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Saturday’s Lunar Eclipse Will Include ‘Impossible’ Sight
(Photo by Images In The Backcountry on Flickr)

This year’s second total lunar eclipse on Saturday (Dec. 10) will offer a rare chance to see a strange celestial sight traditionally thought impossible.

For most places in the United States and Canada, there will be a chance to observe an unusual effect, one that celestial geometry seems to dictate can’t happen. The little-used name for this effect is a “selenelion” (or “selenehelion”) and occurs when both the sun and the eclipsed moon can be seen at the same time.

But wait!  How is this possible? When we have a lunar eclipse, the sun, Earth and moon are in a geometrically straight line in space, with the Earth in the middle. So if the sun is above the horizon, the moon must be below the horizon and completely out of sight (or vice versa).

And indeed, during a lunar eclipse, the sun and moon are exactly 180 degrees apart in the sky; so in a perfect alignment like this (a “syzygy”) such an observation would seem impossible. 

But it is atmospheric refraction that makes a selenelion possible.

Atmospheric refraction causes astronomical objects to appear higher in the sky than they are in reality.

Read the full article from SPACE.com