1. The View from Earth: Introduction to the Celestial Sphere
With the naked eye, we can see more than 2000 stars as well as the Milky Way…at least from Laramie.
Remind students that we often use the term “constellation” to describe a pattern of stars, such as the Big Dipper or the stars that outline Orion. However, technically a constellation is a region of the sky (and the patterns are sometimes called “asterisms”). A useful analogy for students: a constellation is to the sky as a state is to the United States. That is, wherever you point on a map of the U.S. you are in some state, and wherever you point into the sky you are in some constellation.

2. Constellations A constellation is a region of the sky.
Eighty-eight constellations fill the entire sky. We’ll tackle these more in observing and planetarium labs.
Remind students that we often use the term “constellation” to describe a pattern of stars, such as the Big Dipper or the stars that outline Orion. However, technically a constellation is a region of the sky (and the patterns are sometimes called “asterisms”). A useful analogy for students: a constellation is to the sky as a state is to the United States. That is, wherever you point on a map of the U.S. you are in some state, and wherever you point into the sky you are in some constellation.

3. The Celestial Sphere
Stars at different distances all appear to lie on the celestial sphere.
The 88 official constellations cover the celestial sphere.
The illusion of stars all lying at the same distance in the constellations allows us to define the celestial sphere. It doesn’t really exist, but it’s a useful applet for learning about the sky. When discussing this slide, be sure to explain:
North celestial pole
South celestial pole
Celestial equator
Ecliptic
It’s also very useful to bring a model of the celestial sphere to class and show these points/circles on the model.

4. The Celestial Sphere
Ecliptic is the Sun’s apparent path through the celestial sphere.
If you do not have a model of the celestial sphere to bring to class, you might wish to use this slide; you will probably want to skip it if you have a model that you can discuss instead…

5. The Celestial Sphere
North celestial pole is directly above Earth’s North Pole.
South celestial pole is directly above Earth’s South Pole.
Celestial equator is a projection of Earth’s equator onto sky.

6. The Local Sky
An object’s altitude (above horizon) and direction (along horizon) specify its location in your local sky.
Now we move from the celestial sphere to the local sky. The local sky looks like a dome because we see only half the celestial sphere. If we want to locate an object:
It’s useful to have some reference points. Students will probably already understand the horizon and the cardinal directions, but explain the zenith and the meridian; a simple way to define the meridian is as an imaginary half-circle stretching from the horizon due south, through the zenith, to the horizon due north.
Now we can locate any object by specifying its altitude above the horizon and direction along the horizon. A good way to reinforce this idea is to pick an object located in your class room, tell students which way is north, and have them estimate its altitude and direction.

7. We measure the sky using angles.
Point out that in general we have no way of judging true (physical) sizes and distances of objects in the sky -- like the illusion of stars lying on the celestial sphere, this is due to our lack of depth perception in space. Thus, we can measure only angular sizes and distances. Use these diagrams as examples. Optional: You can show how angular sizes depend on distance by having students sitting at different distances from you in the class use their fists to estimate the angular size of a ball you are holding. Students in the back will measure a smaller angular size.

8. Angular Measurements Full circle = 360º 1º = 60 (arcminutes)
1 = 60 (arcseconds)
Use this slide if you want to review the definitions of arc minutes and arc seconds.

9. Thought Question The angular size of your finger at arm’s length is about 1°. How many arcseconds is this?
P. 60 arcseconds
Q. 600 arcseconds
R. 60  60 = 3600 arcseconds
This is a quick test of whether students understand what we mean by arcseconds.

10. Angular Size
An object’s angular size appears smaller if it is farther away.
Use this slide if you want to review the definitions of arc minutes and arc seconds.