THE AUTUMNAL EQUINOX—

To astronomers this is one of the two points on the celestial sphere where the ecliptic and the celestial equator intersect. The other point, naturally, is the vernal or spring equinox.

To the rest of us this represents the beginning of fall in the northern hemisphere and it happens when the sun appears to cross the celestial equator from north to south.

This year that event happens at 5:51 a.m. EDT, Sunday, September 23rd—more or less.

The word “equinox” derives from Latin and means equal nights. On the 23rd this year, day and night are of approximately equal length everywhere on Earth.

While pure science salivates over accuracy, we ordinary mortals have to take these observations at the approximate level. Watch the sun set for example. It takes quite a few minutes from the time the disc touches the horizon until the top of the disc disappears entirely.

When, then, did the sun actually set?

Then, there is the small inconvenience of diffraction, or light rays bending through the earth’s atmosphere—so, what we are seeing isn’t necessarily precisely what we are seeing.

For our purposes, fall this year will arrive about, approximately, between 5:45 and 6 a.m. this Sunday—more or less. That’s close enough.

I am going to enjoy pondering the fact daylight is about even everywhere on this day.

I am not going to enjoy what I know is coming in a couple of months.