 Greetings and welcome to the Introduction to Astronomy. In this week's special topic in astronomy, we are going to talk about the celestial sphere and how that can be used in astronomy. So what is the celestial sphere? Well, the celestial sphere is an apparent sphere around Earth which contains the celestial objects. Which that means is that it's a geocentric construction. So Earth is at the center, and we see everything in the sky as attached to this great celestial sphere and moves around it. So it looks like the celestial sphere rotates causing objects to rise in the east and set in the west. Whereas in reality we know of course that it is the Earth doing the rotating, and that is what causes things to rise and set. However, for the purposes of observations from Earth this is very convenient and a great way to be able to locate objects in the sky. So when we look at the celestial sphere we see a number of different points including the zenith, which is the point straight overhead. So the zenith is here, is wherever you are located, if you look straight up that is what you will see. So an object straight overhead is said to be at your zenith. Your horizon is right around the edge and we see the horizon down here in the pink circle here. That is the horizon so for a person standing at this location that would be the edge of what they could see. Anything below that would be invisible to them being blocked by the Earth. So they would not be able to see objects down below the Earth would be blocking them, the Earth would be in their way. Now we also have things like the poles and the equator. These are related to similar objects on Earth. So the celestial pole, let's look at the north celestial pole here is really just the extension of Earth's north pole out to the sky and that is where it intersects the sky. That would be the north celestial pole. It happens to be close to the star Polaris. The celestial equator is the blue line here which would again just be the Earth's equator. Imagine that stretched out to the sky and that would then give us the celestial equator on the sky. Now the celestial equator, much like Earth's equator divides the sphere into two parts, into two hemispheres. The northern hemisphere in the sky and the southern hemisphere in the sky just as the equator on Earth divides Earth into a northern and southern hemisphere. So let's look at a couple of other terms here. First of all let's look at the ecliptic. Now the ecliptic shown here in the pink is the path of the Sun in the sky. Now we know it is the Earth that is actually doing the moving but from our perspective here on Earth it looks like the Sun is moving and we would watch the Sun follow this path over the course of the year and you would see it in March it would be located here and then in June it would be over in this direction in September over here and then in December. So we can see that the Sun will change its path. This will also affect how high the Sun appears in the sky. So during the summer time here the Sun is well above the celestial equator and will be very high in the sky. In the winter time the Sun is far below the celestial equator and will be very low in the sky at noon time. Now the points where it intersects are what we call the equinoxes and we have the spring equinox that occurs here in March and the fall equinox that occurs in September. So as the Sun moves along this path it will appear higher or lower in the sky because of this tilt. Now that tilt is the 23.5 degree tilt of Earth's axis and that is what causes the seasons to occur. Now the last term I want to mention here is the meridian. The meridian is the line going from the north pole through the zenith and straight down over. So it goes straight around the edges here and that would show us where the Sun is at its highest point. So an object would rise in the east would reach its highest point when it reaches the meridian which is due south and then would reach its lowest point as it sets in the west. So the meridian for the Sun when the Sun crosses the meridian that is when it is at its highest point in the sky. So let's go ahead and finish up with our summary and what we've looked at for the celestial equator is that it is an imaginary sphere around Earth that seems to carry the celestial objects. It is really an extension of locations on Earth into the sky and we looked at some of the different locations on the celestial sphere and have discussed those. So that concludes this special topic on the celestial sphere. We'll be back again next week for another special topic in astronomy. So until then, have a great day everyone and I will see you in class.