One of two times each year when night and day are the same length. 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. 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 correct angle and distance to keep cells from boiling or rupturing with ice. Thankfully, Earth's path is consistent enough.

Pilgrims were exiled as rogue Christians. While they would eventually nurture their own intolerances, they stooped to a 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 reflection of Earth's path in space.

Those who more directly worshipped 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 fit around need: feast before the winter.

Technically, the equinox is a moment in time rather than a whole day: when, around September 22 in the northern hemisphere, the sun crosses the celestial equator moving southward. 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°. Earth circles 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 sunlight. If you are looking at earth from space the shadow 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.

Have you ever seen a chart of the time zones? They seem arbitrary, don't they? The lines zigzag, swerve to envelop clusters of islands, even form pockets. This follows convenience; frequently, our place in a timezone doesn't match our local time, which is sensible. Going by local time, small countries and conceivably cities would find themselves divided between an hour's gap. So, the 6AM/PM figure fails in some places: in 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 is that we lose or gain about eight minutes of daylight, depending on the equinox. In autumn, naturally, you lose eight minutes.

Latitude is also problematic. If you are precisely on the Equator, the aforementioned formula of equality holds true. But move north or south slightly and things bend. 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 move away from the equator, the imperfections grow in a nearly perfect arc. At pole, the sun shines for 24 hours on an equinox day.

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. 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.




Equinox and Solstice

Kansas University Medical Center


Wolfram Research

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