Early risers Saturday morning will be treated to a partial eclipse of the moon. The eclipse happens for Coloradans during morning twilight, beginning at 4:17 a.m. and culminating just as the moon sets at 5:39 a.m. The scene might resemble this image of the partially eclipsed moon rising at dusk May 15, 2004. The moon will be totally eclipsed for all of North America on Dec. 21.

Jimmy Westlake/Courtesy

Early risers Saturday morning will be treated to a partial eclipse of the moon. The eclipse happens for Coloradans during morning twilight, beginning at 4:17 a.m. and culminating just as the moon sets at 5:39 a.m. The scene might resemble this image of the partially eclipsed moon rising at dusk May 15, 2004. The moon will be totally eclipsed for all of North America on Dec. 21.

Jimmy Westlake: A partial lunar eclipse

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Jimmy Westlake

Jimmy Westlake's Celestial News column appears Tuesdays in the Steamboat Today.

Find more columns by Westlake here.

As the Earth and moon perform their perpetual orbital dance, sometimes their shadows are thrown onto each other, creating an event called an eclipse. The word eclipse is derived from the Greek word “ekleipsis,” which means “to abandon.” When the moon’s tiny shadow touches the Earth, that part of the Earth experiences a solar eclipse and the sun temporarily abandons us in the daytime sky. When Earth’s giant shadow engulfs the moon, we experience a lunar eclipse as the moon crosses the darkness of the Earth’s shadow.

In order for any eclipse to occur, the sun, Earth and moon must all lie in the same straight line and in the same flat plane, an unusual circumstance called a syzygy.

The moon’s orbit is tilted about 5 degrees with respect to Earth’s orbit, so the required alignment does not happen every month. In fact, we experience two “eclipse seasons” each year, six months apart, when the eclipse alignments can occur. These eclipse seasons advance through our calendar about 20 days each year. In 2010, the eclipse seasons fall in June/July and December.

Before dawn Saturday morning, beginning at 4:17 a.m., the northern half of the moon will slip into the Earth’s dark shadow, creating a partial eclipse of the moon. The dark bite out of the lunar cookie will be greatest at 5:39 a.m., when about 54 percent of the full moon will be obscured. From Northwest Colorado, maximum eclipse occurs just as the moon sets in the southwest and the sun breaks the horizon in the northeast. From points farther west, more of the eclipse can be seen before the moon sets.

The backdrop for this lunar eclipse will be the stars of the constellation Sagittarius and the star clouds of the summer Milky Way.

While watching the eclipse, you will notice that the edge of the Earth’s shadow is curved and, in fact, forms a complete circle. The Greeks used this observation to argue that the Earth must be shaped like a ball because a sphere is the only shape that always will cast a circular shadow no matter from which direction the light shines.

Another eclipse will occur in the June/July eclipse season, this one a total eclipse of the sun July 11 that will be visible from islands in the South Pacific and the southernmost tip of South America.

When the December eclipse season rolls around, all of North America will be treated to a spectacular total eclipse of the moon the morning of Dec. 21.

Consider this weekend’s partial lunar eclipse a warmup for the main event this December.

Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College’s Alpine Campus. He is an avid astronomer whose photographs and articles have been published all across the world. Check out Jimmy’s astrophotography website at www.jwestlake.com.

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