Look, up in the sky! Is it a moldy orange? A giant celestial penny? A dragon eating the Moon? In fact, it’s a total lunar eclipse, and coming soon to a sky near you….
December 10, 2011 marks your last chance to see a total lunar eclipse—one of the most breathtaking celestial events that you can witness with your unaided eye–until 2014. For us on the West Coast, the drama of the Moon’s occlusion will play out in the early morning hours of Saturday—weather permitting, as always.
A total lunar eclipse occurs when the Moon passes through the long shadow the Earth casts into space. We see a partial lunar eclipse when only part of the Moon grazes the Earth’s shadow and a total eclipse when it is completely engulfed in darkness.
The overall eclipse begins at about 3:33 AM on the morning of December 10, when the Moon first touches Earth’s penumbra, or “half shadow” (the region of space where only some of the Sun’s light is blocked by the Earth). At the beginning, you might be challenged to notice anything different about the Full Moon, unless you’re looking for something—in which case you might start to notice a slight darkening at one edge of the Moon’s disk.
By 4:46 AM, the real show begins: the Moon will begin to enter the umbra, the Earth’s full shadow, in which no direct sunlight shines. Now, a very noticeable “bite” will be taken out of the Full Moon—as if some great celestial creature is nibbling it at the edge. (To the Chinese, this animal was thought of as a dog or a dragon; to the Maya, often a jaguar; and if you mix your myths well, you might imagine those creatures eating green cheese….)
Finally, at 6:06 AM, with the Moon low near the western horizon, it will become completely engulfed in the umbra and will likely turn a dim, coppery, orange, or possibly even reddish color—like a shiny copper penny, or a molding orange. I hope that image doesn’t spoil the experience for you….
This is totality, when the entire disk of the Moon is within the umbra. From Earth, the Full Moon goes very dark during totality. From the Moon, if you were so lucky to be there during totality, the Earth (in its “New” phase as seen from Luna) would be a black disk surrounded by a ring of red or orange light—from the Moon’s perspective, a total solar eclipse.
Why is the Moon lit at all during totality if it’s supposed to be in the umbra where no direct sunlight shines? And why orange and red tones?
The answer is in Earth’s atmosphere, which simultaneously bends, or “scatters,” the sunlight that grazes by the edges, and filters the colors of the sunlight to favor the redder wavelengths passing through. If you’ve seen sunlight shining around the edges of a cloud, making that “silver lining” and shedding light into the cloud’s shadow, then you may have an idea how the sunlight is scattered around the edge of the Earth into the otherwise dark umbral shadow.
And, if you’ve seen the colors of a sunrise or a sunset—orange and red, more or less depending on atmospheric conditions—then you can understand why the light is reddish. Earth’s atmosphere acts like a piece of red glass: white light, containing all the colors of the rainbow, enters the glass, but the bluer colors are absorbed, and only the orange and red tones pass through and shine onward. (What does Earth’s atmosphere do with that stolen blue light? Take a look at a clear daytime sky and you’ll see!)
So, for the 41 minutes of totality, you’ll witness one of the most spectacular partnerships of the Earth and Moon, when the Earth “touches” the Moon with the tip of its shadow and the russet tones of all its sunrises and sunsets acting in concert.
Totality will end at 6:47 AM when the Moon’s leading edge begins to depart the umbral shadow—and at 7:17 the show will be over for us when the Moon sets. Then, it’s another three years until we can see such a sight again, so be sure to catch this one! Weather permitting, we’ll have the Observatory Deck open at Chabot Space & Science Center from 4:00 to 7:00 AM, in case you’d like to watch the event in good company….