 Greetings and welcome to the Introduction to Astronomy. In this section we will talk about the celestial sphere, and that is really what you see when you go out at night or during the day and look up at the sky. It looks like the universe is a great sphere surrounding the Earth, and that is what ancient astronomers would have believed. And when we look out at it, it still looks like everything is stuck on this great celestial sphere because we don't see the three-dimensional nature of the universe. You can't tell when you look at various stars whether they are close or far away. There are measurements that astronomers can do to tell that, but in reality what everything looks like is like it's hanging on this great celestial sphere that we will talk about today because we still use that for determining locations of objects on the sky. So what is the celestial sphere? Celestial sphere is a way of visualizing the sky. So it is a way that we're able to see what there is in the sky, and it is a geocentric model. What that means is that we are looking at it from an Earth-centered point of view. So that is Earth-centered here. That we are looking at it is essentially we consider it Earth-centered. Now we know that's not correct, but it still works for determining positions in the sky because it acts as though we are at the center of this great sphere. And all of the stars, all of the planets, all of the other objects that we see, things like galaxies, all appear to be positioned at various points on the celestial sphere. So how does this celestial sphere look? Well, let's take a look at it from the outside, and we see, for example, here, some of the various terminology that I will go over that includes the different parts of the celestial sphere. And I will define these later. But here is the Earth. Here is the celestial sphere surrounding it. So it's an imaginary sphere, and we see all of the different objects as located on that sphere. So any objects you go and look at in the sky, you can then use coordinates to locate its positioning in the sky. And astronomers use two sets of coordinates. They use the declination, which tells how far you are north or south of the equator. So here is the equator on the sky, and how far you are north or south is measured up and down on that scale. And you get up to 90 degrees would be up at the pole. Zero degrees would be the equator. They also use right ascension. That is a way of measuring the longitude on the sky, how far you are around. Now, on Earth, we measure longitude in degrees. On the sky, we measure it in hours. So it is 24 hours around the entire sky, so the sky is divided up. And you can then measure the various positions of where an object is by how far it is north or south of the celestial equator. That's its declination. And how far it is east or west of a point we call the vernal equinox, which is the reference point for the right ascension. And that can give us an exact location of any object in the sky, and that would allow the astronomer to then point their telescope in the direction that they need to to observe any object. So let's look at some of these definitions here. Some of the different terms to look at. We have the zenith. The zenith is the point straight overhead. That's exactly overhead for you wherever you are on the Earth. So if you lay down on the ground and look straight up, you are looking up at the zenith. There may be an object there, or there may not be. There's not requirement to be any object there, and that will vary depending on where you are on the Earth. So for example, if you have a friend across the country or across the world, and you lay out in your backyard looking straight up, and you see an object straight overhead, and you call them and tell them that there's this object straight overhead, they're not going to see it. And that's because what is at the zenith for you will be at a different position for them. So this very much depends on the location of the observer. Now the coordinates that we talked about on the last slide do not. Those are exactly the same for everybody. Now some of the other ones that are local to the observer is the horizon. If something is on your horizon, then it is on your horizon. It may not be just rising or setting, crossing that border between the Earth and the sky. And it may not do that for somebody else at a different location. We also have the celestial poles. There are two of these, the North and South celestial pole, which are just the projections of the Earth's poles onto the celestial sphere. So there is one in the North and one in the South. And we saw those on the previous slide. We saw the North celestial pole right here. So North celestial pole way up here, and the South celestial pole way down here. Those are just the projections of the Earth's pole onto the sky. So if you imagine the North pole going straight up, it will intersect the celestial sphere there, and the South pole going straight down will intersect the celestial sphere at the lower location. The celestial equator is the projection of the Earth's equator onto the celestial sphere. So just as we did with the poles, we can go back and look at our previous slide. And the equator, you can imagine if the equator were a big stretched band, and you could stretch it outward, there's an equator here on Earth, and if you could stretch that out to the sky, it would fill the celestial equator, this blue line here. So that would just be the Earth's equator stretched out to the sky. The ecliptic is the apparent path of the Sun. Now the Sun, we know, does not move. It's actually the Earth that is doing the moving. But