Nearly the blue moon Wednesday night over D.C., with just a sliver missing. (John Sonderman via Flickr)

With a nod toward celestial mechanics — and how our calendar is designed — we’ll say goodbye to July with a full “blue moon,” a modern nickname for the second full moon within a calendar month.

The moon won’t actually be blue, it will look like all other full moons. The Washington area’s last full moon was July 1 at 10:20 p.m., and the moon officially becomes full again at 6:43 a.m. Friday morning. Our old lunar chum rises in D.C. at 7:37 p.m. tonight, but sets tomorrow morning at 6:21 a.m., just before it becomes completely full. It will appear again in the southeast sky at 8:23 p.m. Friday night – the day of the official full moon.

So why is it called a “blue moon?” The short answer: Don’t ask, since it’s tortuous, tangled and baffling in a Rube Goldberg kind of way. Based on layers of rules – such as farmer maxims, seasonal interpretations, Ecclesiastical calendar dictates and media misinformation passed down through generations – we are left with a veritable vegetable soup of explanations.

The second full Moon of July is just around the corner. According to modern folklore, it is a "blue moon." (NASA ScienceCasts)

In 2006, Donald W. Olson, Richard Fienberg and Roger Sinnott wrote an article for Sky & Telescope magazine called “What Is A Blue Moon?” to explain it. In Sky & Telescope magazine itself, conversant astronomers and editors held a virtual dialogue within its pages for years discussing almanac interpretations. All of that printed chatter morphed into the idea of a second full moon in a calendar month getting the nickname “blue.”

“With two decades of popular usage behind it, the second-full-Moon-in-a-month (mis)interpretation is like a genie that can’t be forced back into its bottle,” conceded Olson, Fienberg and Sinnott.

Actual blue moons

Occasionally, we can spot a real blue moon – or even a blue sun, explain scientists. Our visible blue sky is based on Lord Rayleigh’s law of scattering — without our atmosphere full of tiny particles and gases, the sky would be completely black. Instead, light is scattered by these particles to filter out all other colors except blue.

“On rare occasions, atmospheric aerosols can cause the disks of the sun and moon to appear blue,” explain scientists from the Truckee Meadows Community College in Reno, Nev. “In 1853, the volcano Krakatoa erupted, spewing dust into the atmosphere. For nearly a month afterward, the moon appeared blue over tropical regions of Earth.”

Thanks to the fine dust and light scattering, blue moons and suns also occur in dust-prone areas of Northern Africa and the Arabian Peninsula, they said.

A blue sunset on Mars, captured by Curiosity. (NASA)

Mars enjoys this phenomenon, too. NASA’s rover Curiosity captured a blue sunset on our neighboring red planet, as the first Martian sunset observed in real color by the rover. The image taken April 15, 2015 comes from the so-called left-eye camera of the rover’s Mast Camera, which sees color like humans. Thanks to the Mars’ atmospheric dust, the blue light penetrates the dust quite well – presenting a blue-effect that is pronounced at sunset.

In 2018 we’ll get special lunar circumstances — a blue moon in both January 2018 and March 2018, and no full moon at all in February that year. Sorry, February. And for giggles, nature throws us a colorful curve, since the January 2018 blue moon may be an orange-red due to the total lunar eclipse that evening. (For Washington, that eclipse occurs at moonset.)