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Moon and Companions

February 21, 2014

The last-quarter Moon has a couple of bright companions in the wee hours of tomorrow morning. The planet Saturn is close to its upper right at first light, with the star Antares a little farther to the lower left.

The Moon actually reaches its last-quarter phase a few hours later, when it stands at a right angle to the Sun in our sky.

The name “last quarter” is a bit misleading. At first and last quarter, sunlight illuminates half of the lunar hemisphere that faces Earth. It looks as though someone sliced the Moon right down the middle.

But “first quarter” and “last quarter” refer not to the Moon’s appearance, but to its position in orbit. The beginning point of its cycle of phases comes at new Moon, when the Moon passes between Earth and Sun. About a week later, the Moon is one-quarter of the way through its cycle, so it’s at first quarter. A week later comes full Moon, when the Moon lines up opposite the Sun in our sky and is fully illuminated. A week after that it's last-quarter, then back to new, which completes the 29-and-a-half-day cycle.

Last-quarter Moon produces some of the lowest tides. Ocean tides are produced by the gravitational pull of the Moon and Sun. When the Moon is new or full, the Moon and Sun pull along the same line, so their combined gravity produces high tides. But when the Moon’s at a quarter phase, the Sun and Moon pull at right angles to each other — partially cancelling the other’s pull.


Script by Damond Benningfield, Copyright 2014

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