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Educator's Guide to Eclipses

Courtesy of the Jet Propulsion Laboratory


Astronomical Alignments

Eclipses have long been a source of mystery and spectacle. These events were viewed with fear and dread in the past and, even today, still thrill.

There is a lot of special vocabulary involved in eclipses but there is a way to keep from being confused. The eclipse is named for the object that is being eclipsed, or obscured. In a solar eclipse you observe the Sun (using only safe methods, of course). You will see the Sun with a piece apparently cut out of it. In a lunar eclipse you observe the Moon. A portion of its surface will be obscured.

Another way to avoid confusion is to consider the time at which you will be viewing the eclipse. Because of the geometry described below, you can only view a solar eclipse when the Sun is up, and the Moon is nowhere to be seen. You view lunar eclipses when the Moon is up.

Eclipses occur when the Sun, Earth and Moon line up. They are rare because the Moon usually passes above or below the imaginary line connecting Earth and the Sun. In a solar eclipse the Moon passes directly in front of the Sun. This can only happen when the phase of the Moon is "new." That occurs because, for Earth-based observers, the far side of the Moon is illuminated while the side facing Earth is in darkness. The Moon, like any sphere, casts a shadow. A solar eclipse occurs when that shadow sweeps across Earth. The black cone is called the umbra, as in umbrella. An observer anywhere in that region is completely in shade. None of the Sun is visible from there.

Surrounding the umbra is the penumbra. An observer there will see some, but not all, of the Sun. Outside of these regions, all of the Sun is visible. Note that the tip of the umbra barely touches Earth. At the current time the position of the Moon relative to the Sun is such that the Moon, which is 400 times smaller that the Sun, is 400 times closer! This means that the two objects appear to be the same size in the sky. Only observers at the tip of the umbral cone will see a total solar eclipse. A large number of observers across the globe will see a partial solar eclipse if they are in the penumbra.

An annular eclipse is a special partial solar eclipse. Because the Moon's orbit around Earth is an ellipse, not a circle, the Moon's distance from Earth varies. When the Moon is far from Earth it appears slightly smaller in the sky. (Earth's orbit around the Sun is also an ellipse, and during January, Earth is at its closest point to the Sun. The Sun's size is slightly larger than during the rest of the year.) With a "small" Moon and a "large" Sun the Moon will not completely block out the Sun. The umbra does not touch Earth. An observer would have to be above the surface of Earth to see a total eclipse. For individuals in just the right location, the Sun appears as a ring (annulus) around the silhouetted Moon.

In a lunar eclipse the Moon moves into Earth's shadow. They can only occur when the moon is "full." Observers on the night side of Earth see the Moon take on a reddish hue as it moves into Earth's umbra. If the entire disk of the Moon falls into the umbra it is total lunar eclipse. If only a portion does, then it is a partial lunar eclipse. Penumbral lunar eclipses are very difficult to detect because the Moon dims only slightly while moving through that region. Lunar eclipses are more common than solar eclipses. Total eclipses of the Sun and Moon are partial before and after totality.

Popular astronomy magazines, available on many news stands, always give timely eclipse details.


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Views of the Solar System Copyright © 1997 by Calvin J. Hamilton. All rights reserved.