Total lunar eclipse on Wednesday will be a rare "Selenelion"

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On Oct. 8, Interested skywatchers should attempt to see the total eclipse of the moon and the rising sun simultaneously. The little-used name for this effect is called a "selenelion," a phenomenon that celestial geometry says cannot happen.

And indeed, during a lunar eclipse, the sun and moon are exactly 180 degrees apart in the sky. In a perfect alignment like this (called a "syzygy"), such an observation would seem impossible. But thanks to Earth's atmosphere, the images of both the sun and moon are apparently lifted above the horizon by atmospheric refraction. This allows people on Earth to see the sun for several extra minutes before it actually has risen and the moon for several extra minutes after it has actually set.

As a consequence of this atmospheric trick, for many localities east of the Mississippi River, watchers will have a chance to observe this unusual sight firsthand. Weather permitting, you could have a short window of roughly 2 to 9 minutes (depending on your location) with the possibility of simultaneously seeing the sun rising in the east while the eclipsed full moon is setting in the west.

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