An important celestial event is coming up very shortly and can provide a great opportunity for us and for our children to learn more about the wonders of our solar system.
In the early morning hours of Saturday, Dec. 21, a small part of our moon will pass through the shadow of the Earth. This "partial eclipse of the moon" is not as rare, nor will it be as impressive, as the solar eclipse we witnessed last July, but it's a wondrous event just the same, and it can also serve as a teaching and learning vehicle to help prepare us for some more-spectacular eclipses that will occur during the new year.
To acquaint your children with the mechanics of a lunar eclipse, it may be helpful to have a flashlight on hand, along with several common spheres, such as a marble, a pingpong ball, a tennis ball or a grapefruit, that you can use to represent the Earth and moon. By placing a marble or pingpong ball on a table, and holding a lighted flashlight near the table's surface, you can create a long shadow that extends from the object and narrows like a cone down to a point. (This shadow is called the "umbra," from the Latin word for "shadow," which is the root for our word "umbrella.")
The Earth casts this same type of shadow in space - 24 hours a day - because the Earth is always standing in the path of some of the light from the sun. This circular shadow is widest at the Earth's surface, and when people on the Earth rotate into the shadow, they refer to it as "night."
From there this same shadow stretches some 850,000 miles out into space, but the circle gets smaller and smaller before it narrows down to nothing. In all that immense distance, however, there is only one major object that the shadow can fall upon. Every once in a while, our moon, which is whizzing around the Earth at about 2,280 mph, will cross the path of this shadow, and some or all of the sunlight falling on the moon's surface will be blocked out until the moon emerges from the shadow once again.
At 3 a.m. MST on Dec. 21, the very bottom part of the moon will touch the very top of this shadow, and by 3:33 a.m., almost 10 percent of the moon will be hidden, while the remainder will be bathed in bright sunlight.
By using the flashlight and two of the spherical objects, you can demonstrate why an eclipse can occur only during the time of a full moon. The moon has to be in line with the Earth and the sun in order to enter the Earth's shadow, and so anyone on the "night" side of the earth would see the moon fully lighted until it touches that shadow.
Why don't we have an eclipse during every full moon? Well, first of all, that alignment is tremendously precise. Think of trying to align a grapefruit (the Earth), and a pingpong ball 12 feet away (the moon), and a large house about a mile down the road (the sun). Even more critical than this, however, is the fact that the moon doesn't orbit the Earth on the same plane that the Earth orbits the sun. The moon's orbit is tilted about 5 degrees, and so the moon usually passes above or below the Earth's shadow.
When some ancient Greek thinkers, such as Pythagoras and Aristotle many centuries ago, saw this phenomenon we call a "partial eclipse," they noticed that no matter where in the sky the eclipse occurred, the shape of the shadow on the moon was always curved in the arc of a circle. Through their reasoning and logic alone, therefore, they concluded that the Earth could not be flat and must be a giant sphere.- William F. Russell's books for parents and children include "Classics to Read Aloud to Your Children" and "Classic Myths to Read Aloud." Send your questions and comments to him at Family Learning, 2400 E. Main St., Suite 266, St. Charles, IL 60174-2414.