Regional
4:55 pm
Mon December 20, 2010

NMSU Las Cruces Astronomer Awaits Lunar Eclipse Tonight

Las Cruces – A lunar eclipse is scheduled to occur on the Winter Solstice and the 72-minute show should be visible to about half of the Earth's population.

The eclipse of the moon will begin late in the evening, Dec. 20, and will become a total eclipse early in the morning of Dec. 21. This will be the only total lunar eclipse for 2010; a partial eclipse was visible on June 26.

"No telescopes or binoculars are needed to observe this fairly spectacular event," said Kurt Anderson, professor emeritus in New Mexico State University's Department of Astronomy. "Weather permitting, our clear winter skies should provide an eerily impressive show. Even then, the eclipse is best observed away from the glare of city lights."

A lunar eclipse occurs when the full moon passes into the shadow of the Earth.

Starting on Dec. 20, the full moon will rise in the east at about 4:40 p.m. Mountain Standard Time, but the eclipse will not begin until about 10:29 p.m. when the Earth starts to block out a portion of the sun's light. This phase of the eclipse is not very noticeable to Earthbound observers, Anderson said, and will appear as a slow dimming of the moon's brightness.

The main partial phase of the eclipse will begin as the moon starts to enter the Earth's umbral shadow about an hour later at 11:32 p.m. Observers will see the sharp edge of this darker shadow begin to move across the face of the moon, beginning at its eastern edge. The umbra is that part of the Earth's shadow within which an astronaut on the moon would see the Earth completely covering the sun. The eclipse will become total at 12:41 a.m. on Dec. 21 and totality will last until 1:53 a.m. From 1:53 to 3:01 a.m., the moon will move out of umbra during the second partial phase of the eclipse. Finally, the penumbral part of the eclipse will end about 4:05 a.m., well before the moon sets in the west at 6:26 a.m.

"Watchers should note the circular arc of the Earth's shadow as it crosses the face of the moon during the partial phases of the eclipse," Anderson said. "This offers proof of the Earth's sphericity, as pointed out by Aristotle in about 350 B.C.E.; only a sphere always casts a circular shadow."

During totality, the moon usually assumes a dim reddish hue because of sunlight refracted by the Earth's atmosphere. An observer on the moon during this part of the eclipse would see the Earth as a dark disk completely obscuring the sun, but surrounded by a bright red ring of refracted sunlight. The Earth's atmosphere acts like a prism to bend this sunlight toward the moon, but absorption and scattering in the atmosphere removes almost all but the reddest wavelengths of light.

Anderson said that typically, there are four eclipses in a year. 2011 will be unusual as there will be four solar eclipses and two total lunar eclipses. All of the solar eclipses will be partial and will not be visible in the United States. The first lunar eclipse, on June 15, 2011, will not be visible from the United States and people will only be able to see the beginning partial phases of the second eclipse on Dec. 10, 2011.