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Moon and Companions

July 13, 2010

The Moon races past four bright planets and stars the next few nights. Tonight, they're all to the upper left of the Moon: the star Regulus, the brilliant planet Venus, and the planets Mars and Saturn.

The Moon itself is a thin crescent this evening. Its "cusps" -- the tips of the crescent -- aim to the upper left, toward its companions.

Folklore calls that position a "dry" Moon; if you poured water into the cup formed by the crescent, the water would run out. And when the cusps point straight up, it's a "wet" Moon. In ages past, many folks relied on that angle to tell them if the weather would be stormy or fair.

While some astronomical folklore has a basis in fact, this bit does not. The angle of the cusps depends entirely on the relative positions of the Moon and Sun.

At this time of year, the path that the Sun and Moon follow across the sky is tilted in early evening. So as the crescent Moon sets, it slips sideways instead of straight down. The leading edge of the lunar disk is facing in the direction of the Sun -- in this case, to the lower right. So sunlight illuminates the lower right portion of the disk as seen from Earth. As a result, the cusps of the crescent aim in the opposite direction -- to the upper left.

The cusps point up around the spring equinox in March. The Moon looks like a cup then -- although the folklore that it foretells wet weather just doesn't hold water.

More about the Moon and its companions tomorrow.

Script by Damond Benningfield, Copyright 2010

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