Friday, December 9, 2011
If you happen to be in the Pacific Northwest and also happen to have an unobstructed view of the horizon in both directions, tomorrow morning's lunar eclipse is going to afford a unique and implausible simultaneous view of both the rising sun and the setting eclipsed moon. This is called a selenelion and possible due to atmospheric refraction. For example, when you see the sun or the moon just sitting on the horizon, it isn't really there; the entire disk is actually just below the horizon, but we're seeing the image refracted over the curvature of the earth. With a selenelion both the sun and the moon are actually just below the horizon, but being refracted over it.
If you happen to be in Seattle, the bad news is that the Cascade and Olympic mountains will obscure the sun and the moon at the time of the selenelion, if the rain doesn't. The good news is that for the other 364 sunrises of the year, you also have a view of the Cascades and Olympics.