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When Dragons Eat the Sun

A technology-enhanced astronomy lesson

by Tom O'Haver ( December, 2000

CyberSky is a computer program that shows the stars and planets that will appear in the sky at any time.

1. Launch CyberSky. Select Options/Location, and choose the nearest town and state from the alphabetical pop-up list and click OK. What city and state did you select? (Write your answers on the printed Worksheet).

2. Select Animation/Time Step and set to "minutes", then click the start button. This acts like a time machine, speeding things up so that one hour passes every few seconds. You can see all the stars and planets moving much faster than usual. Why are all the things in the sky moving together?

3. Use the left and right arrow keys on the keyboard to "turn your head" to look in different directions. Turn to the east. What is happening to all the things in the sky?

4. Turn to the west. What is happening to all the things in the sky? Why?

5. Did you notice that the color of the sky changes from blue to black periodically, to indicate night and day. In this program, the stars and planets are drawn so that they are still visible even in the daytime. In real life, only the sun and sometimes the moon are visible during the day. Why can't you see stars and planets during the daytime?

6. Turn to the north. How would you describe the movement of the objects? Is there one star that seems to be standing still? Why? What is the name of this star? (Use the mouse to position the cross-hairs pointer on it to verify that that star is not moving. Then and press the right mouse button. The pop-up menu "About ...." will tell you the name of the star).

In the old days of wooden sailing ships, sailors used to look specifically for Polaris. Why would that star be of greater interest to sailors than, say, Sirius or any other bright star or planet?

We can speed things up even more: select Animation/Time Step and set to "hours"; now each click on the step button advances time by exactly one hour. When you click on the start button, everything zips around pretty fast, and we can clearly see how Polaris stands almost still.

7. Something special happens if we set Animation/Time Step to "solar days", so that each click of the step button advances time by exactly one day. Look at the Local time panel (upper right of the screen) and see that the day changes for each click but that the time remains the same. This is like taking a snapshot of the sky once every 24 hours, after the earth has rotated exactly one revolution. This should make the stars seem to stand still, right? However, when you click on the start button, you can see the stars do continue to move slowly. Why? (The earth also rotates around the sun, so that each day at the same time the stars are at a slightly different position in the sky than the same time the day before).

8. Can you find the moon? It's shown as a white disk. It really zips across the sky when the time step is set to days. How many days does it take the moon to make one cycle through the sky, returning to its original position? (Stop the animation by pressing the stop button or the ESC key, then press the button to advance the moon one day at a time). (The word "month" is derived from "moon").

Note that the moon has "phases" just like in real life. Sometimes it's a crescent moon, sometimes a half moon, sometime a full moon. Why does the moon have phases? (Hint: note that moon is a crescent whenever it is close to the sun in the sky: the closer it gets to the sun, the thinner the crescent; the further the moon gets away from the sun, the fuller it gets). Click here for a cool video loop showing the phases of the moon shown greatly speed up.

Do you notice any planets that have phases? Which ones? Why do only those planets have phases?

9. Select Time/Noon. This sets the time to noon, so you can see how the position of the sun at noon changes throughout the year. Use the left and right arrow keys to turn to the south. The sun is shown as a yellow disk. (When you click on the start button, time passes at a rate of about one month every few seconds, so it should take less than a half a minute to go through an entire year). Follow the path of the noon sun over the four seasons.

10. Now let's really speed things up: select Animation/Time Step and set to "years"; now each click on the step button advances time by exactly one year, so by then the earth has rotated completely around the sun and has come back to its original position, and the stars will seen in their same positions. Try it. When you click on the start button, the stars don't move, but the planets do! Which is the slowest moving planet? It's a real slowpoke. Why?

If you look closely, you might notice that the stars are not completely stationary when the time step is set to "years"; the jump or jerk a little bit You might even say they "leap" (that's a hint). Click on the step button severla time to determine how often this jump occurs. Can you think of an explanation?

11. One of the most dramatic events in the sky happens when the moon gets between the sun and the earth, blocking out the sun in the daytime. This is called a "solar eclipse" and it happens about once a year or so somewhere in the world.

The best eclipses are "total solar eclipses", when the moon completely blocks out the sun and it seems to be nighttime in the middle of the day. Birds stop singing and may go to roost. Daytime flower blossoms begin to close as if for the night. Bees become disoriented and stop flying. The temperature drops. All of Nature seems still and quiet for this brief moment of daytime darkness. It's spooky!

In ancient times, people used to think that a solar eclipse was an evil sign; some thought that a dragon was eating the sun. Once, two unlucky astronomers had their heads cut off because they did not correctly predict a solar eclipse.

The thing about total eclipses is that you have to be in the right place to observe them. Why? Because of the shadow that the moon makes on the earth. Look at the animated picture on the left. It shows the solar eclipse of June 21, 2001. Can you see the shadow of the moon moving across the earth? (Actually, it's the earth's daily rotation that is causing the movement; the moon itself moves much slower than the earth's rotation). The little dark spot is the area of totality, where the sun is completly blocked out. Notice how small it is (compared to the size of the earth). It's only about 200- 300 kilometers across. To see a total eclipse, you have to be in the path of that spot. It's not enough to be in the right continent or even the right country - you'd have to be in the right city! Click here to bring up a close-up map of the path of totality. You can see that the path of totality passes through Angola, Zambia, and Mozambique. From the information given on this map, can you estimate the speed with which the spot moves? Could you keep up with it in a speeding car or ATV (all-terrain vehicle)?

The next total solar eclipse that will be visible in the United States will occur on August 21, 2017. (How old will you be on that date?) To see how this eclipse will look, click on the Local time panel (upper right), type in the date August 21, 2017 (8/21/2017), and click OK. Now use the and buttons to play the eclipse forwards and backwards. Use the buttons (or the PgUp and PgDn keys on the numeric keypad) to zoom in and out to get a better view. Click on the button to center the sun on the screen to make it easier to watch. (Click to unlock the sun and return to a normal display). The eclipse is not total because you are not in the right location.

Now suppose you drive down to Orlando, Florida, to see the eclipse. (By 2017 you'll be old enough to have a driver's license! While you're at it, you can visit Disneyworld). Select Options/Location, and choose Orlando, FL from the alphabetical pop-up list and click OK. Press Play again. Is the eclipse more or less total from Orlando? Hmmm, try some other locations further north and south from you home location and see if you can narrow down the location of the total eclipse. What is the best place you can find to view the total eclipse? If you have ever driven with your family along the eastern seaboard towards Florida, perhaps you have stopped for the night at a motel close to the path of totality. (Give up? Click here for a map).


  1. The CyberSky Home Page (
  2. KidsEclipse (
  3. Solar Eclipse: stories form the path of totality (
  4. Fred Espenak's Eclipse Home Page (
  5. Future solar eclipses (
  6. Solar Eclipse Information (
  7. This Eclipse is History (
  8. Dan's Astronomy Software Collection (
  9. Astronomy with a Stick (