Earthshine

Posted by Rich Talcott
on Thursday, October 26, 2006

Driving home last night during twilight, I was taken aback by the appearance of our nearest celestial neighbor, the Moon. The strikingly thin crescent hung low in the southwest, well away from the pretty pinks and purples clinging to the western horizon where the Sun had recently set. With a little effort, I also saw the star Antares a bit to the Moon’s right and brilliant Jupiter farther in the same direction.

Earthshine beautifully filled out the Moon’s globe. This ashen glow comes from sunlight reflecting off Earth, reaching the Moon, and then bouncing back to us. It shows up nicely on a thin crescent Moon because Earth looks nearly full from the Moon’s perspective and the Moon’s sunlit crescent doesn’t overwhelm the fainter light. Here’s an odd fact to impress your friends: The sunlight we see as earthshine left the Sun 2.6 seconds before the sunlight coming directly from the crescent.

Part of my surprise at seeing the Moon last night came from its distance from the brightest twilight glow. At this time of year, the ecliptic — the Sun’s apparent path across the sky that the Moon follows closely — makes a shallow angle to the western horizon after sunset. The Moon was actually about 45° from the Sun yesterday evening, but appeared only 10° high in the sky. The remainder of the Moon’s solar elongation pushed it well south of the Sun’s setting point. This wasn’t anything I didn’t know, of course, it’s just that I don’t normally get such a vivid reminder of solar system dynamics.

But the real reason I was taken aback by the Moon’s appearance was that it marked the second night in a row I saw it. Here in Wisconsin, particularly these past few weeks, clouds have been the rule. This fall hasn’t been conducive to much observing.

I frequently get asked how an astronomy magazine ended up in Wisconsin. (Our friends at Sky & Telescope in sunny Cambridge, Massachusetts, often hear the same kind of question.) “Why aren’t you headquartered in Arizona or someplace like that?” We can “thank” our magazine’s founder, the late Stephen Walther, who called Milwaukee home. It’s no more complicated than that. Although Stephen may have been more clever than we give him credit for. Imagine a magazine produced by a bunch of people who walk around like zombies every day from lack of sleep.

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