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Skywatch: One aspect of lunar eclipses is hard to predict.

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This Sunday night, the full moon will traverse Earth’s shadow while above the horizon for Idaho sky watchers—a total lunar eclipse.

Eclipses are conspicuous expressions of the celestial clockworks, i.e. they can be predicted centuries in advance. The ability to calculate the future (and past) positions of solar system bodies dates to the early 17th Century, when Johannes Kepler discovered his famous laws of planetary orbits. (With the ink on his monograph barely dry, he calculated a triple Jupiter-Saturn conjunction in 7 BC, which he speculated heralded the appearance of the Christmas Star).

Nowadays, supercomputers predict eclipses to within seconds of accuracy by calculating the mutual gravitational interactions between planets, something Kepler’s equations were incapable of.

So, while there are no surprises about “when” a lunar eclipse will occur, they’re not completely predictable. Because the only thing illuminating the moon during a total lunar eclipse is sunlight filtered through Earth’s atmosphere, the transparency of the atmosphere can have a dramatic effect on how dark the eclipsed moon appears.

About a century ago, French astronomer Andre-Louis Danjon defined a scale of lunar eclipse brightness, ranging from 4 (bright, coppery, red-orange) to 0 (nearly invisible). The historic 1991 eruption of the Philippines’ Mt. Pinatubo put so much ash into the atmosphere (the most since Krakatoa in 1883) that the total lunar eclipse of December 9, 1992 was almost invisible (a textbook “Danjon 0”) at mid-eclipse.

The moon will pass fairly deep into Earth’s shadow on Sunday, but exactly how dark it will be literally remains to be seen. The moon will rise, already partially eclipsed, at 8:45 p.m. Totality is from 9:29 to

10:54 p.m., with maximum eclipse at 10:13 p.m., but the partial phase afterwards (until 11:55 p.m.) is worth watching too.

Next column: What black holes are, and aren’t.

Chris Anderson manages the College of Southern Idaho’s Centennial Observatory in Twin Falls. He can be reached at 208-732-6663 or


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