As I have written here before, since Full Moons occur every 29.5 days, it is possible to have two Full Moons in a month and that second one is popularly called a “Blue Moon.”

We had a Full Moon to launch this month on July 1 (in the U.S.) and now the month will close out with another Full Moon tonight (the 31st).

Why blue? One might think that it goes back to some early person recording a second Full Moon in a month and that particular Moon appeared blue. Particles of dust of a particular size or smoke from large fire or volcanic eruption can cause a moon to look blue in color, but it is certainly not something that is predictable by date and this next Full Moon will probably appear no more blue than the one earlier this month. Moonlight does have more of a blue color (more so for a camera than our eyes) than the reddish light of sunrise and sunset. You often see that in films as a way to indicate night or even film “day for night.”

Actually, the use of the Blue Moon name seems to be quite modern. The March 1946 issue of Sky and Telescope magazine ran an article that defined the term as a second one in a month.

It is an unusual but not very rare occurrence and we can have two Blue Moons in a single calendar year. That happened in 1999 with two Full Moons in January and March and no full moon in February. We will have the next year of double Blue Moons in 2018.

We get a Blue Moon in the month of July every 19 years. This is the Metonic cycle and so in 2034 we’ll again have two full moons in July 2034 and another Blue Moon on July 31, 2034. Mark your electronic calendar.

Why is this? There are 235 full moons yet only 228 calendar months in the 19-year Metonic cycle. Because the number of full moons outnumber the number of calendar months, it means at least 7 of these 228 months will have two full moons. The math is simple enough for even me to understand: 235 – 228 = 7 extra full moons.

To add some complexity to our desire to wrap up our attempts to control the universe and time by making clocks and calendars, take this situation: If a February within this 19-year period has no full moon at all – as is the case in February 2018 – that means this extra full moon must fall within the boundaries of another month, too. In 2018 we will have two Blue Moons.

Anyway, enjoy this July 31st “blue” Full Moon.

For farmers, this was often called the Hay Moon. For Druids and some southern American Indian tribes, this late July Full Moon was a time when the harvest is celebrated.

I used a Cree tribe name for this late July Full Moon – the Moon When Ducks Begin to Molt. The Cree are one of the largest groups of Native Canadians/Native Americans in North America. There are over 200,000 members in Ontario, Manitoba, Saskatchewan, Alberta the Northwest Territories and Quebec. In the United States, this Algonquian-speaking people historically lived from Lake Superior westward, but today, they live mostly in Montana, where they share a reservation with the Ojibwe (Chippewa).

Like most birds, ducks shed, or molt,their feathers. They do this twice each year, with the first molt in early summer. New feathers grow in and push out the old ones. Ducks molt very quickly and in a few weeks, they lose all their feathers and grow a whole new plumage. During molting, they need to find a safe place to stay, because this is a dangerous time because they can’t fly. Molting ducks spend most of their time hiding in tall grass or floating out in deeper waters.

Ducks lose all their feathers during the first molt of the year and then have their summer feathers for a few months. Around September, they molt again, but only the body feathers fall out.