The moon takes on a blue appearance due to tiny water droplets in the air in Watauga, Texas in October, 2003. (The moon is not actually blue) (NASA/Tom King)

Here in Washington, we get two full moons in August. One is tonight (Aug.1) and the next on Aug. 31. In recent decades, the popular-notion, folksy people define a blue moon as the second full moon in a month. But in order to have a second full moon in a one-month period, you must have a first.

Officially, the moon becomes full tonight Aug. 1 at 11:27 p.m. (If you’re attending the London 2012 Olympic games, then it’s Aug. 2, 3:27 a.m., Universal Time.) The second full moon falls on Aug. 31 at 9:58 a.m. here in Washington, while in London, it’s at 1:58 p.m. (Source: Observer’s Handbook, Royal Astronomical Society of Canada.)

Barring clouds, tonight’s full moon should be easy to spot. It’s a negative 12 magnitude and it’s loitering within the constellation Aquarius. Late August’s blue moon can be spotted in the constellation Pisces.

The blue moon isn’t a serious subject. It’s not even really blue. Our calendar is a tool devised by humanity and it happens that two full moons fall within that given monthly space.

But hanging out at the Astronomy Café, you can learn amazing things about blue moons. In 1999, you may recall we had two full moons in January and March. And February had no full moons at all.

In 2018, we’ll enjoy double blue moons again in January and March – and once again February is devoid of official lunar plump. Odenwald – the author of “The Astronomy Café,” “Back to The Astronomy Café,” and “Patterns in the Void” – explains on his website that the lunar month is 29.53 days long. The largest number of days in February is 29 days, so February will never see a blue moon.

Rare among the rare: Blue moons can occur in months with only 30 days – and that happened Nov. 30, 2001, Odenwald says. We have two more decades before that happens again on Sept. 30, 2031.

On many different levels, blue moons have a bright, festive future: Get your Halloween costume ready for the Oct. 31, 2020 and the Oct. 31, 2039 events. You can ring in 2029 with a blue moon on Dec. 31, 2028.

“Once in a blue moon”: This saying is in reference to the rarity of the blue moon. The famous 1934 love ballad “Blue Moon” by Richard Rodgers and Lorenz Hart was also likely inspired by this uncommon event.

Sturgeon moon: While second full moons are conversation starters, we still have our first full moon to name. For August, it’s the Sturgeon Moon, so says the Old Farmer’s Almanac. The venerable almanac explains that fishing tribes near the Great Lakes are credited with naming this moon – identifying the season when ample sturgeon were caught.