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This weekend is one of those four special times during the year that our ancient ancestors celebrated as a holiday even before there were calendars. What they were celebrating was, perhaps, the best optical illusion of all time - one that continues to play tricks on our eyes unless we learn to know what is really going on.

The sun appears to move along a circular arc in our sky. Each day that arc changes slightly so that the sun's path in the winter is very low, while in the summer the arc rises until it passes, at noontime, almost over our heads. Well, on Friday, this arc will be as high as it will ever be during the entire year, because Friday marks a time known as the summer solstice. Solstice is a combination of the Latin words "sol," meaning "sun" and "sistere," meaning "to stand still." Early humans knew that on this day the sun would stop climbing higher and higher in the sky, and, having reached its highest point, would "stand still" before it steadily carved out lower and lower arcs in the sky.The illusion, of course, is that the sun isn't really doing any moving at all. It is the movements of the Earth alone that account for all the changes we see in the position of the sun and the other stars. Our language even supports the illusion when we speak about the sun "rising" in the morning and "setting" at night. In fact, the sun "stands still" every day of the year, but lest we be too quick to laugh at our ancestors' ignorance, think for a minute about how things would appear if the sun actually did move around the Earth. Exactly the same.

The point here is that children will be fooled by this wonderful illusion unless we use every opportunity - especially days like the solstices and equinoxes - to give them the knowledge that will lead them to a different interpretation of what they see.

If you have a basketball-size globe of the Earth in your home or at your local library, you can show your children the pattern of light that exists during the day of the solstice. Position the globe so that its axis is tipped toward you. Now hold the flashlight near enough to the globe so that the entire Arctic Circle is bathed in light, but so that no light at all falls on the Antarctic Circle. When you have achieved this pattern, you will find that your flashlight, just like the sun itself on the summer solstice, is directly above the Tropic of Cancer, 23 1/2 degrees north of the equator.

So, if you lived anywhere along the Tropic of Cancer, the sun would be directly above your head at noon on the day of the summer solstice. Turn the globe and see how many countries you can name that lie along the Tropic of Cancer. The people who live in these countries, if they ventured out into the noonday sun, would cast no shadow at all.

Have your child hold a short pencil straight up from any of these countries as you shine the flashlight from above. No shadow. Now move the pencil to the area where you live while holding the flashlight still. Is there a shadow? Yes, an erect object, such as a person or a telephone pole or a street sign, will cast a shadow at noon every day of the year in the latitudes where we live, a fact that you can confirm with your child by picking out such an object and then marking and dating the length of its noontime shadow.

And now is a great time to begin this marking because that shadow will be getting longer and longer every day; it will never be shorter than it is on the summer solstice.