also see here by the way that stars have different colors and we'll talk more about that later the sun is so bright that you can get serious eye damage just by looking directly at it for a few seconds during so don't do it during daytime when the sun is in the sky we cannot see any other stars in the sky because they're just not bright enough compared to the immense brightness of the Sun so this is why we can only see the stars during the night or during a solar eclipse so during a solar eclipse the sun is blocked by the moon and since it's behind the moon we don't see the light from it and that allows us to see all the much smaller and dimmer light of the other stars the sun is also bright enough to turn the color of the sky from black to Blue and we learn learn how that works later now the Earth rotates in two different ways one around its own axis and two around the Sun now these two different forms of rotations have different periods of rotation so a day is the period during which the earth completes one rotation around its axis and it is around 24 hours and a year is the time it takes the air to complete a full revolution around the Sun and that is around 365 days the plane of the Earth's rotation around the Sun is called the ecliptic so let's look here this is the Earth and here is the ecliptic now the sun isn't actually in this uh illustration this illustration is just the celestial sphere around the earth so we don't have the sun in this illustration but imagine the Earth just going in a big orbit around the Sun that is offscreen so um the plane of that orbit is the ecliptic the reason it's a plane is that any orbit is inside the plane so the Earth rotates around the Sun but it never goes above or below that plane it always stays within that plane the line perpendicular to the ecliptic intersects the celestial sphere here at the ecliptic poles so I have the ecliptic over here and I have a line perpendicular to the ecliptic over here and at the end of that line when it intersects the celestial sphere that is the north ecliptic pole similarly if you continue this line down here would be the South ecliptic pole the plane of the earth rotation around its axis so the second type of rotation intersects the celestial sphere at the celestial Equator the equator is this black line on Earth and if you extend that you will find this dashed white line which is the celect Celestial Equator so again it's a plane the equator always stays within the same plane but the two planes the ecliptic and the equator differ by an axial tilt of 23.4 De so you can see that over here so here is the ecliptic is the plane of all Orit around the Sun here is the equator that's the plane of the orbit around the Earth's own axis and between these two planes is an angle of 23.4 De equivalently if you look at the line that is perpendicular to each plane so here I have the line perpendicular to the ecliptic here I have the line perpendicular to the equator which is the line that passes through the North and South Poles between these two lines there is also an angle of 23.4 De you also notice there are two intersections of the ecliptic and the celestial Equator so here's the ecliptic again here's the celestial equator and they intersect at exactly two points one over here and one over here these two points are called the equinoxes and the Sun is at the equinoxes around March 20 and September 23rd so that means that on September 23rd the Sun is going to be not actually here because remember this is just the celestial sphere around the Earth the Sun is not on this sphere the sun is very far away but when we look at the sky the Sun is going to appear at this position on the sky at the position where the ecliptic intersects the celestial equator now the Sun appears to move along the sky during the year but again I remind you the Earth is the one that actually moves the sun just stays in place and the Earth moves around it now this means there are different constellations behind the sun on different times for example in June the Earth is over here and when we look at the sun we will see that I guess we won't really see because you can see what's going on behind the Sun but uh behind the sun would be the constellation Taurus later in August we have traveled a long way along the orbit so now when we look at the sun we're looking at a completely different direction in space now when we look at the sun we will see the constellation cancel behind it now these constellations I should stress again they are this is not to scale right so these constellations are actually many many light years away hundreds or even thousands of light years away that is the distance to the different styles in these asterisms that Mark the constellations the constellations themselves don't move like I explained before over a very long period of time you will see the stars move but over a short period of time just a few years a few decades you can consider them to be fixed in place so it's clear that since we are much closer to the Sun than to any of these Stars the sun when we look at the sky will appear to move and be in different places in the sky while the constellations are going to be at the same place place now in total there are 13 constellations on the ecliptic the Sun appears to move in a circle Eastward and comes back to its original spot after exactly a year again it only appears to move because we are moving around it but when you look at the sky and you mark the location of the Sun every day you will see it will move in a circle there are 360° in a circle and around 365 days in a year these numbers are very close together so the Sun appears to move around one degree per day in the sky with the naked eye we can see five planets Mercury Venus Mars Jupiter and Saturn because you can see them with the naked eye that means that thousands of years ago before telescopes that is all people knew about of course you can also see the sun and moon in the sky with the naked eye and so therefore the sun moon and these five planets were known in ancient Greece as the seven classical planets now don't get confused the word Planet means Wanderer in ancient Greek these seven objects seem to wander around the sky while the Stars stay in place but the sun and moon in modern terminology are not planets so the world planet now only applies to large objects that orbit the sun now the sun follows the ecliptic like I said on the celestial sphere the paths of the moon and the planets are close to the ecliptic but not exactly on it the orbits of the planets around the Sun and of the Moon around Earth they all lie on their own planes but these planes turn out to be very close to the ecliptic plane you can think of it as the whole solar system basically rotating within the same plane roughly speaking although of course there are exceptions that we'll talk about later so the sun moon and planets are always within 8 to 9 degrees north or south of the ecliptic this belt is called the Zodiac if you imagine there's the circle on the sky that's the ecliptic and then just imagine a belt around that Circle so kind of like a belt going around the earth on the celestial sphere that is the Zodiac and the sun moon and planets are always inside that belt on the sky Now zodiac means circle of animals many of the 13 constellations on the Zodiac are named after animals so that's how it got its name so this lecture focused on things that can be seen with the naked eye Stars asterisms and constellations we also learned about many imaginary points and lines in the sky such as the Zenit the celestial poles the ecliptic the equinoxes and so on now the stars are basically the only thing here that actually exists the asterisms and constellations are just patterns that we decided on and all of these points are just points we use to orient ourselves to review what I taught in this lecture read the textbook Open Stacks astronomy section 2.1 and there will be practice questions posted on teams like we had for the previous lecture