The autumnal equinox arrives Wednesday. Here’s what to know about it as Philly’s days shorten before our eyes.

The sunsets are becoming evermore glorious, and soon the leaves will be rivaling them for color. The planets are putting on an amazing celestial show nightly, and the region is about to host some of the briskest bird traffic of the year.

But on Wednesday, the equinox day when the astronomical autumn arrives in the Northern Hemisphere at 3:20 p.m. Philadelphia time, for reasons that transcend the fact that we are yet again entering a pandemic fall, not everyone is in the celebratory spirit.

We are in the period when the curtains of darkness are closing at their most rapid pace of the year. The sunrise-to-sunset time on Wednesday will be more than 2½ minutes less that it was on Tuesday, and close to three hours less compared with the solstice day in June.

“It’s amazing how quickly it starts to happen,” said Phil Gehrman, a clinical psychologist at the Hospital of the University of Pennsylvania who specializes in sleep and circadian rhythm disorders.

The loss of light clearly affects some people, he said, adding that a certain sluggishness and a desire for more sleep would be common responses, which some studies have linked to light-related hormonal changes.

Gehrman is more circumspect.

“It’s more the effects of the light exposure on our behavior, as opposed to a direct effect on our physiology,” he said. He said it could be a diluted variant of what animals experience: “There’s probably just some small corollary of hibernation.”

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He said the explanation for an increased sleep appetite might be simpler: “People normally are not getting enough sleep. They just may be getting closer to what they should be doing 12 months of the year.”

He advises people to just be aware that the light is eroding radically and to take some simple steps, such as sitting by windows and going outside more often. And there is a lot to see out there these days.

Visually, not much; you won’t feel a thing; and your shadow will look like it does on other days at 3:20 p.m. Astronomically, however, this is mega-event foreshadowing significant changes in the weather.

At the instant of the equinox — and it’s only an instant — the sun will beam directly above the celestial equator, that imaginary line that divides the planet in two. At the summer solstice, direct light was about 1,500 miles to the north, on the Tropic of Cancer, thus the longer days and the stronger sun up our way. Winter will begin when it’s over the Tropic of Capricorn, 1,500 miles to the south.

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At the equinox — derived from the Latin for “equal night” — day and night aren’t precisely equal, but it’s close. At Philadelphia’s latitude the closest we get to even split is Sept. 26, when the day will be a minute and 16 seconds longer than the night. From then until the winter solstice, nights will increase their advantage.

While pondering the skies, you can see Jupiter and Venus every evening these days, along with Saturn. Venusis apparent in the southwest sky, and the sun is politely getting out of the way earlier and earlier.

To the southeast “creamy colored” Jupiter is dominating, with Saturn in the background, and they are coming ever closer together, says Derrick Pitts, astronomer at the Franklin Institute.

He points out that Saturn is about 885 million miles from Earth, and Jupiter, 484 million.

“The gulf between them is almost the same as the distance from Earth to Jupiter,” Pitts said. “People now get a more three-dimensional view of the solar system. It’s no longer dots on a velvet curtain.”

Jupiter and Saturn are roughly the same size, yet Saturn appears so much smaller, attesting to the distance.

Closer to Earth, this is the harvest season for local bird-watchers, says Keith Russell, a program manager for Audubon Pennsylvania. “It’s a pretty wonderful time in the world of birds,” he said.

The last week in September and the first week in October “are really the height of fall migration,” he said.

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“Hawks are migrating in great numbers,” he said. He highly recommends a visit to Militia Hill at Fort Washington State Park.

He also advises keeping an eye on the weather forecasts. Passing cold fronts are great importers of migrators. “That’s when we get really big numbers,” he said. “Those are the best days to go look for them.”

In the Southern Hemisphere, the astronomical spring arrives at 3:20 p.m., but evidently most Australians won’t be partying. By official decree, spring begins on Sept. 1, which is the first day of the meteorological spring down here, and meteorological fall up this way.

Once upon a time this was New Year’s Day in France. The French revolutionaries overthrew both the government and the Gregorian calendar, and in 1792, Sept. 22, the day of the autumnal equinox, became day one of the new year.

Like the new French Republic, it didn’t last. Napoleon might have preferred wearing his New Year’s Eve party hat on Dec. 31. In any event, the Gregorian system was reinstated on Jan. 1, 1805.