The moon, like many children, goes through phases. And, just as children’s phases have names (“terrible twos,” puberty, etc. ) the moon’s phases have names too, though we rarely use them nowadays.
Three terms are easy: When the moon is fully in shadow and not readily visible at all, it’s a “new moon”; when the whole surface facing the earth is alight, it’s a “full moon.” Just before and after “new” moon, we have a “crescent moon,” so called because only a small arc is visible.
It’s the between phases that have the unusual names.
As the surface of the moon becomes more lighted each day on the way to full, it’s “waxing.” “To wax” means to increase in size gradually, and used to be a frequent synonym for “grow,” according to the The Oxford English Dictionary. It was first used to describe the moon around 970, the OED says.
And as the surface becomes less lighted each day on the way back to “new,” it is “waning,” which, not surprisingly, means to decrease in size gradually. And, also not surprisingly, it appeared at the same time (and in the same place) as “waxing” did in reference to the moon. “Wax and wane” have always gone hand in hand, like “rise and fall.”
Then there’s the “gibbous moon,” when it’s more than a semicircle. “Gibbous” means “convex, rounded, protuberant.” The OED says “gibbous” was first applied to the moon in 1690, though it came into English more than 200 years earlier.
So we can have a “waxing gibbous moon” and a “waning gibbous moon,” though we see those terms outside of astronomical contexts only once in a blue moon.
The expression “once in a blue moon” means something that happens rarely, like a blue moon. Some news reports have said a “blue moon” is the second full moon in a single calendar month, but that’s a recent explanation, and a more common phenomenon. The truth is older and a little more complicated, as Sky and Telescope magazine has reported.
We have four seasons, each three months long (though winter always seems longer), so most seasons have three full moons. Full moons have names, in Native American and farmer lore, the most important ones being the full moons falling closest to the equinoxes and solstices that signal the change of seasons: The last full moon before winter (that will be the first weekend in December this year) is called the Moon Before Yule, or the Cold Moon in Native American and farmer lore. But because the lunar cycle, at 29.5 days, is shorter than all our calendar months, a fourth full moon occasionally appears in a season. To avoid confusion, the name of the full moon closest to the change of seasons needs to stay the same, even if it’s the fourth one. So the extra full moon is the third full moon that occurs in a season when there are four full moons. That’s the “blue moon.”
A “blue moon” doesn’t happen often, or regularly. The next “blue moon” will be in July 2015, and it will be both kinds: the second full moon in a calendar month and the third in a season with four full moons. Set your calendars now!