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The other day I looked out my window and I could see the ‘dark side’ of the moon really well. The Moon was a whopper, swollen by the well-known illusion that makes moons near the horizon seem big. But that wasn’t what grabbed my attention. The wonderful thing was the way the “dark” part of the Moon was faintly glowing. The phase of the Earth changes reciprocally with the moon, so the illumination is greater for thin crescent moons; and specular reflection off the oceans increases the brightness of a near-full Earth in the moon’s sky. The brightness of the sky also has an effect: it can easily drown out the subtle illumination of earthshine. So for thin crescents, there’s a trade-off with the position of the sun below the horizon.

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