Terrestrial Earthshine

I was on my deck looking for comet Pan-STARRS on March 12th, when it was passing close to the young Crescent Moon. Although I managed to get a photo of it, the image was too disappointing to use in a Sky Lights post. But I did observe an interesting optical effect while scanning for the comet through binoculars.

The top image shows a closeup of where the arrow points in the bottom image. This is a view to the west, where the Hieroglyphic Mountains define my horizon. The time is just after sunset. That tiny peak from a more-distant mountain range normally blends in with the Hieroglyphics as both ranges darken at sunset. That’s what the bottom image shows, albeit with less magnification. When seen in silhouette, you can’t really tell that they’re different ranges. But this time, something looked unusual and caught my eye.

As you can see in the top image, that distant peak is distinctly brighter than the closer mountains. You are looking at the east sides of both ranges. And those ranges are only about 3 miles apart, so both should be equally dark as the Sun sets in the west. The question raised in my mind was: Where is that light coming from?

After some thought I realized what was happening. Although the Sun had set for me, it was still shining on the west face of the Hieroglyphics. That light was in turn reflected back to the east side of the distant peak. It probably helped that Lake Pleasant is between those two ranges, since water reflects light even better than mountains do, and there was still a lot of diffuse light coming from the sky.

This is the same effect that allows us to see the “dark” part of the Crescent Moon. Light from the Sun reflects from the Earth and illuminates the part of the Moon it doesn’t directly strike. See my Feb 15 2007 Sky Lights for more on the topic of earthshine. But in this case, earthshine was illuminating another part of the Earth.

The effect only lasted about 10 minutes. And I’m sure it happens every sunset to some degree. It’s just one of those things you don’t notice under normal circumstances. Since I was scanning with binoculars, it was immediately apparent to my eyes and begged for an explanation. Science isn’t always about spectacular phenomena. The role of science is to explain any phenomenon that raises questions in the mind of a careful observer.

Next Week in Sky Lights ⇒ Of Easter, the Equinox, and Balancing Eggs

Q&A: The Setting Crescent Moon
Q&A: Of Easter, the Equinox, and Balancing Eggs