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Penumbral Eclipses of the Moon

The Shadow of the Earth

Like any opaque object illuminated by the sun, the earth casts a shadow behind itself, as you can see in the diagram below.

earthUmbra picture

The shadow of the earth consists of two parts, an area of deep shadow called the umbra, and a much brighter area of partial shadow, called the penumbra.

Lunar Eclipses

Eclipses of the moon occur when the moon in its orbit enters the shadow of the earth. The kind of eclipse that results depends upon what area of the earth's shadow the moon passes through.

Types of Lunar Eclipses

When the moon grazes the umbra, we have a partial lunar eclipse. If the moon becomes immersed in the umbra, the eclipse is total. In a penumbral eclipse, the moon passes through the penumbra only and never contacts the umbra.

Penumbral Eclipses

Because the penumbra of the earth is only partially shadowed, it still receives sunlight. As a consequence, the moon continues to receive solar illumination inside the earth's penumbra. In a penumbral eclipse the decrease in the brightness of the moon is not easily noticeable.

A penumbral eclipse is usually about the least impressive eclipse that you can imagine. Penumbral eclipses of the moon usually pass unnoticed by the general public and unreported by the news media.


Copyright © 1998 - 2004 by Arnold V. Lesikar,
Professor Emeritus
Dept. of Physics, Astronomy, and Engineering Science,
St. Cloud State University, St. Cloud, MN 56301-4498

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