This lab uses a different piece of software than we have been using. The software here is SkyGlobe. I have version 2.04 on several computers around campus. Version 3.6 and version 4.0 can be downloaded. Use whichever version you would like. The screen shots and movies shown are from version 2.04. Your screen may look different. All commands are version 2.04 specific also. The keystrokes may be slightly different on the newer version.

Precession was discovered by Hipparchus about 2000 years ago. It is the change in the pole position on the celestial sphere due to an alteration in the direction the Earth's axis points. Please see the discussion on precession on my website.

In order to view the position of the North Celestial Pole (NCP), we must first be facing North. To do this, click the N in the lower portion of the screen or press the "N" key on your keyboard. You should now have a view to the north. Notice the lines of right ascension flair out from the NCP like spokes on the wheel of a bicycle. To clean up the area of sky, turn off the horizon by clicking the horizon button () on the right hand tool bar. In order to view more of the sky zoom out to about Z:1.5. You can zoom out by right clicking the zoom factor (see the Z:2.5 toward the top of the screen) or by pressing shift-z. Do this until you reach a 'Z' factor of 1.5. If you right click, pause between clicks or SkyGlobe will proceed to the lowest zoom level automatically. Once you have a zoom factor of 1.5 you should print your view by choosing File/Print from the menu bar. You will use this printed copy to mark your changing NCP. The screen should be similar to figure 1 below.

Figure 1: A typical SkyGlobe view to the North

In order to advance time in sufficient increments, SkyGlobe allows you to move in 100 year steps. You can do this by clicking the J button on the left hand toolbar or pressing the "J" key on your keyboard. Again, if you click the J button too quickly, SkyGlobe will begin to advance in 100 year steps automatically. Right clicking or shift-J will reverse the 100 year steps. Stepping in 100 year increments, watch the position of the NCP change and record the new NCP on your printed sheet with a penciled 'x' every 1000 years. (Use a pencil so you can erase mistakes.) It will be very helpful to get your bearings if you rotate your sheet of paper with the rotating view on your screen. Continue advancing until the NCP returns to about the original position at Polaris.

The movie here shows the process above. The mouse clicks are used so that you may see the button locations. Some clicks are right clicks. The movie advances only 1000 years (10 centuries).

precession movie (avi format)
precession movie (QuickTime format)
precession movie (Real Player format)

You may need to rerun the steps above in order to answer the questions below.

1. What kind of path was made by the 'x's?

2. With what year did you begin?

3. At what year did you stop?

4. Subtract the starting date from the ending date. How many years did it take (this is the period for precession)?

5. Which constellations fall on or very close to this path?

6. At approximately what date will Vega be closest to the path?

7. What are some other bright stars that are on or close to the path? You will need to zoom in to see the names of the stars labeled. What are the dates for these stars? You should be able to identify at least three.

8. How does precession effect the celestial coordinates (Right Ascension and Declination) of objects in the night sky?

One interesting thing about precession is that we can use the celestial coordinates from the path to calculate the angle at which the Earth's axis is tilted.

Procedure: Divide the period (see question 4 above) by two. Add this to the starting date. Move to that date in SkyGlobe by using the "J" key on the keyboard (if you are still at the end date use shift at the same time as you use the "J" key). For this date, find the declination of Polaris. Subtracting this angle from 90 degrees (the declination of the NCP) gives the angular separation from the NCP today to the NCP at that future date. This should be twice the angle of the Earth's tilt.

9. What is your answer and how does it compare to the actual value?

10. What can you conclude from this laboratory?

You will need to mail or drop off the printout with your NCP markings. This is in addition to answering the questions above.

This page was last updated on 06/13/01.