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February 20, 2008: Total Lunar Eclipse

Be sure to circle February 20 on your calendar, for this will be the last total eclipse of the Moon until December 21, 2010. The partial phase of the eclipse begins at 8:43 p.m. Eastern Standard Time, and the Moon is totally engulfed in shadow by 10 p.m. Mid-eclipse falls at 10:26 p.m. and the total phase ends at 10:51 p.m. Thereafter, a partial eclipse remains in view until the curtains are drawn by 12:09 a.m. (February 21). If you wish to know these eclipse times for another time zone, click here.
When a lunar eclipse is approaching, people often ask me what the moon's phase will be during the eclipse. The answer is always Full Moon. It's only at Full Moon that a lunar eclipse is possible. In fact, if the Moon orbited the Earth on the same plane that the Earth orbited the Sun, we'd have a total lunar eclipse every month at Full Moon. As it is, the geometry is such that the Moon's orbital plane is inclined at about 5 degrees to the Earth's orbital plane.

Eclipse Geometry

As the Moon revolves full circle around the Earth, it travels north of the Earth's orbital plane for half the month, and south of it for the other half of the month. Twice a month, the Moon in its orbit crosses the plane of the Earth's orbit. When passing from north to south, the Moon is said to be at its descending node. When swinging from south to north, the Moon passes through its ascending node. Thus, when the Full Moon is appreciably close to one of its nodes, a lunar eclipse is not only possible, but inevitable.
On the night of Feb. 20-21, the descending node is a bit to the west of the Full Moon. Nonetheless, the alignment is close enough for the Moon to pass through the southern half of the Earth's dark umbral shadow. (See illustration.) If the alignment of Full Moon and the descending node were closer to exact, the Moon would pass through the central part of the Earth's shadow and the total eclipse would last more than twice as long. Take for example, the total lunar eclipse of July 27, 2018, the longest total lunar eclipse in the 21st century.
Last month, there was no lunar eclipse because the descending node was too far east of the Full Moon, so the Full Moon passed to the north of the Earth's shadow. Next month, the descending node will be too far west of the Full Moon, so it'll sweep south of the Earth's shadow. The mean time period between successive Full Moons is around 29.53 days, but it only takes some 27.21 days for the Moon to return to the same node. Relative to the backdrop of stars, the Full Moon advances about 29.5 degrees eastward along the Zodiac monthly, while the nodal point retrogrades about 1.5 degrees westward.

Seeing the Eclipse from the Moon

If you were to watch this eclipse from the Moon, you would see a 50-minute total eclipse of the Sun! As viewed from Earth, the longest total solar eclipse in all the 21st century is only 6 minutes and 39 seconds long.

copyright 2008 by Bruce McClure

January 2008 Feature * March 2008 Feature

Yearly Eclipse Cycles