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On the evening of March 3, 2007, the eastern part of the United States will be watching a total lunar eclipse, starting at sunset. For more information on this eclipse, go to Fred Espenak's eclipse page, a treasure chest of information.
A total eclipse of the Moon can only happen at Full Moon, when the Moon is exactly opposite the Sun in our sky. If the orbit of the Moon around the Earth were on the same plane as the Earth's orbit around the Sun, then we'd have a total lunar eclipse every month at Full Moon. As it is, the Moon's orbital path is inclined by about 5 degrees to Earth's orbital plane. (For reference, the angular diameter of the Moon equals about 1/2 degree.)
The geometry is such that the Moon in its orbit travels north of the "ecliptic" (Earth's orbital plane) for half the month, and south of the ecliptic for the other half of the month. Twice a month the Moon crosses the plane of the Earth's orbit at points called nodes. When the moon is going from south to north, it's called the ascending node. When the Moon is crossing the ecliptic from north to south, it's called the descending node.
For a total eclipse to occur, two things are necessary: first, the Moon must be full, and secondly, the Full Moon must be appreciably close to one of its nodes. Most of the time, the Full Moon eludes the Earth's shadow, by orbiting above it or below it. On March 3, however, the Full Moon will be appreciably close to its (north to south) descending node, so a total eclipse will be inevitable. The descending node (intersection of the Moon's and Earth's orbital planes) will actually be a bit east of the eclipsed Full Moon, as shown on this illustration from Fred Espenak's eclipse page. Consequently, the Full Moon will pass through the northern part of the Earth's dark umbral shadow, not through the center.
For the last six months, the Full Moon has passed to the NORTH of the Earth's shadow, and for the next six months, it'll pass to the SOUTH of it - until the next lunar eclipse on August 28. On March 3, look for the eclipsed Moon in front of the constellation Leo the Lion, showing you right where the sun will stand amidst the stars six months thereafter. Last month, the February Full Moon shone in front of the constellation Cancer the Crab - about 30 degrees to the west of the March Full Moon's place upon the zodiac. Next month, the April Full Moon will shine in the constellation Virgo the Virgin - about 30 degrees to the east of the March Full Moon.
by Bruce McClure
February 2007 Feature * April 2007 Feature
|Constellation Chart by Jim Kaler|
|Eclipse Animation by Shadow and Substance|