Rare Blue Moon Rises in the Sky This Week: How and When to Watch

First Posted: Jul 27, 2015 08:33 AM EDT
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If you only do something once every blue moon, you better start preparing. The end of July will feature one of these moons as the second full moon of the month graces the skies.

Once in a while, two full moons occur within a month. This is due to the fact that while calendar months are about 30 days, a lunar month is 29 days. This second full moon is actually called a "blue moon," which isn't named for its color.

A blue moon occurs every two and a half years, on average. While this moon won't actually appear to be blue, though, there are instances in which it will.

In 1883, an Indonesian volcano named Krakatoa exploded. Plumes of ash rose to the top of Earth's atmosphere and made the moon appear to be blue for months and years afterward. A similar phenomenon can be seen during large forest fires, which also send particles in the air that color the moon at night. For this to happen, the particles need to be slightly wider than .7 micron.

So why is the moon blue if it isn't actually blue? It actually stemmed from a March 1946 issue of Sky and Telescope magazine, which contained an article called "Once in a Blue Moon." The author of the article actually simplified the definition of the moon, and it was later used in a radio show in the late 1970s to talk about a second blue moon in a month.

The full moon will actually occur July 31 at 6:43 a.m. Make sure to look to the skies the night before on July 30 to catch a glimpse.

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