Dark Side

What is the distance between the dark and light side of the moon?

At the place where the dark side of the moon turns into the light side… how long does it take to go from dark to light? Is it a gradual changing? or is it a very sudden change as in a line of light?

On the moon, it is a fairly sharp line – if you were in the bottom of a meteor crater, light would hit the sides, and reflect down into the crater, fuzzing the line between light and dark, a bit, but it is really a fairly sharp line. In contrast, the Earth’s terminator (the line between dark and light) is really fuzzy because of the Earth’s atmosphere – light refracts around a bit, making the terminator quite broad – so it takes roughly a half hour from sunset to dark – at the midline of the US it is probably around 250 miles wide or so. where the moon’s terminator is much sharper – if you were in a perfectly flat spot, you could see the line travel by. (much slower than the Earth, as it rotates once a month rather than once a day for the Earth!)

Kelly Clarkson – “Dark Side”