Earthshine, and what Leonardo got wrong

The moon over this house right now is a waning crescent, a quarter lit, and it will still be hanging around in the blue well after sunrise at around 05:00 — a waning moon rises before the sun and then trails it all morning, pale and ignorable, which is why most people think the moon is a night object. Twenty-five percent is the wrong phase for the thing actually worth looking at, though. What I want is the other three-quarters: the part that isn’t dark.

It’s called earthshine — sunlight that hits the Earth, bounces, and lands on the night side of the moon, so the whole disc shows up faintly inside the bright hook. Leonardo worked out the mechanism around 1510 and wrote it down in the Codex Leicester. He got the geometry exactly right and the reflector exactly wrong: he decided the sea was doing it, because water shines and land doesn’t. It isn’t the sea. Oceans are close to black from orbit — flat water is a mirror, and a mirror pointed at the sun throws light in one narrow direction, not everywhere. The reflector is cloud. The brightness of that ashen glow on the moon is essentially a reading of how much of this planet is overcast at that moment.

Which makes the moon an instrument. People at Big Bear ran that measurement for years — point a telescope at the dim part of the crescent, compare it to the bright part, and you’ve got Earth’s albedo averaged over an entire hemisphere in one go. I like that better than the ceilometer I was reading about last week, which fires a laser straight up at a pencil-width of air above a runway, watches for half an hour, weights the last ten minutes double, and calls the result the sky. One of those takes a sample and infers a hemisphere. The other takes the hemisphere directly, off a rock, second-hand.

The catch is that the crescent is thousands of times brighter than the earthshine beside it, so a quarter-lit moon mostly drowns the thing out. New moon is four or five days off. The best view of the whole moon arrives when there’s almost none of it lit.