Saturday, March 20, 2010

How complicated can it be?

240px-Ecliptic_path My optician frowned into the computer screen: “It’s very complicated.”

No, really, it’s just a lens that keeps slipping out of a cheap pair of reading glasses.  ‘tightening a screw and a dab of glue?

He sighed, reached for a screwdriver.  “No, not that.  It’s the lentenachtevening.”

Lente I recognized: spring season, similar to Lent Term at Cambridge.   I leaned in: his screen held several paragraphs from


  He gestured, nodding.  “What day is the equinox?  It says it can be between the 18th and the 22nd?”

It’s today,  the 20th, the first day of spring.

I taught astronomy at Cascadia College: it’s not complicated.  Easter is complicated.

Equinox is the precise moment that the  sun shines vertically above the equator, twice each year.  Days and nights are of equal length, 12 hours each, at every location on earth.  Yes, there are refractive and scattering effects of the atmosphere that make this not exactly true, but close enough.

Equinox advances by roughly six hours each year, occurring at 12:44 last year, 18:32 this year, 00:21 next year.  In four years it advances a full day, but the date (almost) always remains on the 20th.  This is the cumulative effect of there being 365.25 days in a year, with the leap year adjustment working to hold the date steady.

He fiddled with my glasses, adjusted the frame and the lens, handed it back; I was still reading the Dutch Wikipedia article.  You know, darned if it didn’t say that there was a span of possible days, along with a  technical note about solar precession.

“Complicated.”, he affirmed, shaking his head..

So, I had a fun afternoon trying to track this all down, no luck yet finding the reason.  But there is, indeed, a steady shift of the Gregorian calendar relative to the tropical year.  As  a result, vernal equinox moves progressively earlier, always fallingg on March 20 or 21 between  1797 and 2043, but falling on March 18 for the first time in AD 4092.

So, yes, it’s complicated, but only when talking about it with the Dutch, taking the very (very) long view….

1 comment:

Ladybird & butterfly said...

interesting! Thanks for sharing this with us :D It makes me curious now too...