 Greetings and welcome to the Introduction to Astronomy. In this video we are going to look at the rotating Skylab and the various positionings of the stars that we can use in this. Now we will have, looking at the horizon coordinate system, that is, altitude and azimuth. And we will also look at the celestial coordinate system, which is right ascension and declination. And you'll be able to look at both of those a little bit in these. And then what we're going to want to look at is the rotating Sky Explorer right here. That's the thing we're going to look at for the lab exercises that you will need. So let's go ahead and get started here. And as we open up the rotating Sky Explorer, we will find that it has a number of windows. And, in fact, it has a celestial sphere view, and it has the horizon diagram view. So this is looking down on Earth from space, but seeing the celestial sphere and looking down on Earth, and this one we are on Earth, and looking outward towards the celestial sphere. We can adjust the observer's location, start the animation to allow the sky to run forward. There's a number of different settings here that you may need to use. And you can control the stars. There are different patterns you can put on. You can put random stars on that you can adjust the positions are. And, of course, you can clear off all the stars if you start getting too many there. Now, what you can do with this is, again, to really learn about the positionings of the stars. So if we do one example and take at a star randomly, then a star will appear on the sky. And if you select that star, it will now tell you its coordinates. So in the horizon diagram view, it will give you the azimuth and altitude of it. The altitude, how high it is above the horizon, and the azimuth, how far it is around from the north point of the horizon. And for the same star in the celestial view, you will get the astronomical coordinates here, and again in the boxes here, and that this is 16 hours of right ascension, and negative 9.8 degrees of declination. So if you need to get those numbers, you can just look them up by selecting your active star. Now, the other things we can look at are some of the star trails. So if we actually let this run with star trails and let the animation run, turn on long star trails here and start the animation, then the star will leave a trail behind. You can see that the horizon coordinates are changing throughout the animation. The celestial coordinates do not. Those are fixed to the sky, so the star always remains, keeps those same positions. And remember that the reason all of this is happening is because the Earth is rotating, not because anything is changing in the sky. Now, as we have our full circle there, we can then see, again as we turn this, you can then see how that pattern goes, that it will rise at a certain position. And in this case, our star is rising a little bit to the south of the western, or setting a little bit to the south of the west, and over here rises a little bit to the south of the east. Now, we could add a bunch more stars if we liked. So let's add a number of random stars here. And you can run a whole bunch of star trails, so we could start the animation, let all of these run. It's still showing the coordinates of our originally selected star. You could select one of the other stars if you wanted to, to be able to follow its coordinates. But for this point, we just really want to look at how things are changing as we look at different areas. So let's go ahead and pause the animation. And then as we turn this, we will now see that some of these stars make trails that never cross the horizon. So if we look here very close to the North Pole, there are a number of stars. This one, this one, and this one, and even this one, just making it not quite crossing the horizon. And these are what we call circumpolar stars. So circumpolar stars, and you can actually check the checkbox here to show the circumpolar region on the area. It'll be shaded now. And those are all those stars that would be circumpolar that never actually will rise or set. They are always above the horizon. Now, depending on where you are on the Earth and you can change your location, if you go further and further north, you'll see that more stars are becoming circumpolar. And in fact, if you go all the way up to the pole and at the North Pole here and watch the animation, stars do not rise or set at the North Pole. They all just move along in circles parallel to the horizon. If you take this down to the equator and go down to zero degrees and the animation is still working here, and you'll see now as we turn that that all stars are rising and setting. So from the equator, you could see every star. From the poles, you could see half of the stars, but only for a certain amount of time. And as I said, there were circumpolar stars, but if we go back up here again, we will also look and you can see that there are some stars like this one that will never get up above the horizon. They come very close or not so close. These ones closer to the South Celestial Pole. And therefore, there are some stars that can never be seen from certain locations. So if you are very far north, you can't see things like the Southern Cross. And we can remove all stars and put on the Southern Cross pattern. And if you are at 41 degrees north latitude, you can see the Southern Cross making its pattern there. And it will never get above the horizon. And things like the Big Dipper will always be above the horizon and are always visible. So depending on where you are on Earth, certain stars may or may not be visible. Now, if you travel further south, what you'll note is that things change. Now, the Southern Cross becomes more visible and the Big Dipper becomes less. So if you go down to 42 degrees south, you will see that the Southern Cross is now a circumpolar constellation. It is always above the horizon and that the Big Dipper never rises. So if you go down that far south, such as the southern tip of South America, you would not be able to see the Big Dipper at all but would see other constellations that are not visible here. So you'll be playing with a number of those, being able to add stars, remove stars, look at the star patterns. You may need to look at some of the different sections here to show if they can highlight things for you. And you'll certainly be adjusting the observer's location and looking at both of these charts. So that concludes this video on the rotating sky simulation. We'll be back again next time to look at another simulation. So until then, have a great day, everyone and I will see you in class.