Blue Moon

StarDate: December 30, 2009

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The full Moon casts its glow across the end-of-year sky the next couple of nights. And it's a special one: a Blue Moon -- the second full Moon in a calendar month.

There are actually lots of definitions for Blue Moon. The most common is just "a long time;" something that happens "once in a blue moon" is a rare event.

Another definition -- one that dates to the 19th century or earlier -- is the fourth full Moon in a three-month period. And another is more literal -- when the Moon actually looks blue. That can happen under certain atmospheric conditions, like when there's a lot of ash from fires or volcanic eruptions in the air.

The "second-full-moon-in-a-month" definition is more recent -- it's only been around since the 1980s, and in common usage since the '90s.

Like all the others, this type of Blue Moon is rare. The Moon's cycle of phases lasts about 29 and a half days. That means that most months have just one full Moon. In a 31-day month like December, a Blue Moon occurs if you have a full Moon on the 1st or 2nd, and another on the 30th or 31st. This month the Moon was full on the 2nd, and it'll be full again tomorrow at 1:13 p.m. Central Time.

And for skywatchers in Alaska, there'll be a partial lunar eclipse early tomorrow, as part of the lunar disk just dips into Earth's shadow. The rest of the United States will miss out on the eclipse. We'll just have to settle for another astronomical rarity: a Blue Moon.

Script by Damond Benningfield, Copyright 2009

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

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