Have you ever been outside on a clear night, when there was a full moon? If so, you probably noticed that it was incredibly bright, almost ten time brighter than a half moon. How can that be? Shouldn't a half moon be half as bright as a full moon? To find out, you will need:

• a ball, or some other round object
• a lamp or flashlight

The lamp will simulate the Sun, and the ball will simulate the moon. You are going to be the Earth. Lets start with a full moon. Darken the room by turning off the lights, and closing the window shades. Turn on the lamp, and sit with your back to it. Hold the ball out in front of you, so the entire surface of the ball seems well lit. This is how the moon is positioned during a full moon, on the opposite side of the Earth from the Sun. Notice how the light from the Sun (the lamp) is reflecting off the moon (the ball) back to you, making it look very bright.

OK, now lets switch to a half moon, also known as either a first quarter moon or a third quarter moon. To see that, turn in your chair so that the Sun (the lamp) is directly to your left. Again, hold the ball out in front of you. The side of the ball that faces the lamp is still fully lit, but you can only see half of it. The side of the ball that is away from the lamp is dark, and you can see half of that. It should look much like the photograph of the half moon.

Notice that even the lighted part of the ball is not as bright as it was when you simulated the full moon. That is because most of the light is still reflecting back towards the lamp, just as it was before. The difference is that you are not between the lamp and the ball, so that reflected light is not coming towards you.

If you want to compare the actual brightness of the different phases of the moon, do an internet search for "printable eye chart", or make your own. It should have very large letters at the top, and they should get smaller as you go down the page. Print that page, and find a place outside where there are no lights shining on you except for the moon. Notice what phase it is in, and then see how far down the chart you can read, using moonlight for illumination. On a clear night, with a full moon, you should be able to read several of the top lines of letters. A week later, at the half moon, try it again. Be sure that the moon is about the same height in the sky. You will find that it is much harder to see the letters, because there is much less light. You might even try it every night, to see how much it changes from day to day. Does it change the same amount every day? Can you figure out why? Might make a good science fair project.

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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Scale Model of the Solar System

A simple model for seeing how large the solar system really is.

Make your own craters, and compare them with photos of the real thing.

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Lunar Illusion

Exploring a Ponzo illusion.

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.