because of our reference point on the Earth, we see the Sun as apparently moving, and it moves across the sky on a path called the ecliptic. And one of the positions that it reaches is what we call the vernal equinox. The vernal equinox is where the Sun is on the first day of spring. So the Sun is there on the first day of spring, and that's when it crosses the celestial equator heading North. It will cross it again in the fall as it heads South. So let's look at those two on the chart here. The ecliptic is the red line here, and that is the ecliptic. That's the apparent path of the Sun. Notice how it's tilted relative to the celestial equator. That is because of the tilt of the Earth's axis. And the vernal equinox is just that point. It's a point where the ecliptic and the celestial equator meet. This is the vernal equinox. On the other side, way over here, you'd have the autumnal equinox, and that would be the first day of fall. So when the Sun goes there, that would be the first day of fall when the Sun crosses the celestial equator, but this time heading South instead of heading North. So those are some definitions of the different terms that are related to the celestial sphere. So what are some other things that we see here? Well, we have, when we look at the way things move on the celestial sphere, is we have circumpolar stars. These are stars that never appear to set. A common example of this would be Polaris. Polaris is the star near the North celestial pole, and it's actually right there, the bright object there. Polaris is not the brightest star in the sky by far. It's about the 50th brightest star. But it is prominent because of where it is in the sky that it happens to be located, not at the celestial pole. You can see it's probably a little bit away, celestial pole being right in the center here. But it's very close to it, and it highlights what would other be an invisible object. So what we see is that stars will revolve in circles around the celestial pole. So each of these we're seeing just a part of an arc here, but they will make an entire circle over time. If you could leave a camera lens open for an entire day and it never got light out, then you would be able to see these make a complete circle around the pole. Now, depending on the location of the pole in the sky, some of these circles will be completely visible. So some of the ones that are closer will actually make complete circles, and that means that stars at these locations, like Polaris, will be circumpolar. That means they are always visible, and they never drop below the horizon. So some examples of this would be things like the Big Dipper, maybe one that you're aware of if you're in the northern hemisphere. The Big Dipper is a set of seven stars, and if you're far enough north, then those never dip below the horizon. You can always find the Big Dipper. If you get further away, then those stars will cross the horizon, and that means that they will rise and set, and they are no longer circumpolar stars. It is only those stars that are close enough to the pole that they never actually rise or set. Now the location of the pole has been very important in history for navigation. Navigation, studying the stars was very important for doing things like navigating, and when we look here, what we find out is that how high the pole star is, its altitude, how high it is above the horizon, tells you your latitude. Where are you north or south of the equator? So as an example, if you are at a latitude of 40 degrees north, then the pole will be 40 degrees above the northern horizon. Because Polaris is a prominent star, you're then able to locate the north pole, and sailors back hundreds of years ago could use that as a way to determine where they were north or south. Were they heading too far north or too far south? They could use that, measuring where the pole star was. So if the pole star was getting higher in the sky, they were heading further north. If it was getting lower in the sky, they were heading further south. So in the days before GPS and more accurate navigation, it gave a very good way to be able to determine where you are when you were at sea with no other landmarks. Now the pole is at 40 degrees for the horizon. It will be 40 degrees for an latitude of 40 degrees. It will be on the horizon if someone is at the equator. It will be at the zenith straight overhead if you're at the north or the south pole. So what that also means is that you cannot see certain objects. The north celestial pole, this does not work when you are south of the equator because the north celestial pole is not visible. There is a south celestial pole and all of this applies to it just as well. The problem is there's no bright star near the south celestial pole, so nothing that stands out to be able to make the south celestial pole easily visible or to help with navigation in that way. So let's look at these, some of the motions that we get on the celestial sphere. We get a daily motion which is rising in setting which is caused by the rotation of the earth. So the celestial sphere seems to orbit around the earth once a day, turn it spinning. It really is not, that's really the earth spinning from west to east and make the celestial sphere objects on it appear to rise in the east and set in the west. There is also an annual motion. The annual motion is caused by the revolution of the earth around the sun. And what that means is that over the course of the year, every day the sun moves one degree relative to the stars. So while the stars and their patterns remain fixed, the sun does not. The sun and the planets and any other