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September 23, 2011

The Moon swings past Regulus, the bright heart of the lion, the next couple of mornings. Regulus is well to the lower left of the Moon tomorrow, and closer to its upper left on Sunday.

The Moon passes Regulus like the hand of a giant clock — once every 27-and-a-third days. The time it takes the Moon to make one full turn against the background of stars is known as the sidereal month. That same interval applies to the Moon’s passage by any of the stars along its path.

Not many cultures have relied on the sidereal month as part of their calendar system. But many have relied on another lunar interval: the cycle of the Moon’s phases, which is a couple of days longer. By the time the Moon returns to any given star along its path, Earth and the Moon have moved a good distance around the Sun, so the Sun appears in a different direction related to the more-distant stars. It takes a couple of more days for the Moon to catch up to the Sun and begin a new cycle.

The modern western calendar is based not on the Moon’s motion across the sky, but on the Sun’s. And on that calendar, today is an important date for the Sun. It’s the September equinox — the date the Sun crosses the equator headed from north to south. The precise moment of the equinox, by the way, is 4:05 a.m. Central Time.

The equinox marks the beginning of autumn in the northern hemisphere, and spring in the southern hemisphere — one more marker on the celestial calendar.


Script by Damond Benningfield, Copyright 2011


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