By Chris Capps 12/11/11
Here's a brainteaser that left some sky-watchers scratching their heads when the moon suddenly and unexpectedly went black during the daytime in an impossible astronomical display. Why is it impossible to see a Lunar Eclipse during a perfectly cloudless day? Conventional wisdom suggests it's because Lunar eclipses happen when the Earth, the Sun, and the Moon make a straight line. So what often overlooked law of physics made it possible for witnesses in the Rocky Mountains to see both the Sun and the Lunar Eclipse at the same time?
The phenomenon is called a selenelion, and it's a rare but not impossible time when both the eclipsed moon and the sun are visible at the same time. The phenomenon happens because of refraction from the Earth's atmosphere. And to understand it, we need look only at a simple pool of water. From the surface of the water we are seeing images that seem fairly consistent and correspond directly to our point of view. But this is an illusion. When we place our hand in the water and look at it, the hand looks almost as though it has slightly curved forward or backward. Conventional wisdom in this case tells us that the hand has not changed shape, but that the light refracting from the water has altered it and made it appear so. This phenomenon is one of the common problems among spear fishermen who have to adjust for the refraction of the water. But what does this have to do with the Moon?
The Earth's atmosphere is a refracting sphere as well, which is why we can see the moon despite the fact that it is appearing just as the sun is rising in the East. Also called a "horizontal eclipse," a selenelion is visible only in specific areas – often favoring high altitudes or ocean where the horizons can be broadened as much as possible. So while it looked like the laws of physics were crashing down and flying in the face of basic geometric logic, it was in fact simply an incredibly rare occurrence.
Additionally, it should be noted that during the eclipse the moon turned a particularly deep shade of blood red, almost as red as how we would picture an alien planet or Mars. This is partially because of the amount of dust in the atmosphere, causing the dawn to turn a bright and beautiful shade of red just like the moon. And no doubt NASA will have some incredible images of the event as it was seen aboard the International Space Station.
And because of the rare event, the next total lunar eclipse will not become visible (or rather invisible) again until 2014 when once again sky watchers will break out their cameras and telescopes to catch a glimpse at something unique to remind us that we are all part of a vast cosmic experience full of mystery and surprises.