This week's experiment comes from Sheldon Schafer, Director of Science Programs and Facilities at the Lakeview Museum of Arts & Sciences in Peoria, Illinois. (http://www.lakeview-museum.org) He told me that much of the USA would have a partial solar eclipse on Christmas Day and suggested this experiment as a safe and easy way to follow the eclipse. If you miss the eclipse or do not live in an area where it will be seen, there are other ways you can see how this works. I will need those, as the eclipse will not be visible from where I am. You will need:

- a solar eclipse
- a piece of aluminum foil
- a pen or pencil
- a colander or a piece of pegboard
- a smooth, flat piece of paper

When viewing an eclipse, DO NOT LOOK AT THE SUN! There is a reason for this. First, you should never look directly at the sun, as it can damage your retina. During an eclipse, the visible light is much less, but there are still plenty of harmful rays. Since the visible light is less, the pupil in your eye stays open wider, letting in more of the harmful rays, making eye damage much more likely. The same goes for filters. They may filter out the visible sunlight, giving the illusion that they are protecting your eyes, when in fact they may be letting most of the harmful, invisible rays enter the eye.

To do this safely, you want to look at an image of the sun, instead of the sun itself. We will use a pinhole, similar to the pinhole we have used in past experiments (camera obscura, paper glasses). Cut a piece of foil about 6 inches square. Use the point of the pen to make a small hole in the center of the foil. Hold this about a foot above your piece of paper, with the sun (or a lamp) shining through the hole onto the paper. This will form a circle of light on the paper. Now, you may be thinking that the round circle is due to the round hole we made. To investigate that, watch the circle of light. Hold your finger about a foot above the foil and move it around. Looking at the circle of light, you will see the image of your finger moving over it. That is similar to what will happen during the eclipse, with the moon taking the place of your finger.

To see this with many images, hold the colander about a foot above the paper, letting the sunlight shine through the holes. On the paper, you will see lots of tiny circles of light. Now you might think that the dots were round due to the round shape of the holes. Many colanders have square holes, but you still get round circles of light unless you hold the colander very close to paper. What you are seeing is an image of the sun (or the light bulb in the lamp). As we have seen in past experiments, a small hole can act as a lens, either letting you see more clearly, or focusing an image as with a camera obscura. We are doing the same thing with multiple holes to give multiple images. As the eclipse begins, you will notice something happening to the circles of light. You will see the shadow of the moon slowly eat away into the side of each circle.

I first saw this in a much more amazing way. I was walking in the woods during an eclipse. If you have walked in the woods, you will notice that the light shining through the leaves forms lots of small circles of light on the forest floor. The spaces between the leaves form the "pinholes". That day, the forest floor was covered with crescents of light, letting me safely follow the eclipse while continuing my hike.

This week's experiment uses a computer CD or DVD. If you have ever looked closely at either a music or computer CD, you have probably noticed that they produce rainbows. If you hold the CD with the shiny side up and let light from a lamp reflect off of it, you will see a very nice rainbow of colors. If you are using a regular incandescent bulb, you will see all of the colors of the rainbow: Red, orange, yellow, green, blue, indigo, violet. If you use other kinds of bulbs, you may find some colors missing. To try this, you will need:

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Try this easy way to visualize how gravity differs from planet to planet.

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Use a globe of the Earth to learn about seasons, day and night, and the scale of the solar system.

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This week's experiment comes from my new "How Do You Know That?" show. How do we know how far away the planets are? Did someone take a tape measure and stretch it from the Earth to each planet? Did someone get in their car and drive there, to check the mileage? No, but we still know how far away they are. To see how this is done, we will need:

- a thumb
- 2 eyes
- something to look at

Look at something that is far away. It might be a tree or a building. Be sure it is something that is sitting still, not a car going down the street. Now, close one eye or cover it up with your hand. Stretch your hand out in front of you and stick up your thumb. Move your hand until your thumb covers up the distant object. Now, change eyes. Open the closed eye and close the open one. Look at your thumb and the distant object. It has moved!!!! Did that tree really move? No. Did your thumb move? Not unless you moved it. What changed was the path you were looking along, because your eyes are an inch or two apart.

Holding your thumb in front of the object again, (good exercise isn't it), switch back and forth with your eyes. Right eye, left eye, right eye, left eye, etc. It will seem that your thumb is jumping back and forth. Notice how far it jumps. Now move your thumb close to your nose. Start switching your eyes again. Right eye, left eye, right eye, left eye. How far does your thumb seem to jump now? Much farther. The closer an object is to your eyes, the more it jumps. The farther away it is, the less it jumps. This can be used to tell how far away something is.

But what if something is VERY far away? Then it would move so little, we would not be able to tell. For that we need to move our eyes farther apart. How can we do that? Wouldn't that hurt? No, instead of taking out our eyes and moving them apart (ouch!), we use one eye from two different people. One person looks at the planet in one city, and another person in another city far away looks at the same planet, at exactly the same time. By comparing the planet’s position from the two spots, we have moved our eyes very far apart.

This is fine for measuring the distances to planets in our solar system, but stars are so far away that we have to move our eyes much farther apart. How can we do that? Imagine the Earth going around the sun. During summer, we are on one side of the sun, and in winter we are on the other side. If you compare the position of a star in summer and winter, that moves your eyes a couple of hundred million miles apart. And even that is not far enough for distant stars. If the star is over 200 light years, then we have to use other methods to estimate its distance.