Total Lunar Eclipse 08-28-07. Credit: Conrad Jung

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….

Charles Burckhalter's 1900 solar eclipse image with stars marked for Relativity analysis. Credit: Chabot Space & Science Center

Elevendy-one years ago (that's a hundred and eleven to the non-Shireborne), Chabot Observatory Director Charles Burckhalter set forth on an expedition across the plains of India, risking bandits, tigers, famine, and plague, on a hunt for big game: a rare meeting of the Sun and the migrating Moon in a total solar eclipse. Little could he know, years later his then-world-class astrophotographs of the event would be used in an effort to prove or disprove the General Theory of Relativity published by Albert Einstein in 1916.

Einstein's General Theory of Relativity is the one that describes gravity, that attractive force between objects, not as some kind of invisible bungee cord but as a distortion, or warping, of space (and time) by matter. Other objects within the warped, "curved" space accelerate "downhill" along the curve, toward the mass producing the distortion. Likewise, objects moving through the gravity field are deflected, their trajectories curved toward the mass.

The trick was how to prove this theory with observational evidence. That gravity is a fact is easily demonstrated by holding up an object and then letting go of it. Gravity, whatever gravity is, accelerates it "downward"—in our case, toward the center of Earth's mass.

But how to demonstrate that gravity, as Einstein theorized, is the effect of space-time warped by matter causing objects to slide down the "uneven ground," (yeah, using a lot of quotation marks, I know) was a bit like trying to prove that gnomes are responsible for missing keys (as we all know they are, but just can't prove!)

So, an experiment was proposed to try to sort out the culprit of gravity as curved space-time, or "merely" an invisible bungee cord. If space-time is in fact warped around a massive object, then not only other material objects, but non-material forms of energy should follow along the contours of the warp and also be deflected. Light was the handiest observable, non-material form of energy available.

As the theory went, the greater the mass of an object, the greater the deflection, and the Sun was by far the most massive nearby object available. If the stars behind the Sun's vicinity could be observed to shift their apparent position—as a consequence of their light rays being deflected as they flew past the Sun on their way to Earth—then General Relativity could be observationally confirmed. And there were astronomers in both camps—pro-Relativity and pro-Invisible-Bungee-Cords—trying to observe the effect, or the lack thereof.

A total solar eclipse in 1919 made itself available only three years after Einstein published, and so became a very important eclipse to science. (And, coincidentally, occurred on almost the same day of the year, May 29, as the May 28, 1900 eclipse captured by Burckhalter–which meant that the stars whose positions were being measured, the Hyades cluster in Taurus, were the same in both cases.) In the early 20th century, before space-based satellite observatories existed, stars could not be observed close to the Sun's disk, the Sun being so bright as to drown them out completely.

But during a total solar eclipse, the Moon temporarily blocks the Sun's bright disk, briefly giving astronomers a glimpse of the starry background surrounding the Sun. Photographs of the stars' positions taken during the eclipse could be compared to those of the same stars taken at a different time of the year, when the Sun wasn't present in that location, and so the space-time distortions of gravity (if there were any) might be observed in the deflection of the stars' apparent positions.

Charles Burckhalter's expedition photos from the 1900 eclipse were accessed as part of the observational experiment—including by those on the bungee-advocacy side of the aisle. His innovations in a technique for photographing solar eclipses made his image plates quite valuable among the 1900 sets.

In the end, General Relativity was, in fact, demonstrated observationally, by Arthur Eddington and others, which opened up a whole new universe of astounding possibilities, including the origin of the universe in the Big Bang, and the existence of the mind-bending, light-devouring objects called Black Holes.

But, back in the day, Burckhalter was probably more concerned by the sounds of tigers creeping through the bush and reports of the surrounding plague epidemic as he snapped his shots of the darkened Sun above….

Total Lunar Eclipse Dec 20/21 2010

One of the most striking and beautiful examples of the Earth-Moon relationship takes place during a total lunar eclipse, when the Moon passes through the Earth's shadow, transforming in a couple of hours from the stark brilliance of the Full Moon to the dark ruby-hued wonder of "umbral occlusion"—or totality.

Monday evening, December 20th, starting at about 9:30 PM, the Moon will begin to enter the Earth's partial, or "penumbral," shadow. Around 10:30, it begins to enter the umbra (full shadow), and by 11:40 will be completely engulfed: "totality." Totality will last until 12:53 AM Tuesday morning, when the Moon begins to leave the umbra.

While the extended weather forecast at the moment doesn't look favorable for the SF Bay Area, there are always freak changes in weather to hope for. Also, we'll be having a Lunar Eclipse celebration at Chabot Space & Science Center, rain or moonshine, which will be a lot of fun: Lunar Labs, planetarium shows, sci-fi movie reels, and every Moon-related song we could find—hope to see you there!

Though a total lunar eclipse is a rare event to see, this one is rarer still–not the least reason being that for the Western US it will be one of the highest lunar eclipses you can see, with the Moon reaching its apex for the night over 75 degrees from the horizon (practically overhead) close to mid-totality. For our latitudes in the Bay Area, the Moon can't get much higher than that. So, we get High Moon when the eclipse is at its best (weather permitting).

What makes this eclipse rare among the rare is the fact that the Moon is crossing several important features in the sky simultaneously. First, it's crossing the Ecliptic, the path of the Sun's apparent motion over the course of a year, cutting through the 12 constellations of the Zodiac. In essence, the Ecliptic is the projection of Earth's orbital plane onto the sky. Is it a coincidence that the Moon will be crossing the Ecliptic during this eclipse? Actually…not at all. By virtue of the geometry of a lunar eclipse, the Moon must be on the Ecliptic in order to pass through Earth's shadow, since the Earth's shadow, cast by the Sun's light, always runs along Earth's orbital plane, and so too the Ecliptic.

Another line is crossed during this eclipse because it happens on Winter Solstice. On this day, the Sun is located in Sagittarius, and so the Earth's shadow is cast toward the opposite point on the sky, in Taurus. Halfway around the circle of the Ecliptic from the Winter Solstice point you find the other solstice point, the spot on the Ecliptic where the Sun is located at Summer Solstice.

The Moon will also be crossing the Galactic Equator: the line representing the plane of the Milky Way Galaxy. This alignment is a bit more tangible than those with the Ecliptic and the Solstice point since the Milky Way is a visible sky feature—at least in areas not impacted by urban light pollution. If you live in a place where you can normally see the Milky Way on a dark night, you have an extra wonder to marvel at during this eclipse: when the bright Full Moon enters totality and goes dark, the subtle light of our galaxy will be revealed, with the Moon set like a darkling gem in a diamond bracelet….

Well, we can only hope for clear skies—but in either event, come up to Chabot and celebrate with us this midnight delight….

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The footage of the August total eclipse that we got from the Exploratorium and used in our story is incredibly beautiful. The red images of the sun that you'll see are created by a telescope that views the sun in a wavelength of hydrogen gas. "The sun is mostly made of hydrogen gas," said Doherty, "and this is a deep red wavelength called hydrogen alpha." You can watch the entire China 2008 eclipse on the Exploratorium's web site. And Doherty recommends trying to view the next two total solar eclipses live. They will be visible on July 22, 2009, from Shanghai, China, and on July 11, 2010, from Easter Island. But if your budget doesn't allow you to travel that far, you can always wait until 2017, when a total eclipse will be visible in Washington and Oregon and all the way across the United States to South Carolina. Or you can plan to visit the Exploratorium or the Chabot Space and Science Center on any of those three days, to watch the eclipse through their live feed.

Watch the Eclipse Chasers television story report online.

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