Friday, October 14, 2011

Total Penumbral Eclipses

The last of the three types of lunar eclipses is the penumbral. The moon passes through the earth’s penumbral shadow, resulting in a slight darkening of a portion of the moon. Because this effect is not that impressive at all, it’s hard to detect and mostly are of academic interest. These eclipses are only observed for the purpose of learning (academic reason), but are a great chance for eclipse enthusiasts to witness them. Only total lunar eclipses are worth seeing to most people. But, another interesting thing occurs with penumbral eclipses. A rare subset of penumbral eclipses can be viewed rarely (approximately less than two percent of all eclipses). It is known as a total-penumbral eclipse, or just another penumbral. 

Total Penumbral eclipse of March 3, 1988

If you consider this eclipse to be a part of the others, then there are four types of lunar eclipses. The last one occurred on March 14, 2006, and you won’t see another one until August 29, 2053. The moon is totally immersed in the penumbral cone of the earth, without even touching the umbra. But occasionally, the penumbra is too small to contain the whole moon. So, the moon would have to be at its apogee (farthest point) in order to fit. When you calculate the timings of the moon being at apogee, being at full moon, and being on the ecliptic, you don’t begin to realize how rare this is.

In Total Penumbral Lunar Eclipses written by Jean Meeus, this is how he describes the frequency of total-penumbral eclipses: “the total-penumbral eclipses are unevenly distributed with time; their frequency varies in phase with that of tetrads and that of total eclipses in the umbra.” Taken from the abstract of the document, Meeus says in simpler terms that these eclipses are rare, compared to who often the moon eclipses in the umbra. Tetrads are just a set of four total umbral eclipses within two years. On another note, the color of the whole moon slightly darkens. It is neither as dark nor red like the total lunar eclipse, but you will be able to tell the color has been altered. Though only for theoretical interest, total-penumbral is nothing spectacular. “Unlike umbral eclipses, the ‘total’ type of penumbral eclipse should not be considered to be an important and beautiful phenomenon!” writes Jean Meeus on the viewing of these special eclipses.  They’re nothing special—but rare!

[This is a special interest story--images can be found here.]

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