ECLIPSES AND SHADOWS

Background Information:

Lunar eclipses occur whenever the full moon passes into the shadow of the earth. Solar eclipses occur when the new moon slips in front of the sun and blocks it from view. The moon's shadow then falls upon the earth.

Voyagers refers to a lunar eclipse which Columbus observed while marooned on the island of Jamaica at Santa Gloria Bay (today's Saint Ann's Bay). The eclipse occurred on Leap Day, February 29, 1504. Ten years prior, on September 15, 1494, Columbus had seen another lunar eclipse from the Dominican Republic. Columbus probably consulted an astronomical almanac such as the Calendarium of Regiomontanus to obtain full details of both eclipses.

Over 2000 years before Columbus' voyages, the ancient Chinese and Babylonians had discovered a cycle that could be used to predict both lunar and solar eclipses. According to this cycle, called the Saros, identical eclipses repeat over a period of 18 years and 11 days.

Activity:

1.) To create a lunar eclipse in your classroom, illuminate an earth globe with an overhead projector, as show in Figure 1. A smaller sphere, like a styrofoam ball, can represent the moon. The shadows of both objects are easily observed against the screen. As the styrofoam ball is passed through the globe's shadow, a lunar eclipse will be produced.

Lunar Eclipse Model

A solar eclipse can also be created by positioning the styrofoam ball in front of the globe. The styrofoam ball's smaller shadow will appear on the earth globe. Anyone living where the shadow falls on earth would witness a solar eclipse.

2.) As mentioned in Voyagers, Aristotle proved the earth was round by noting the curved shadow that it cast upon the eclipsed moon. He pointed out that this shadow was round no matter where the eclipse took place, whether the moon was high in the sky or low near the horizon. The only object that can cast a round shadow from every angle is a sphere.

To prove this, have students place objects of different shapes into the light beam of an overhead or slide projector. When the objects are rotated, the shape of the shadows will change. For example, the shadow of a disk (like a coin) will be circular when illuminated face-on, but will appear as a straight line when viewed edge-on. In between these two positions, the disk will cast shadows in the shape of an ellipse. The only object that will cast a round shadow from every angle is a sphere.

3.) Figure 2 depicts the lunar eclipse which Columbus observed from Jamaica. This picture appeared in Washington Irving's Life and Voyages of Christopher Columbus (1892). It is ironical to note that while Aristotle used lunar eclipses to prove the earth is round, it was this very book that created the myth that people in 1492 believed the world was flat and that Columbus was doomed to fall off its edge.

Columbus Lunar Eclipse

Further Reading:

Olson, Donald, "Columbus and an Eclipse of the Moon," Sky & Telescope, October, pp. 437-440, 1992.

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