Saturday, September 22, it’s all over (actually early morning, 1:54AM UTC on the 23rd, but that’s late Sat in North America…).  Summer is gone, and fall or autumn (whichever you prefer) is upon us.  This transition is marked by the Autumnal Equinox, which comes from latin – aequi (equal), and nox (night).  “Equal Night”, so yeah, this is the day that the day and the night are equal.  Except they aren’t.

A quick look at my “Sun-Moon” app (there’s an app for everything) shows that the duration of daytime – the period between sunrise and sunset – at my location is 12 hours, 9 minutes on the supposed day of “equal night”.  But given a 24 hour day, that means night will only last 11 hours, 51 minutes.  That’s an 18 minute difference!  I thought they were supposed to be equal!

Well, now hold on, is the day actually 24 hours?  No, as it turns out.  The varying speed of Earth in its orbit, along with the tilt of the Earth’s rotational axis with respect to the path of its orbit around the Sun make the length of an actual day, a full rotation, vary over the course of a year.  In fact, earlier this week, we had our shortest full day of the year – at only 23 hours, 59 minutes, and 40 seconds.  (This variation is very nicely explained by fellow blogger, The Science Geek, here.)

But still, we’re only talking about 20 seconds – that’s not nearly enough to account for my 18 minute night-day difference on the so-called Equinox.  So what’s going on here?  If I look at daylight duration, I won’t actually see day and night of roughly equal length (11:59 and 12:01, respectively) until the 26th – four days late.

If I look at the daylight duration for a location in the southern hemisphere – in fact, my position, but South latitude instead of North – I see that the daylight duration crossed 12 hours on the 19th, three days EARLY!  Down south, they’ve already had an “equal night”, well before the Equinox!

But there’s a clue here, surely?  If I look at what’s going on at the equator, they should match, so…. running the math….. carry the two (ok, actually just plug in zero latitude on the daylight calculator)…. and… What the WHAT?  12 hours, 7 minutes.  All year.  At the equator, day is ALWAYS longer than night.

Brain short-circuiting, picturing geometry, spherical earth, tilted axis, elliptical orbit.  Mind being blown…

And then it hits me…  Sunrise is measured from the first appearance of the Sun.  Not only does this mean we start the clock when the edge of the Sun is visible, which is well before the center rises, but we actually do it a little earlier than THAT, because sunlight bends around the Earth, lensing through the atmosphere – we can see the Sun before it is in a position to be seen via a straight line.  (Weird, I know – but the sunrise would look a LOT different without an atmosphere).

Similarly, sunset is measured when the last light disappears, a little bit AFTER the apparent position of the Sun pulls it below the horizon, and almost 3.5 minutes after the CENTER of the Sun has set.  The result – 7 minutes of extra daylight on the Equinox, even though the setting and rising of the CENTER of the Sun is (close to) an even 12 hours apart.  And this is true, roughly everywhere-ish.

And that reference to 1:54 am UTC on the 23rd?  That’s the precise time the Sun will cross the celestial equator.  The time when its path along the ecliptic (equatorial plane) intercepts our Earth’s equator.  Not a day, or a night, but a precise moment that defines when everything is equal.  More or less.

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