As the Sun drops from sight this evening, and the bright blue sky fades to the pastel shades of late twilight, the Moon will pop into view high in the south. It looks like somebody sliced it like a ripe melon, with a clean line down the middle.
That line indicates the Moon is at first quarter -- it's one-quarter of the way through its month-long cycle of phases. If you drew lines from Earth to the Moon and Sun, they'd form a 90-degree angle. At that angle, sunlight illuminates exactly half of the lunar hemisphere that faces our way, so half is in sunlight, the other half in shadow.
There's quite a difference in those two halves. On the dayside, temperatures can climb to around 250 degrees Fahrenheit. And at night, they plummet to 200 below zero. This extreme range has been tough on spacecraft that have landed on the Moon; more about that tomorrow.
The difference is so large in part because the Moon has no atmosphere. On Earth, clouds in the atmosphere reflect a lot of sunlight back into space, so the surface doesn't get too hot. And at night, the atmosphere acts like a blanket, preventing heat from the surface from escaping into space. The atmosphere also distributes heat around the planet, so Earth doesn't experience such great extremes of heat and cold.
Incidentally, if you don't want to wait for sunset to watch the first-quarter Moon, you don't have to. It rises around midday, and climbs the eastern sky during the afternoon.
Script by Damond Benningfield, Copyright 2007
For more skywatching tips, astronomy news, and much more, read StarDate magazine.