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Volume 14, Issue 52 ~ December 28 - January 3, 2006

Sky Watch

by J. Alex Knoll

Riding the Celestial Merry-Go-Round

It is we who spin like creatures within a sphere

Not so long ago, people imagined the heavens to be a dome above us, while the sun, moon and planets, the stars and constellations all circled within, rotating around a stationary earth. Think back to a time before Newton’s laws of physics and Galileo’s telescope, and it’s not so far fetched.

Just look to the sky: There, revolving like the figures on a merry-go-round, are the major celestial objects: the moon and Saturn, plus the zodiacal constellations Pisces, Aries, Taurus, Gemini, Cancer and Leo. Conversely, the sun, as explained here last week, arcs low in the sky, reaching its southern-most point on solstice.

Every 365 days the winter sun reaches its nadir, while the full moon nearest solstice reaches its zenith. At summer solstice, the opposite is true. Where it takes the sun one year to complete its north-to-south-and-back cycle, the moon completes its own cycle in 281⁄2 days.

Rather than a spinning sky above earth as envisioned by our early ancestors, it is we who spin like creatures within a sphere, better yet a gyroscope, revolving at a 231⁄2-degree tilt. It’s this tilted rotational axis that causes the solstices, at which point the sun reaches its lowest or highest point, 231⁄2 degrees below or above the celestial equator.

The moon, too, revolves around a tilted axis, in this case five degrees, wandering five degrees north at its peak and five degrees south at its nadir.

Every 19 years, the moon’s 281⁄2-day cycle coincides with the sun’s 365-day cycle, producing a winter moon that climbs exceptionally high — 281⁄2 degrees above the celestial equator, the combined maximum shift of the 231⁄2 -degree solar peak and the five-degree lunar peak. Tuesday’s full moon marks the peak of that 19-year cycle.

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