Something new about the Equinox

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The Equinox happens Sunday at 4:44 in the afternoon. It’s a familiar story: Autumn begins. Day and night are sort of equal. Night expands at its fastest rate. Nearly everyone knows those basics.

Then there’s the next tier of subtler knowledge. It includes these facts: The day of the Equinox, the Sun moves in a perfectly straight line across the sky. It rises precisely in the east and sets due west. And it’s the rare day when everyone in the world has more minutes of sunshine than solar absence: kind of a smiley-face occasion. (Perfect day/night equality happens a few days later, the middle of next week.)

You also hear wilder stuff, like those who regard it as a day of equilibrium, and imagine that eggs can balance on end. (Try it. It’s ridiculous). But there really is a kind of balance or symmetry that day, and it’s easy to picture why.

Imagine Earth on Sunday, as seen from space. On that day, the line separating day from night, the Terminator, runs exactly from pole to pole. Each pole barely sits in full sunlight. That’s the symmetry. This means that no matter where you live on our spinning globe, you meet the Terminator perpendicularly. This is what ensures that the Sun rises and sets due east and west.

But let’s get even more geeky. To do so, you have to teleport yourself back to fifth-grade Earth Science, when they talked about latitude. The latitude at the pole is 90 degrees, and it’s zero degrees at the Equator. Each latitude line is about 69 miles from the next. The exact 42nd parallel cuts through West Hurley and passes between Kingston and Saugerties. You can tell because your car hits a big bump on Route 28 as you cross the line.

Actually, I made that up. Nonetheless, if like me, you’re somehow obsessed with recognizing the 42-degree spot, it’s just past Route 375 heading west on Route 28, marked by a self-storage place on the right. Imagine that: You can stash your stuff at the precise 42nd Parallel.

How does this relate to the Equinox? Well, on Sunday, the midday height of the Sun, its distance from the exact overhead point, exactly equals your latitude – meaning, at the Equator, latitude zero, the Sun hovers zero degrees from the zenith: straight up. At either pole, latitude 90 degrees, the Sun hovers 90 degrees from overhead, which means that it sits on the horizon there. In our neck of the woods, the meridian Sun stands about halfway up the sky this weekend.

And now, I suspect, you’ve had more than enough, all things being equal.

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