The first of August’s two full moons occurred on August 1st as a preview for a so-called “blue moon” rising on Aug. 31. The next time we see two full moons in a single month will be July 2015.

High levels of dust or ash in the atmosphere can make the moon take on various hues, like the orange “harvest” Moon, but “blue” moons aren’t named for their color– they usually look like any other full moon in the sky. Astronomers refer to the second full moon in a single month as a “blue moon.”

The term “once in a blue moon” has long been used to denote rare occurrences, as blue moons appear on average once every 2.7 years.

The existence of blue moons is a result of the fact that lunar months don’t exactly match our calendar 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.

Here’s a lunar fact for you: in 1999 sky watchers saw two blue moons in the span of just three months!


NLSI Associate Director Doris Daou clearly explains the Phases of the Moon in this ‘Ask an Astronomer’ episode.

Posted by: Soderman/NLSI Staff
Source: http://eclipse.gsfc.nasa.gov/phase/phases2001.html

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