objects will also move slowly through the sky. But things like stars and galaxies which are at much greater distances actually will stay at the same spot. So the pattern of the constellations that we see will be the same. But where the sun is located will be different. And that is because when we are here at this location, we then see the sun against these constellations. Three months later, in December, we now see the sun against these constellations. Here we'll see it against this constellation and finally against these constellations. That means that the constellations that we see at night will be different. And you may recognize many of these constellations. These are what we call the constellations of the zodiac. So the zodiac constellations of the zodiac are important because they are the ones that the sun passes through. And planets and other objects that happen to be near the path of the sun, near the ecliptic, will also pass through those constellations. They are not necessarily the most prominent constellations. In fact, many of them do not have really bright stars at all. Things like Libra and Cancer and Aries, et cetera, a few others do not have any really bright stars. But they are prominent still because they are one of those 12 constellations that the sun will pass through over the course of a year. So what else do we know? There are also seven objects that were known that moved among the stars. So another motion that we have is that the sun gave us the day and the year. So we have the sun here giving us the day and the year that we've already talked about. The motion of the sun over the, which is the rotation of the earth, but the motion of the sun, because of that rotation, gives us the day and the year. The motion of the moon gives us the month of about 30 days. And the week, where does the week come from? Well, the week is because of the seven objects that moved among the stars. So we have the day of the sun, the day of the moon, and the day for each of the five planets that were known to the ancient astronomers, Mercury, Venus, Mars, Jupiter, and Saturn. So these were the objects that were known. So how did we end up with a week of being seven days? Well, that's because there were seven objects that wandered through the sky, five planets, the moon and the sun. These are the ones that did not stay fixed relative to the rest of the stars. The planets, the name comes from wanderers, so they are wanderers, they wandered among the stars and moved through the stars. So in the image here, you can see that the planet here is moving through Capricorn and then moves into Aquarius. It makes a little loop and then it comes back and heads into Pisces. This is what we call retrograde motion, this little loop. This is something that is distinct to the planets only. So only planets will undergo retrograde motion. The sun and the moon do not do this. They move through the sky just like this. They go through those constellations, but they just make a completely straight path. The planets will at specific times form this little loop, which was very important for our understanding of the motions of the planets. Now the other thing that we see there are the constellations. Constellations is something that our groupings of stars and the definition of them has changed. Long ago, there were simply groupings of bright stars. Not every part of the sky was part of a constellation. There may be no real bright stars there. Nothing else of importance, so it was not part of a constellation. They were named in honor of mythological figures, so you may recognize some of these if you've taken a class on mythology when you talk about things like Perseus and Hercules and Cassiopeia and many others, Andromeda. You may actually recognize many of those from mythology. So they were areas of relatively bright stars that were named from mythological figures, but areas with fainter stars or very few stars visible were not part of a constellation. The modern definition done not quite a hundred years ago now is that there is a very specific boundary to each of the constellations and the sky is divided into 88 regions. And no matter where you look on the sky, every single part of that sky is one of those 88 constellations. So if you point out to the sky, you are pointing to one constellation or another. So finishing up here, we looked at briefly at the celestial sphere and some of the different terminology that is used and how that is used as a way of visualizing the sky. We said that the appearance of the sky will depend on the observer's location, so what you see will change depending on where you are on the earth, where the north pole will be will change. And since that's changing, that's changing the entire celestial sphere. So there are stars that are visible when you go south at the equator that you can never see from the north. And similarly, someone in Australia can never see Polaris the north star unless they travel north of the equator. The planets and the stars move differently on the celestial sphere. The stars move as a bigger group all together and the patterns that we see within them, the constellations stay the same. The planets will wander through those groupings of stars and wander through the constellations over the course of a year. And then finally, we talked about constellations and how that definition has changed in the modern definition, being specific regions of the sky versus the ancient definition where it was just that grouping of bright stars. So that concludes our discussion here on the celestial sphere. So until next time, have a great day everyone and I will see you in class.