Eighteen years, eleven and one-third days — that's a Saros. It's the result of a cosmic close-coincidence among three independent astronomical quantities: the lunar month, the precession of the plane of the moon's orbit, and the time between lunar perigees. Every 6,585.3+ days the relative configuration of the Sun, the Moon, and the Earth repeats, almost precisely. (Footnote: whether it's 11 days or a wee bit less past the 18-year mark varies, depending on the number of leap-year days involved.)

So every 18+ years "the same eclipse" happens again. The Earth has turned an extra ~8 hours, however, and eclipse tracks relative to locations on the surface of the planet are thus likewise shifted by ~120 degrees of longitude. And there are other slight misalignments which make for a further slow drift away from exact repetition. Those errors accumulate enough that after about a dozen centuries a new eclipse family begins.

It's an astounding discovery: clear patterns in dramatic heavenly events that extend across decades and beyond. The Saros Cycle was recognized by ancient Babylonian astronomers. Some have speculated that Stonehenge is related to a triple-Saros 54+ year multi-cycle. And modern astrologers have not been shy about taking advantage of the Saros for their petty purposes.

But real events are infinitely more awe-inspiring than pseudoscience could ever be. Cicadas emerge in massive cohorts every 17 years (and other prime-number cycles) to avoid or overwhelm predators ... annual meteor showers wax and wane with planetary perturbations of comet orbits ... sunspot activity and solar flares move in 11 year patterns, driven by magnetohydrodynamic forces ... the alignment of the Earth's axis changes, both on timescales short (Chandler wobble) and long (precession of the equinoxes) ... the Earth's magnetic field varies, as do climates, as do ocean currents, and as do countless other phenomena great and small.

As Gerard Manley Hopkins says (in "God's Grandeur"):

This weekend, on the night of 8-9 November 2003, a lunar eclipse will occur — as one did near the end of October 1985, and as will another in the later part of November 2021. My twins were infants last time; I hope to be happily retired next. The prior instance of this eclipse saw me entering adolescence. The previous time to that, my wife was a baby; my parents were still in high school.

Saros — bridges to past and future ...

And in 2004 Venus will transit the Sun — for the first time in well over a century. More on that anon!

(see also NoiseAndPredictability (14 Sep 1999), AllSunsets (26 Apr 2000), GlassDarkly (1 Jan 2003), ... )

TopicScience - TopicPersonalHistory - TopicPoetry - 2003-11-06

(correlates: BovineMind, ElusivePimpernel, SemiAstrophysicist, ...)