Introductory Astronomy: Positions on the Celestial Sphere

hello and welcome to astronomy we’re going to do our first lecture it is the night sky position this is the content that is covered in the lecture tutorial on page 1 of your lecture tutorial look and it’s called position we live on the earth and as you may know our earth is roughly spherical it rotates on an axis and you and I most likely if you’re listening to this you live in the northern hemisphere on the earth specifically you live in Westchester County in New York what you’re looking at right now is a satellite composite image of the Earth from NASA this is true color this is what the earth would actually look like from space if you were to fly far enough away from it now if you also fly from the earth and you’re stationed roughly so that you are not moving with respect to the earth you’ll see it rotate and let me show you what that looks like and so the earth it spins on an axis that has a north and a South Pole I’ll show you the movie one more time the Earth spins on an axis it’s got a North and the South Pole because you live in the northern hemisphere you will see different things in the sky than you would if you were living at the North Pole the South Pole or even in the southern hemisphere but because you’re standing on the earth because you’re moving with it as it rotates you don’t actually feel it move and so what that means is as the Earth rotates the things that are in the sky appear to move around you now we know this isn’t true it’s because the earth is the thing that is rotating but from our vantage point everything that’s out there in space appears to be going around us and so for thousands of years humans have actually used a model to describe these motions and that model is called the celestial sphere and it’s a model that we’ll use here in astronomy class so what we do is that we pretend that the sky is a sphere that surrounds the earth and it allows us to estimate and where things in the sky will be and what their motions will look like so on the slide that you see here you’ll see that the earth is spinning and it’s surrounded by a celestial sphere the grid might have particular coordinate systems on it and you see that their stars are painted on the inside of the celestial sphere a few points to note on the celestial sphere it’s got a North Pole much like the earth has a North Pole and the North Pole of the celestial sphere is called the North celestial Pole it’s the projection of Earth’s North Pole onto the sphere so if you were standing at the North Pole of the earth and you looked directly overhead the North celestial pole would be there likewise if you were standing at the South Pole of the earth and you looked directly overhead you’d see the South celestial pole if you were standing at the equator of the earth anywhere on the equator and you looked overhead you would be standing beneath the celestial equator which is just earth’s equator projected out onto the celestial sphere for those of us who are living in the northern hemisphere we can actually see the North celestial pole on any clear night the reason being is that there just happens to be a star at this moment in time that’s located on the sphere at the same place that the North celestial pole is and you may know what the name of the star is the North Star or Polaris something you may also know about is the concept of rising and setting so you may know that the Sun rises in the east and then sets in the West but so does the moon the planets and pretty much everything else in the sky but depending on where something is in the sky it may appear to move but never rise or set and we’ll give you some examples of that in the lecture here so let’s talk about that a little

bit so rising and setting rising and setting is always relative to what is called your horizon which is a line that is 360 degrees all around you and it separates the land or the surface that you see from the sky and so the horizon is as far as your eye can see and so I’ll point at it with the mouse in these images so here’s an image of the ocean with the Sun and so that line separating the surface or where the ocean is with the sky to that limit of your vision that’s the horizon also you’ll have a horizon if you were on the land and so you can see that there’s a line that separates the land from the sky that’s the horizon sometimes your horizon is blocked because there might be higher elevation features on the surface such as mountains often we’re going to be working with horizon diagrams horizon diagrams show up because they allow us to trace or mark the positions of things on the celestial sphere the difficulty with horizon diagrams is that you’re not used to dealing with them they involve some spatial reasoning that you may not have practice with and so the first couple tutorials that you’ll be doing in this course will get you to really think deeply about the celestial sphere using these diagrams part of the lecture today is going to be comprised of me giving you advice on how to think about these diagrams so this is the diagram figure one that you’ll see in the lecture tutorial it looks kind of scary let me point out some pertinent features first of all you’ve got a person standing on the earth this is not to scale at all here’s a person they’re in the northern hemisphere of the earth and you can tell that they’re in the northern hemisphere because he’s not standing at the North Pole or at the equator he’s somewhere between also that dome that’s above him is basically his sky the horizon is drawn as an imaginary line that stretches off of the earth so that you can get to the Dome of the sky or the celestial sphere and then there’s also some star paths that are drawn on onto this diagram this is a two-dimensional representation of a three-dimensional concept the celestial sphere and so things that are into the screen are shaded with a dashed line or a lightly shaded line and things that are out of the screen or out of the paper in your book are a bold line that’s not dashed the directions on here should become apparent very soon if this person were to face towards the North Pole then they would be able to see the North Star up in their sky and that North Star is above their northern horizon in this diagram the northern horizon is right on the left of the diagram so for you looking at this image North is on the left part in there since the horizon is the line that I’m pointing at with the mouse you can see it there completely opposite that is the southern horizon to the right side of the diagram and into the paper or into the screen is East because if I’m facing north and I turn to the right in this case into the screen that would be East and out of the screen or out of the paper would be West alright let me give you some help with this I’m switching over to a computer view so here is a celestial sphere and what I can do with it is I can rotate it around you can see that the earth is inside the blue lines coming out of the celestial sphere represent the North celestial pole and the South celestial pole I can rotate this around and you see that there’s a white dot on the earth that white dot represents somebody’s location on the surface I could change the latitude so that I could be at the equator I could be at the South Pole or in Australia or I could be somewhere say in the northern hemisphere like Western Europe or I could be completely at the North Pole of the earth for now let’s stick with the northern hemisphere and let’s go at a latitude that’s roughly similar to Westchester about 41 degrees how do I change from a celestial sphere view to a horizon diagram view so what I’m going to do is I’m going to hit the

