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As the Sun sets, its lingering light creates a zone between daylight and darkness: the twilight zone. It typically lasts for an hour or so, as Earth's atmosphere scatters some sunlight back into the region where the Sun has already set.
There's a twilight zone on the Moon, too, but it's quite different from the one here on Earth. There's a sharper boundary between day and night, with little sunlight reflected into the nightside.
The line between day and night is known as the terminator -- a line that's obvious if you watch the Moon descend the western sky this evening. It's bringing daylight, so more and more of the lunar disk will be illuminated over the next few days.
Here on Earth, the terminator moves at more than a thousand miles an hour at the equator. But the Moon is much smaller than Earth is, and it turns much more slowly. That combination means you could actually outrun the terminator, as it glides along at just ten miles an hour.
There's no atmosphere on the Moon to speak of, so the terminator is quite sharp. As it passes, it's almost like flipping a light switch on or off -- it's dark one second, and daylight the next.
But there is a bit of twilight. That's because radiation from the rising Sun can make tiny grains of moondust float high into the sky. These grains form a glowing layer that reflects a little sunlight back into the nightside -- giving the Moon a "twilight zone" of its own.
More about the Moon tomorrow.
Script by Damond Benningfield, Copyright 2010