The Science of Sustainability

Midnight Delight: Total Lunar Eclipse

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Total Lunar Eclipse Dec 20/21 2010
The Moon and the Earth have a very special relationship in the Cosmos. The Moon is close enough to us to tug our oceans into tidal swells, and even to make you (very slightly) lighter when it's overhead. You can even "touch" the Moon, electromagnetically, by aiming a flashlight at it and pressing the button: about a second after you do, the photons you launch physically contact the soil and rock on the Moon's surface (and the way a flashlight beam spreads out, you don't even need good aim). And of course the Moon is the only place in the Universe we've personally visited.

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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Category: Astronomy, Partners

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Ben Burress

About the Author ()

Benjamin Burress has been a staff astronomer at Chabot Space & Science Center since July 1999. He graduated from Sonoma State University in 1985 with a bachelor’s degree in physics (and minor in astronomy), after which he signed on for a two-year stint in the Peace Corps, where he taught physics and mathematics in the African nation of Cameroon. From 1989-96 he served on the crew of NASA’s Kuiper Airborne Observatory at Ames Research Center in Mountain View, CA. From 1996-99, he was Head Observer at the Naval Prototype Optical Interferometer program at Lowell Observatory in Flagstaff, AZ. Read his previous contributions to QUEST, a project dedicated to exploring the Science of Sustainability.
  • Carmen

    I love this article!

  • Ben Burress

    Thanks! Are you coming up to Chabot for our Midnight Delight eclipse party?

  • Ben Burress

    It'll be very high in the sky in LA as well–in fact, a little higher than here in the SF area. As for a path of totality map, here is a link to NASA's: http://eclipse.gsfc.nasa.gov/OH/OH2010.html#LE2010Dec21T. That's the path that the Moon takes through Earth's shadow, of course. Anyone on Earth who can see the Moon at all during this eclipse will see the eclipse, and were in the best situation here on the West Coast. Hope your weather is better than our's is forecast to be….

  • KEVIN PERALTA

    Great article. This is just the astrologicical view though. What effect will it have on the enviroment/nature though? So many things happening at the same time. I'm curious if this event will have an affect on California's geological faults. I've noticed a few postings on the web today refering to recent sizable quakes in New Mexico and the 1971 Sylmar quake that took place just after a lunar eclipse. Interesting theory. With this particular event, I guess we will find out if this theory has any validity. To bad we having rain down in So Cal too.

  • Ben Burress

    Actually my article outlined the astronomical, not astrological, features of this eclipse :) As for earthquakes, I doubt the eclipse would have any effect on this beyond what the Moon's gravitational influence might already exert. It's a fact that the Moon's gravity distorts the Earth's oceans into tides, and the same tidal effect does tug at Earth's solid body as well. But, there would be no additional or anomalous effect from the Moon's gravity whether there is an eclipse or not. The eclipse is an effect of light and shadow, not gravitational disturbance.