Moon and Jupiter

StarDate: June 12, 2009

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The Moon and the planet Jupiter keep company in the wee hours of tomorrow morning. Jupiter looks like a brilliant star to the lower left of the Moon.

The Moon is in its "waning gibbous" phase. "Gibbous" means that sunlight illuminates more than half of the lunar hemisphere that faces Earth. And "waning" means that the sunlit portion is growing smaller every night.

The line that divides night from day is known as the terminator. When the Moon is waning, that line marks the sunset and the start of the long lunar night.

A single day on the Moon lasts the equivalent of a full month here on Earth. Divide that in half, and you get two weeks of daylight, followed by two weeks of darkness.

During the day, the ground can heat up to about 250 degrees Fahrenheit at the equator. But there's no atmosphere to trap the heat. As the Sun sets, the heat quickly radiates into space, so nighttime temperatures can plunge to more than 250 degrees below zero.

On the hemisphere that faces Earth, lighting conditions at night can vary by quite a bit. That's because Earth goes through a full cycle of phases, just as the Moon does. A "full" Earth is close to 50 times brighter than a full Moon, so it really lights up the night. But when Earth isn't full, it's darker.

And since there's no air to scatter the earthlight, the sky remains completely dark. The stars don't twinkle, either -- they shine steadily through the long, cold lunar night.

Script by Damond Benningfield, Copyright 2009

For more skywatching tips, astronomy news, and much more, read StarDate magazine.

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