Sky Talk December 2010: Total Eclipse of the Moon

On the early morning of December 21st (or beginning late evening the night before on the West Coast), skywatchers throughout North America will be treated to one of nature’s grandest spectacles — a total eclipse of the Moon! It’s been nearly three years since the last total one, so this event is being widely anticipated.

Finally, another total lunar eclipse to enjoy! This cosmic spectacular begins with the full Moon entering the Earth’s dark inner shadow (the “umbra”) at 1:32 a.m. EST on December 21st and continuing to become ever-more fully immersed in it for the next 68 minutes. Totality itself will occur at 2:40 a.m. and last well over an hour, after which the Moon begins to slowly emerge from the cone of darkness at 3:54 a.m. in reverse order. Adding to the beauty of the overall scene at this eclipse (especially during totality itself, while the sky is dark), the Moon will be seen surrounded by many of the bright winter stars, including Capella, Castor, Pollux, Betelgeuse and Aldebaran! (Use your Scientifics’ Star and Planet Locator to identify them.)

There are several interesting things to notice as you watch this event unfold. Most obvious are the darkness of the eclipsed Moon and the range of colors displayed, both of which vary from one eclipse to another. These depend on the clarity of the Earth’s atmosphere at the time, which refracts or bends sunlight around into the umbral shadow. There have actually been eclipses so dark that the Moon remained all-but invisible during totality — and ones so pale that you had to look carefully to see that there was even an eclipse in progress! Among the colors that have been seen are shades of reddish-orange, brown, copper, rose, and even blood-red. Notice, too, that the Earth’s shadow is curved at all phases of the eclipse, as the Moon passes through it. This is direct proof that the Earth itself is round — something recognized by many early skywatchers. And finally, realize that you’re actually seeing our lovely satellite move eastward in its orbit — as it first passes into, through, and then out of the shadow at roughly its own diameter each hour.

While lunar eclipses can certainly be enjoyed with the unaided eye alone (as they have been throughout most of history!), they are best-seen in binoculars. An ideal pair for this would be a 7×50 or 10×50 glass, the first number indicating its magnification and the second the aperture in millimeters. Telescopes themselves, with their relatively narrow fields of view, typically don’t provide enough sky coverage around the Moon to get the full effect of its globe being suspended in space. An exception here, however, is Edmund Scientifics’ amazing Astroscan wide-field reflecting telescope. Providing a 3-degree actual field of view at it low magnification of 16x, it takes in an astounding six full-Moon-diameters of sky — something many have described as looking through the porthole of a spaceship!

PS: The annual Geminid meteor shower peaks on the night of the 14th, with a fairly bright first-quarter Moon lighting up the sky. As one of the year’s richest displays of “shooting stars,” it should still be well worth watching.

–James Mullaney
Former assistant editor at Sky & Telescope magazine and author of eight books on stargazing.