The next full moon will be on Friday, August 31, and will be quite special because it is the second within the calendar month. (The first one was on August 1.) This full moon is called a ‘blue moon,’ and will happen in the U.S. at 9:58 am EDT, or 6:58 am PDT.

The time between one full moon and the next is close to the length of a calendar month. So the only time one month can have two full moons is when the first full moon happens in the first few days of the month. That means a blue moon has come to mean something rare. In fact, the next blue moon won’t happen until July 2015.

Can there be two blue moons in a single calendar year? Yes. It last happened in 1999. There were two full moons in January and two full moons in March and no full moon in February. So both January and March had blue moons.

The next year of double blue moons is coming up in 2018.

But wait, there’s more!

There are two other definitions for blue moon. It can be the third of four full moons in a single season. The Old Farmer’s Almanac defined a blue moon as an extra full moon that occurred in a season. One season – winter, spring, fall, summer – typically has three full moons. If a season has four full moons, then the third full moon may be called a blue moon.

The next blue moon by this definition will fall on August 21, 2013.

Or you might actually see a blue-colored moon one day, but it’s pretty unlikely. Unusual sky conditions can create them as they did in 1883, when an Indonesian volcano named Krakatoa exploded. Plumes of ash rose to the very top of Earth’s atmosphere. And the moon turned blue. Some of the ash-clouds were filled with particles about 1 micron (one millionth of a meter) wide–the right size to strongly scatter red light, while allowing other colors to pass. White moonbeams shining through the clouds emerged blue, and sometimes green.

But don’t be misled by the photo above. The secret to photos like these lies in special photographic filters.

What does all this have to do with Neil Armstrong?

Friday, the day of the blue moon, is also the day of  a private funeral service for Armstrong, the first man to walk on the moon, who died last Saturday in Ohio at age 82.

Serendipitous timing!

Armstrong’s family has suggested paying tribute to him by looking at the moon and giving the astronaut a wink.