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Jimmy Westlake: Saturday’s brush with totality

Coloradans will get to see only the first half of Saturday morning’s total lunar eclipse. Last December

Coloradans will get to see only the first half of Saturday morning's total lunar eclipse. Last December, we got to see the whole thing, as captured in this montage of images. The next total lunar eclipse visible from Colorado won't come until April 15, 2014.

By the time 2011 comes to a close in a few weeks, there will have been six eclipses in the preceding 12 months — four partial solar eclipses and two total lunar eclipses. North America, however, is on the wrong side of Earth for all of these eclipses save one — on Saturday morning, the western U.S. will get to see the first half of a total eclipse of the moon.

An eclipse of the moon occurs when the moon passes through Earth's long shadow cast into space, either partially or totally. This can occur only during the full moon phase when the moon is opposite the sun in our sky, but it doesn't happen at every full moon because of the 5-degree tilt of the moon's orbit relative to the Earth's orbit. The full moon usually passes slightly above or below the shadow of Earth.

Total lunar eclipses are infrequent but not rare. According to NASA's Five Millennium Canon of Lunar Eclipses, there will be 3,479 total lunar eclipses in the 5,000 years between 2000 B.C. and 3000 A.D. That means, on average, we experience a total lunar eclipse somewhere on Earth once every 1.44 years. From any given location, you can expect to see a total eclipse of the moon about once every 2 1/2 years.

When the full moon is immersed in the dark shadow of Earth, sunlight reddened and bent by Earth's atmosphere illuminates the shadowed moon with a coppery red color — the equivalent of alpenglow projected onto the moon.

Unlike the spectacular total lunar eclipse we enjoyed Dec. 21, the eclipse Saturday will set for Coloradans just as the total phase of the eclipse begins. Early risers can watch the eclipse begin in a dark sky at 5:46 a.m., when the top edge of the moon will begin to darken as it slips into Earth's shadow. Then, as the eclipse progressively gobbles up more and more of the full moon, the sky will begin to brighten with the morning dawn. Eventually, the sky will become so bright that the eclipsed moon likely will fade from view as it sinks toward the west-northwest horizon. Totality begins at 7:06 a.m., the sun rises at 7:18 a.m. and the moon sets at 7:21 a.m. — lousy timing for folks living in Northwest Colorado, but perfect for folks living in Alaska and Hawaii. They will get to see the entire eclipse.

This is the closest that Colorado will come to a total lunar eclipse until April 15, 2014, so you'd better take advantage of this brush with totality.

Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. Check out "Jimmy's 2012 Cosmic Calendar" of sky events on his website at http://www.jwestlake.com. It features 12 of his best astrophotographs and a day-by-day listing of cool celestial events that you and your family can have fun watching in 2012. Proceeds from the sale of Cosmic Calendars support the CMC SKY Club.