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The equinox–vernal in March, autumnal in September–is the time the sun crosses the Equator, officially changing the seasons. Today’s will take place at 9:04 p.m. Eastern time. The term “equinox” means “equal night.” In theory, sunrise and sunset are 12 hours apart, and we get equal hours of daylight and night.

But as this post at space.com points out, equality is in the eye of the beholder. All over the world.

How do you measure the moments of sunrise and sunset? When the center of the sun hits the horizon? When the leading edge touches, or the trailing edge? Or when the first (or last) glow shines? That alone will affect the timing by a few minutes.

And then you have to account for the effects of geography.

Our Connecticut home has a hill to the west. Sunsets in our backyard are about two hours earlier than at our place in Vermont. There, the horizon is low on the western plains past the far shore of the lake.

Then there’s the matter of twilight. Day and night are cut-and-dried on the moon, where there’s no atmosphere. There, you’re either in light or shadow. Here on Earth, light diffuses through the atmosphere even after the sun is down. What is the moment night starts?

Finally, the length of the day depends on your latitude. In the higher latitudes, inside the Arctic and Antarctic circles, perpetual twilight sets in before the final moment of winter darkness.

Yes. Winter nights are longer the closer to the poles you go. We see that in Vermont, but the night is nowhere near as long or dark as in the Arctic.

On the flip side, our part of Vermont gets long summer days, though nowhere as long–and perplexing–as those farther north. I leave you with these lyrics from Stephen Sondheim’s “A Little Night Music.” It’s based on Ingmar Bergman’s 1955 film “Smiles of a Summer Night,” set in the northern reaches of Sweden in mid-summer.

The sun sits low
Diffusing its usual glow
Five o’clock…
Twilight…
Vespers sound
And it’s six o’clock…
Twilight
All around…

But the sun sits low
As low as it’s going to go

Eight o’clock…

Twilight…

How enthralling!
It’s

Nine o’clock…

Twilight…

Slowly crawling
Towards

Ten o’clock…

Twilight…

Crickets calling…

The vespers ring
The nightingale’s waiting to sing
The rest of us wait on a string
Perpetual sunset
Is rather an unset-
Tling thing

The sun won’t set
It’s fruitless to hope or to fret
It’s dark as it’s going to get
The hands on the clock turn
But don’t sing a nocturne
Just yet

“The Sun Won’t Set” (Night Waltz I), “A Little Night Music” by Stephen Sondheim