The rarity of a wet moon is not universal; it is highly dependent on geographical location.
How Rare Is a Wet Moon?
The occurrence of a wet moon, sometimes called a "Cheshire Cat moon" due to its upward-pointing horns resembling a smile, varies significantly based on one's latitude on Earth. It is a common spectacle in some regions while being quite rare in others.
Geographical Occurrence
The primary factor determining the frequency of a wet moon is the angle at which the Sun and Moon rise and set relative to the horizon.
| Region | Wet Moon Rarity | Reason |
|---|---|---|
| Tropics | Routine | The Sun and Moon rise and set nearly vertically in these regions. |
| Polar Regions | Rare | The Sun and Moon rise and set at a glancing angle, or may not rise or set at all, making the specific celestial alignment for a wet moon uncommon. |
In the Tropics:
In areas near the Equator, such as tropical regions, wet moons are a routine and common sight. This is because the celestial mechanics dictate that the Sun and Moon's paths across the sky cause them to rise and set at a nearly perpendicular (vertical) angle relative to the horizon. This vertical orientation allows for the crescent moon to appear with its "horns" pointing upwards, creating the characteristic "wet" or "U"-shaped appearance.
In Polar Regions:
Conversely, as one moves closer to the Earth's poles, wet moons become rare. In polar regions, the Sun and Moon rise and set at a much more oblique or "glancing" angle. In some cases, during certain times of the year, they may not rise or set at all for extended periods. This shallow angle of ascent and descent makes it highly unlikely for the moon to be seen in the upward-facing crescent orientation required for a wet moon.
What is a Wet Moon?
A wet moon is an optical phenomenon where the crescent moon appears to be tilted on its side, with the tips (cusps) of the crescent pointing upwards. This gives the impression that it could "hold water," hence the name. This visual effect occurs when the ecliptic—the apparent path of the Sun and Moon across the sky—makes a large angle with the horizon during sunset or sunrise, allowing the illuminated sliver of the moon to align vertically.
