Lunar Eclipses

Wednesday morning, October 8, 2014, from about 3:15 a.m. to 6:30 a.m. a total eclipse of the moon occurred. At this point, you either saw it or you didn’t. Even then, seeing an eclipse and understanding what is happening are not necessarily the same thing.

A lunar eclipse occurs when a full moon passes through some portion of the shadow of the Earth. For that to occur, the sun, Earth and moon must be in perfect alignment. That is not as easy as it might seem as the moon is offset from the plane of the Earth’s orbit by about five degrees. It spends most of its time either above or below the Earth’s plane. The moon only passes through some portion of the Earth’s shadow 2-4 times a year.

The Earth actually has two shadows, one nested inside the other.  The umbral or complete shadow which is roughly the diameter of the Earth blocks all direct light from the sun.  The penumbral shadow, which sits like a slice of pie on either side of the umbral shadow, only partially blocks the light from the sun.

A penumbral eclipse is very subtle. Only an astronomy aficionado is likely to notice one. Partial and total lunar eclipses are much more interesting and observable because the moon passes through the umbral (darker) shadow and the effect is very obvious.

A total eclipse such as occurred on Wednesday comes on like a tide. It builds almost imperceptibly at first, as the moon first enters the penumbral shadow. Then it slowly edges up the beach into the umbral shadow. At this point, a portion of the moon may go dark. But as the moon fully enters the umbral shadow, it doesn’t disappear, but seems to turn rusty red. The moon stays within the umbral shadow for about an hour. Then the tide eases back out and the process reverses, taking a little more than three hours from start to finish.

 The moon of a full lunar eclipse is called the Blood Moon because of its red color. Through the centuries it has been credited with many superstitions. However, the origin of the red color is nothing more than the sun peeking around the edges of the Earth and indirectly illuminating the moon. The sunlight passes through the Earth’s atmosphere where most of the blue light is filtered out, just like at sunrise and sunset. What is left is light in the red to orange spectrum.

Usually the three types of lunar eclipses come in no particular order. However, Wednesday’s show was the second of what astronomers call a tetrad: four consecutive total lunar eclipses occurring within a one year period. It started in April 2014 and the final two will occur on April 4 and September 27, 2015. So, if you missed Wednesday’s show, you still have two more chances to catch an encore.

I find it fascinating that colossal hunks of mineral hurling and spinning through space can be so precisely predictable. It is comforting to know that despite all the surface chaos created by human beings, the greater workings of our Universe never miss a single step.