One of two times each year when night and day are of the same length all over the world. The beginning of autumn.
Equinox is derived from the Latin aequinoctum — equal night. In the northern hemisphere it denotes the transition from summer to autumn, after which night becomes longer than day.
Pilgrims lived by astronomy. Plants and animals live by astronomy: the movement of the Earth and the stars. That we are alive to spend our days talking about equinoxes is thanks to a fluke of momentum, which keeps the planet wheeling around the sun at the right angle and distance to keep burgeoning cells from boiling, or bursting from ice crystals forming in their cytoplasm.
Thankfully, Earth's path is consistent. Astronomy is an exact science.
Pilgrims were exiled as rogue Christians. While they would eventually engage in their own forms of intolerance, They stooped to a crude form of Paganism when it was to their benefit. They watched the sky enough to see the autumnal equinox coming; it signaled the year's second harvest, and the last feast before the lengthening night. Thanksgiving is a by-product of Earth's path in space.
Those who more overtly worshipped the heavenly bodies referred — refer — to the day as Mabon. Of course, the Pilgrims wouldn't catch on to the idea until the 1600s. In either case, traditions formed according to need: feast before the winter.
Like I said, astronomy is an exact science. Technically, the autumnal equinox is a moment in time rather than an entire day: when, around September 22, the sun crosses the celestial equator moving southward. That's for the northern hemisphere. South of the equator, it's around March 20, when the sun crosses the equator going north.
The sun crossing the celestial equator sounds elegant, but it doesn't have much meaning without explanation.
The planet is tilted. 23.44° to be exact. Earth passes around the sun. About half the time, the top half of Earth is tilted toward the light; about half the time, well, it's the bottom half.
Twice during its orbit, the angle between sun and earth are such that the northern and southern hemispheres receive equal amounts of the sun's light. If you are looking at earth from space, the shadow of the sun runs perpendicular to the equator. This is what equinox means: what it would be if the world weren't tilted. The autumnal equinox precedes the northern hemisphere's tilt away from the sun — winter. Because the seasons are reversed between the hemispheres, autumnal equinox isn't really an appropriate name. After all, only half the earth is actually moving into autumn. This is why it's alternatively called the southward equinox.
All over the world on an equinox, the sun rises at 6AM and sets at 6PM. Twelve hours of daylight. But it's not exact. Astronomy is exact, sure — but it's not needle-fine. It's good enough to plan a party around, but it's not chemistry.
Have you ever seen a chart of the timezones? They seem arbitrary, don't they? The lines zigzag, swerve to envelop clusters of islands, even form pockets. They are a product of our need for convenience; frequently, our place in a timezone doesn't match our local time. Which makes sense. Going by local time, small countries and conceivably even cities would find themselves divided between an hour's gap. So, the 6AM/PM figure falls to pieces in some places: some places, the sun on an equinox can rise at 8am and set at 10pm.
Even when you disregard human intervention, you still find a few minutes slipping into day or night. As the earth circles the sun, its speed changes. The result: lose or gain about eight minutes of daylight, depending on the equinox. In autumn, naturally, you lose eight minutes.
Now, did I say equal day and night all over the world? Whoops.
If you are precisely, smack dab on the Equator, yes, this formula of equality holds true. But move north or south a little bit, and you start to find some discrepancies. In moderate latitudes — think the UK, the northern US, and some of Canada — you find a difference between day and night approaching twelve minutes. As you get further away from the equator, the imperfections grow in a nearly perfect arc: at pole, the sun smiles for 24 hours on an equinox day.
A chart1 approximating the autumnal equinotical periods, past, present and future. Times are GMT.
9/22/1980 21:01 | 9/23/1990 07:06 | 9/22/2000 17:11
9/23/1981 02:50 | 9/23/1991 12:54 | 9/22/2001 23:00
9/23/1982 08:38 | 9/22/1992 18:43 | 9/23/2002 04:48
9/23/1983 14:27 | 9/23/1993 00:32 | 9/23/2003 10:37
9/22/1984 20:15 | 9/23/1994 06:20 | 9/22/2004 16:25
9/23/1985 02:04 | 9/23/1995 12:09 | 9/22/2005 22:14
9/23/1986 07:52 | 9/22/1996 17:57 | 9/23/2006 04:02
9/23/1987 13:41 | 9/22/1997 23:46 | 9/23/2007 09:51
9/22/1988 19:29 | 9/23/1998 05:34 | 9/22/2008 15:39
9/23/1989 01:18 | 9/23/1999 11:23 | 9/22/2009 21:28
More interesting stuff?
Both equinotical periods mess up communications equipment. Briefly, the sun is exactly behind, say, a geostationary satellite, and the signals are overwhelming enough to degrade hardware.
snopes.com houses an article detailing the old European folk tale which posits that, on an equinox day, you can balance an egg on its point. Not true, obviously. But it bespeaks the then-new fascination with gravity.
Briefly, the autumnal equinox was the French Republican Calendar's New Year's Day — specifically, between 1793 and 1805. When French Republicans usurped the monarchy on (significantly) September 21, 1792, they declared the day the start of the new year.
1 Chart ganked from Eric Weisstein's World of Astronomy. The figures came from a mathematics application in software package Scientific Astronomer. Weisstein's site derived slightly different times than those calculated by the US Navy — sometimes with up to twelve minutes' difference — but they're close enough.
Sources
Equinox and Solstice
http://www.equinox-and-solstice.com/html/autumnal_equinox.html
8/5/2006
Kansas University Medical Center
http://www3.kumc.edu/diversity/other/autumnal.html
8/5/2006
Wikipedia
http://en.wikipedia.org/wiki/Equinox
8/5/2006
Wolfram Research
http://scienceworld.wolfram.com/astronomy/AutumnalEquinox.html
8/5/2006
For the Quest
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