Down to our last 10 days!
We're running a membership drive this month – our goal is 150 new members – and as a part of it we're including this blatant clickbait series... Besides, it's a chance to tell some of the funny stories that come up during cloudy nights at the VATT!
The old astronomer tells the tale... There is an effect called the Opposition Effect where solid bodies such as the Moon and Mars get significantly brighter when they are opposite the Sun. Part of the reason is the fact that the whole disk looks brighter when there are no shadows; the Full Moon is 40% brighter than the Moon even only one day before or after. (Check this out for yourself!)
But when the the light is virtually exactly behind you ("zero phase angle", or at least an angle much less than one degree) there is a second effect that kicks in, called coherent backscatter, which is due to quantum effects and the wave nature of light and... well, it's interesting, and potentially a powerful way of learning about the intrinsic nature of the material on the body's surface just from measuring how much brighter it gets as the phase angle goes to zero.
So we had the bright idea of observing the Moon on a night when we'd calculated that the phase angle would in fact drop exactly to zero. As the night progressed, and the Moon slowly moved into position exactly opposite the Earth from the Sun, the Moon did get brighter and brighter. But then, to our surprise, our detectors recorded it starting to fade, radically, almost as if it were disappearing!
So finally we went outside to see if there was a cloud or something between us and the Moon. Nope; the sky was crystal clear. And the Moon was going into the shadow of the Earth, beginning its lunar eclipse.
(If we'd thought about it, we'd have realized that this always happens to the Moon when you try to observe it at zero phase angle. Fortunately, Earth's shadow does not reach out to the asteroids, where this sort of orientation – while rare – can on occasion let us make observations at very low phase angle.)
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