Mazen Saadeh
Mazen Saadeh took this image of the July 3 full moon from Bchemoun, Lebanon.
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updated 8/1/2012 2:32:46 PM ET 2012-08-01T18:32:46

The first of August's two full moons occurs Wednesday night, offering a lunar preview for a so-called "blue moon" rising at the end of the month.

The moon will reach its full phase at 11:27 p.m. EDT tonight, then come back big and bright with a full blue moon on Aug. 31. Night sky observers won't have a chance to see two full moons in a single month again until July 2015.

While high levels of dust or ash in the atmosphere can make Earth's nearest neighbor take on various hues, blue moons aren't named for their color. They usually look like any other full moon in the sky.

The term has always been associated with an "extra" full moon, but its meaning has changed over time. Earlier this century, for example, it referred to the third full moon of a season — spring, summer, winter or fall — that contained four full moons instead of the usual three.

But that definition, found in the Maine Farmers Almanac, was misinterpreted several times over the years, and today we call the second full moon in a single month a "blue moon."

The term " once in a blue moon " has long been used to denote an absurd or unusual occurrence, but blue moons aren't really that rare, coming on average once every 2.7 years.

The existence of blue moons is a result of the fact that calendar months aren't exactly lined up with lunar months.

It takes 29.5 days for the moon to travel around the Earth, during which time we see the satellite go through all of its phases. But all calendar months (except for February) have 30 or 31 days, so it's possible to squeeze two full moons into a month every now and again.

In 1999, in fact, moon watchers saw two blue moons in the span of just three months.

If you snap any good photos of tonight's full moon and would like them to be considered for a future story or gallery, please send them to Space.com Managing Editor Tariq Malik at tmalik@space.com.

Follow Space.com senior writer Mike Wall on Twitter  @michaeldwall  or Space.com  @Spacedotcom.  We're also on  Facebook  and  Google+.

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Video: Once in a blue moon

  1. Closed captioning of: Once in a blue moon

    >>> one of them london showers there is the moon over london and tonight's full moon is special because it is the first of two in the month of august. the second one comes on the 31st and as such makes it a blue moon . this two moons in a month event won't happen again until

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