You probably know what a Blue Moon is, but what about a wet moon? No, the term doesn’t refer to the question of whether there’s water on the Moon. (There is, albeit in different forms than you’ll find it on Earth.) You’ve probably seen a Wet Moon for yourself at some point, even if you didn’t know what it was called at that time.
Although only one Moon orbits the Earth, it’s sometimes called different names based on a variety of factors, including the time of year it is, what phase it’s in, whether it’s undergoing a celestial event, and more. The Wet Moon is one such name—and even though it might not be as familiar as a Blue Moon or as ominous-sounding as a Blood Moon, it’s notable all the same.
Here’s how it works.
The Eight Phases of the Moon
First, it’s worth remembering that the Moon doesn’t generate its own light; what we call “moonlight” is really sunlight reflecting off of the Moon. The Moon is always half-lit by the Sun and half in shadow—how much it appears to be lit to those of us on the Earth’s surface depends on the orbital positions of the Moon around the Earth, and the Earth around the Sun.
Sometimes, we see half of the Moon lit in its entirety. When we do, we refer to it as the Full Moon. Sometimes, however, the half that’s entirely in shadow faces the Earth. When the moon is in shadow, we can’t perceive it at all—that moon is called a New Moon. In between the Full and the New moons, we see portions of the lit side—which are called the Waxing Moon and the Waning Moon and are described as either Crescent or Gibbous. When exactly half of the lit side of the Moon is visible, we’re seeing the Moon’s First or Last Quarter.
These are what we call the Phases of the Moon, or the Lunar Phases. The Moon cycles from being a New Moon to a Waxing Crescent, from a Waxing Crescent to First Quarter, from First Quarter to Waxing Gibbous, from Waxing Gibbous to Full Moon, from Full Moon to Waning Gibbous, from Waning Gibbous to Last Quarter, and from Last Quarter to Waning Crescent every lunar month—equal to about 29 and a half days.
What Makes a Moon “Wet?”
We have different names for the Moon beyond its eight phases, based on what else is going on in the sky or according to our calendar at any given time. If there are two Full Moons in a month, for example, the second one is called a Blue Moon. Meanwhile, if there’s a second New Moon in a month, it’s called a Black Moon. And, in certain circumstances, we see what we call a Wet Moon.
The orientation of a Crescent Moon’s pointy ends, or cusps, vary depending on a few factors, including latitude and the time of year. Additionally, as NASA’s Star Child site explains, the tilt of the Earth on its axis—which you’re probably aware is responsible for the Earth’s changing seasons—affects the path the Moon appears to take in the night sky when we’re viewing it from Earth. Sometimes, the Moon looks as if it’s traveling towards the horizon at an angle, while at other times, it appears as if it’s moving towards the horizon straight down.
When these factors are combined, the cusps of a crescent moon can appear to be oriented in a way that we perceive as being “up,” as the Moon travels straight down towards the horizon. And when that happens, the Crescent Moon looks to us on Earth as if it’s U-shaped like a cup or a bowl—that is, like a vessel that can hold water. Ergo, this kind of Crescent Moon is sometimes referred to as a Wet Moon. Some also call it a Cheshire Moon—to fans of Alice in Wonderland, it resembles not a water vessel, but the Cheshire cat’s disembodied grin.
There is, by the way, such a thing as a Dry Moon, as well. The term Dry Moon refers to the Crescent Moon when its cusps appear to us on Earth not to be pointing upwards, but pointing to the sides—that is, the moon looks more like a letter C than a letter U. And, if a water vessel is thought of as U-shaped, well, a C-shaped container wouldn’t hold water; it would enable it all to spill out. For this reason, this C-shaped moon is called the Dry Moon.
Exactly when the Wet and Dry Moons occur depends on where you’re located on Earth. In the Northern hemisphere, the U-shaped Wet Moon makes its appearance in the Spring, while the C-shaped Dry Moon pops up in the Fall. In the Southern hemisphere, though, it’s the opposite—the Wet Moon occurs in the Fall, while the Dry Moon happens in the Spring. And, near the equator, the Wet Moon shows up twice—once in the Winter and once in the Summer.
A Crescent by Any Other Name
It’s worth noting that there’s some debate over which crescent moon cusp orientations that the “wet” and “dry” terms denote. The most commonly-cited one describes the U-shaped moon as the Wet Moon and the C-shaped moon as the Dry Moon. However, some prefer to think of the C-shaped Moon as the Wet one, because if it were holding water, then the water would be pouring out of it all over Earth. Meanwhile, the U-shaped moon holds in the water, keeping the Earth from getting wet, therefore making it the Dry Moon.
No matter what you call it, though, the next time you look up at the sky and see a Crescent Moon, take a look at which way the cusps are pointing. It may tell you more about the universe than you think.