Southern Californian sky watchers were blessed with an amazing total lunar eclipse just before sunrise on Saturday, December 10, 2011. This will be the last total lunar eclipse visible from the west coast until 2014.
The timing of this particular lunar eclipse produced an extremely interesting event known as selenelion. In order to appreciate the unique circumstances that produce a selenelion, it's important to first understand the basic geometry that creates a total lunar eclipse. A lunar eclipse can only occur when the full moon and the sun are aligned 180 degrees apart with the Earth in between. The Earth's shadow temporarily blocks the light of the sun, causing the full moon to darken, and as the moon passes deeper into the the Earth's shadow, sun light refracted by the Earth's atomosphere causes the moon to glow orange. On December 10th southern California sky watchers witnessed the rare sight of a fully eclipsed moon setting in the west as the sun rose in the east - a selenelion. This event lasted only a few minutes, but it was a spectacular sight to see.
How is it possible to see the sun in the sky at the same time as the eclipsed moon? It's simply an optical illusion caused by atmospheric refraction. Earth's atmosphere bends light from the sun and the moon like a lens, making them appear higher on the horizon than they actually are. We see this happen every day but we never give it a second thought. It's one of the little miracles of atmospheric physics that we take for granted. When we see the sun rising above the horizon in the east, it really isn't there -- it's still below the horizon. When we see the moon setting in the west, it isn't really in that exact location either. It has already set.
Because this eclipse occurred at moonset, it made imaging difficult because the moon was in the thickest part of the atmosphere. When viewed through a telescope objects low on the horizon are distorted by atmospheric turbulence. They boil and wiggle and swirl, blurring the image. Meade's low cost Lunar and Planetary Imager (LPI) acts like a web cam, capturing many images and discarding those that fail to meet your specified quality threshold. The Autostar Suite software automatically creates a composite image by stacking exposures until you tell it to stop. Compositing improves overall quality and contrast. The images below were captured using a Meade ETX 60 telescope and LPI. Each image is a composite of 50 individual images stacked in Meade's Autostar Suite software. The images were further processed and the orientation was corrected with Adobe Photoshop:
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