A wet moon (also called a Cheshire moon) is a lunar phase when the "horns" of the crescent moon point up at an angle, away from the horizon, so the moon's crescent takes on the appearance of a bowl or a smile. A wet moon occurs when the crescent moon is near the horizon at a point more or less directly above the sun's (invisible) position below the horizon. This in turn is determined by the earth's and moon's positions in their orbits, the inclination of these orbits relative one another and to the earth's axis, and observer's latitude on the earth. Wet moons occur routinely in the tropics (where the sun and moon rise and set nearly vertically), and rarely in polar regions (where the sun and moon rise and set at a glancing angle or not at all).
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The terms wet moon and dry moon originate from Hawaiian mythology, where it was thought that the moon appeared as a bowl which would fill up with rain. The period where this is most common, January 20 - February 18, corresponds with Kaelo the Water Bearer in Hawaiian astrology and makes the moon known as the "dripping wet moon". As the year passes into summer, the crescent shape shifts, pouring out the water and causing the summer rains. After the "bowl" empties, it dries out and rights itself, creating the "dry moon".
During the course of 26 weeks, the moon's path appears to move slightly southward in the sky, relative to the stars. This "movement" of the moon slowly changes its relative place to the horizon, as the motion of the Earth changes its tilt relative to the plane in which the moon orbits.
During summertime in the northern hemisphere, when the hemisphere tilts towards the Sun, the side of the Earth at nightfall is pointed away from the moon's orbit, making the lunar path appear closer to the southern horizon. This effect also causes the moon to appear lower in the night sky.
Wet moon/Cheshire moon
This process is reversed during wintertime in the northern hemisphere, causing the moon's path to appear nearly vertical to the western horizon, as well as causing the moon to appear higher in the night sky.
- StarChild Question of the Month for March 2002, NASA, accessed February 2006
- Ask the Astronomer: How often does a 'Dry Moon' happen with the horns of the Moon pointed straight up?, Astronomy Cafe, accessed February 2006
- "Wonderquest: Cheshire Moon, speedy sunset, languid eventide", USA Today, June 10, 2004
- http://www.kellscraft.com/ReadingWeather/readingweather09.html (Dry moon reference)
- Cheshire Moon by Jim Buckels (painted circa 1993)
- Cheshire Moon, a set of songs by Bruce (2004)