switch button and it will change it to a horizon diagram view in this simulation what what will happen is that the view will zoom in to a person standing at that white dot and because we are not standing at the North Pole of the earth when we zoom in the person will turn right side up in the diagram which will make the North celestial pole tilted in our view so let me hit switch and I’ll do it a couple times so you can get a feel for how these two figures are related we’re zooming in on the person standing there on the earth and the celestial sphere is still around them the blue lines represent the North and the South celestial poles but we see that the horizon has been extended out so that we’re really zoomed in on this person I can move the view around so I can look at from look at this person from almost any direction but let me make it similar to what we have in our book horizon diagram in our remember in our book horizon diagram the North celestial pole is somewhere in the upper left portion of the diagram which means that the northern horizon is over to the left so if I were to look at this person completely from top down or over head north would be on the Left South would be over on the right east would be at the top and West would be on the bottom but I’m looking at them from the side in the book diagram so what that means is East is into the paper or into the screen West is out of the paper or out of the screen south is to the right and north is to the left if you are that person you wouldn’t see this remember we’re outside of the sphere or outside of the diagram looking in also note something here North is not straight up and North is not too directly to the left North it I’m sorry the North celestial Pole is not straight up or it’s not directly to the left North is to the left and also keep in mind South the southern horizon direction is not beneath us when we talk about directions we’re always talking about directions on the horizon so for this person they would only see anything that’s above their horizon and that’s where this concept of rising and setting comes in all right let me switch back to the celestial sphere view and here we have the earth surrounded by the celestial sphere also note that we have a yellow line here that appears to be the celestial equator because it roughly lines up with where earth’s equator would be now if I switch back to our horizon view our person standing there we are zooming in we make them right side up again that yellow line could also represent the path of a star on the celestial sphere as it rotates around us and notice that that line intersects with the eastern horizon the western horizon and that will be important a little bit later all right let’s go back to the presentation just for a moment and I want to tell you one more thing if something is below the horizon and it moves in the sky from below the horizon to above that’s called rising if it goes from above the horizon so I’ll move my cursor if it goes from above the horizon to below the horizon that’s called setting many objects in the sky appear to rise and set but some of them do not in fact there’s some things that from our position on the earth will never see rise or set because either they are always in the sky or they are always below the horizon and never get above it so let’s go back to our horizon diagram what we’ve got here is a person standing in the northern hemisphere North is to the left South is to the right on the horizon East is into the screen or into the paper West is out of the paper or out of the screen so I’ve got two paths of different stars star B which has a path which is always completely above the horizon and then the path of star a which if you remember is very similar to the yellow line from our simulation in star a has a path that intersects with the eastern horizon and the western horizon if you were this person and you were watching star a move across the sky you’d notice that it goes from the

eastern horizon gets higher in the sky until it gets to its highest point above the horizon right here at position two and then it goes down and then sets somewhere right here on the western horizon and then it spends some time below the horizon during that time you’d not see it what direction does this person have to face in order to see star a when it is at position number two well remember if right in the diagram is south so I’m I’m hovering over the southern her and that looks like point number two for star a is exactly halfway between the dashed line and the bold line for this path it looks like it’s right on this line above the southern horizon and so viewer this person you would be facing south high in the sky to see star a when it gets to its highest point most things that rise and set get to their highest point above the southern horizon let’s go back and do another simulation exercise with the celestial sphere so here is a simulation that allows you to see the celestial sphere view of the earth and a horizon diagram view at the same time I’ll also be able to add some things onto the sphere such as stars and then we’ll see how they move as the Earth turns so the first thing that I want to do is is just notice that we’ve got a person here in North America which is where we live and this is a horizon diagram of that same person so I’m going to orient the diagram so it’s just like our view in the book and I’m going to add a star pattern to this and in this case I’m going to add the constellation Orion which is something that you can see during the winter so on the celestial sphere Orion just happens to be located here at this coordinate when the Earth turns it causes that constellation to move in our sky now where is it in relation to the person on our horizon diagram well if I move their sphere around you’ll see that Oh Ryan appears to be far below the horizon it’s it’s nowhere in this person’s visible sky at the moment but maybe when I hit play we’ll see it move above the horizon so let me do that and let’s see what happens to it I’m going to add a star trail to this when it gets above the horizon so we can see roughly what its path looks like for this person so I’m going to start the animation so as Earth turns the celestial sphere appears to move around this person sky you see that Orion has moved above the horizon so this person could see it and now it is sent it’s gone from above to below the horizon so now it is completely below the horizon and turning around us soon it’s going to go from below to above rising notice that it rises right here almost directly east and so what that means is as this constellation continues to appear to move in the sky it will move up and to the right from if you’re facing east so if this person we’re standing there facing east on this night over the course of several hours they would see Orion move up and to the right which is towards the south and then Orion would get to its highest point in the south so here we are so now Orion is at its highest point up here high above the southern horizon this would be similar to star be at point two in our lecture tutorial book I’m going to add another star pattern to the mix I’m going to add the Big Dipper so the Big Dipper is a completely different set of stars and the celestial sphere it’s much closer to the North celestial Pole which means it’ll be closer to the Polaris in our night sky now what does it look like to this person well this person would have to face north in order to see the Big Dipper and watch what happens when the celestial sphere appears to rotate the Big Dipper gets high in the sky but not

