Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2004 July 23
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Tonight: A Blue Moon
Credit & Copyright: Vic Winter (ICSTARS)

Explanation: How often does a full moon occur twice in a single month? Exactly once in a Blue Moon. In fact, the modern usage of the term "Blue Moon" refers to the second Full Moon in a single month. Tonight's Blue Moon will be the first since November 2001. A Blue Moon typically occurs every few years. The reason for the rarity of the Blue Moon is that the 29.53 days between full moons is just slightly shorter than the number of days in the average month. Don't, however, expect the moon to look blue tonight! The term "Blue Moon" has recently been traced to an error in a magazine article in 1946. It is possible for the Moon to appear tinged by a blue hue, sometimes caused by fine dirt circulating in the Earth's atmosphere, possibly from a volcanic explosion. The above picture was taken not during a full moon but through a morning sky that appeared dark blue. The bright crescent is the only part directly exposed to sunlight - the rest of the Moon glows from sunlight reflected from the Earth. In this dramatic photo, however, the planet Jupiter is also visible along with its four largest moons.

Tomorrow's picture: cosmic mayonnaise

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