December: Lunar Eclipses and Blue Moons

December 2010

Lunar Eclipses and Blue Moons

By Calvin L. Chrisman

December brings us the first total lunar eclipse visible in North America since 2008. This one will be late at night (or early in the morning depending on your point of view) with totality beginning at 2:41 a.m. on Tuesday December 21st. It will be visible from beginning to end, weather permitting. The next total eclipse visible here will not occur until 2014, so you may want to get up to check it out.

Eclipses occur when the object that is eclipsed (in this case the Moon) falls in the shadow of another object (in this case, the Earth). Many people do not realize it, but the Earth actually casts two shadows. It has a lighter shadow called the penumbra consisting of light that is only partially blocked by the Earth; and a darker shadow called the umbra which is completely blocked from the Sun. Think of them as concentric circles with the penumbra being the larger outer circle and the umbra being the smaller inner circle. As the Moon passes through the Earth’s shadow, it first enters the penumbra and then the umbra. The penumbral shadow will first touch the Moon at 12:55 a.m. Eastern time on Tuesday morning the 21st. Its effect is a subtle dimming and is easily missed. The umbra will first touch the Moon at 1:33 a.m. and will totally eclipse the moon at 2:41. Totality will last for 72 minutes and then the process will reverse itself ending at 5:01 a.m.

If you have never watched a lunar eclipse, you may be surprised to see that, during totality, the Moon will have a reddish hue instead of being black or dark grey. It is quite striking and is an effect of Earth’s atmosphere. Light that passes through the Earth’s atmosphere is bent as though it had passed through a lens and is deflected into the umbral shadow. The light is bent toward the red end of the spectrum and so gives the umbra and the Moon a reddish tint. If you are observing the eclipse through a telescope or binoculars, you may want to try and time the first appearance of the penumbra and umbra on the Moon. It is also interesting to watch the effect as the umbra touches various mountains and craters on the surface of the Moon. Finally, watch the sky for dim stars that were obscured by the light of the Moon. They will be quite visible during totality.

A final interesting note on the Moon during this season: By the earliest definition of the term “Blue Moon”, the full moon of last month was a Blue Moon. Current usage of the term refers to a blue moon as the second full moon in a month, but this did not happen in November. This definition came into common usage in the 1940’s but was a misinterpretation of the actual rule. A blue moon was originally called such if there were four moons in a calendar quarter. The monks in charge of the calendar when the lunar cycle still determined dates were quite frazzled in the rare years when there were 13 full moons instead of 12. It was just not tidy and 13 was an unlucky number. Moons were referred to by season as “early” “mid” and “late” as in early summer moon, etc. What were they to do with a fourth moon? The solution was to keep the early, mid, late nomenclature and call the third of four moons a blue moon.

By this definition, the full moon last month was the third of four during the fall and therefore a blue moon. Now someone is going to raise their hand and say that the eclipsed moon of December is on the 21st and is therefore the “early moon” of the winter season. That’s close, but incorrect because the winter solstice (marking the beginning of winter) does not occur until 6:38 p.m. on the 21st which is about 15 hours after the moon becomes full at 3:13 a.m. This makes the eclipsed moon the “late moon” of fall. See if you can explain this to your friends while you are watching the eclipse.

Clear Skies!

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