quite directly overhead and gets fairly low in the sky but doesn’t go below the horizon so what I’m going to do is I’m going to actually rotate the horizon diagram view so that we could see the northern part of the sky as this person would so North is now into the screen the constellation is fairly high up in the sky at this point now the constellation is in the north part of the sky low in the north part of the sky and heading up to the high in the Northeast and so what would this look like if in our in our book view what that what it would look like is the Big Dipper would be high in our North East sky and it would probably be one of those dashed or lightly shaded lines since it’s into the board or in the northeast part of the sky when the Big Dipper finally rotates back around and it’s in the northwest part of the sky in our book diagram that would be a bold solid line so here it comes back around I’m going to pause that it’s high up above our North West part of the sky I’ll rotate the sphere around so that you can see this North is into the screen west is to the left and the Big Dipper is somewhere between that but high in the sky in our book view in our book diagram the Big Dipper would be out of the paper or out of the screen let’s take a look at that diagram again and see if there is a position that corresponds roughly to where the Big Dipper is located at this moment that I’ve paused the simulation so the path of star B is very similar to the path that the Big Dipper takes in the sky it never goes below the horizon it’s always above the horizon which means it never rises and never sets it’s always above the horizon and the poet and the place that I’ve paused the simulation where the Big Dipper is above the northwestern horizon high in the sky is similar to position 3 for star B notice that it’s on the bold line which means it’s out of the paper or out of the screen and so that’s high in the northwest so north is right here West is where I’m circling the cursor at the moment and position number 3 is between that high in the sky when you do the lecture tutorial you’ll be able to practice with these positions quite a bit I’m going to change up one more thing in this diagram and I’m going to show you one more star pattern and that is the Southern Cross the Southern Cross is very close to the South celestial pole so here it is here it is on the celestial sphere so what I’ll do I’ve got to rotate this around again what I’ll do is start the animation you’ll notice that the Big Dipper always stays above the horizon for a person in the northern hemisphere will Ryan rises gets high in the southern sky and then sets but the Southern Cross is on a place on the celestial sphere that never gets above our horizon so it never rises and that’s just a product of where we’re standing we could move to a different location on the earth and see different stars perhaps let me give you an example of that so I’m going to move our location on the earth so notice how we’re in North America I’m going to move to a latitude that is on the equator if I go to the equator then notice what happens here I can see the Big Dipper I can see the Southern Cross and I can see Orion but watch what their paths are like when I hit start their paths take them around my sky and they basically have straight up and down paths notice how all Ryan is going straight up instead of up and to the right as it was in the northern hemisphere and then it sets straight down and the Big Dipper and the Southern Cross both rise and set so if you’re standing at the equator you can see the stars of both celestial hemispheres and they always rise or set that’s the only

place on earth that you can do that is somewhere on the equator we’ll talk more about different views of the sky and their motions in the next lecture alright so back to our horizon diagram view what if you’re that person what if you wanted to know what view that person has well here’s what it looks like here’s a person and in our diagram in Figure one they’re actually facing south what if they what if they turned around and they faced north their view would be a would be the diagram that you see on the right so this is a diagram from your book it’s actually in the second tutorial but they’re related this person is standing there the horizon is all around them and if they’re facing north then the North Star would be high above their northern horizon and there’s a path drawn in this diagram and that’s the path of star B and there are actually X’s at locations that are matched up with the positions that are marked with numbers in figure one part of your job in the tutorial would be to matchup which X goes with which number so just to summarize a little bit here for you we have a horizon which extends all the way around you things can rise or set depending on where they’re at on the celestial sphere in a horizon diagram you’re only concerned with things that are above your horizon not things that are below it if they’re below it you can’t see it mainly because you can’t see through the earth a horizon diagram you always have to remember that you are outside looking in and think of yourself if you were that person what directions you’d have to face in order to see particular points on the celestial sphere this takes practice in the diagram in the book we start off with a horizon diagram where North is on the left because the North Star is above the northern horizon and that’s on the left side of this diagram South is opposite so the southern horizon is where I’m circling my cursor at the moment and the light lightly colored dashed lines are into the paper or into the screen that’s the eastern part of the sky the western part is out of the paper or out of the screen with the bold line all right I think that’s enough to get you started on lecture tutorial number one it’s on page one of your book and it’s called position good luck