Thursday was the first day of fall, also known as the autumn equinox. But even though “equinox” comes from Latin words meaning “equal” and “night,” night and day weren’t exactly 12 hours each.

“It’s kind of a misnomer that the equinox is the day of equal daylight and equal darkness,” said Brian Brettschneider, a climate scientist with the National Weather Service Alaska Region. “But there is an actual day where it’s approximately equal daylight equal darkness.”

The date when they’re closest in length — the equilux — is still ahead of us.

Why aren’t day and night equal?

As the Northern and Southern hemispheres are tilted toward and away from the sun, there are two times each year when they are oriented such that they receive equal amounts of sunlight — the equinoxes.

“In the equinoxes we go between leaning in and leaning out and we’re sort of sideways to the sun,” said Andrew Fraknoi, an astronomer with the Fromm Institute at the University of San Francisco.

While day and night are about the same length on the autumn equinox, there is still a several-minute difference across the U.S.

In 2022, the autumn equinox was at 6:03 p.m. on Sept. 22. At this point, the center of the sun was shining directly on the Earth’s equator.

“The sun passing from one side to the other — from the point of view of Earth — is the equinox,” said John Erickson, who previously directed the planetarium at the Lawrence Hall of Science in Berkeley.

Bay Area residents, for example, had an extra eight minutes of daylight Thursday.

There are a few reasons for these extra minutes of sunlight.

The equinox is defined based on the position of the center of the sun. But the sun is not just a point in the sky. It appears as a disc, so the sun provides light to Earth before and after its center crosses the horizon — when we see the sun rise or set.

“By definition, if any part of the sun is above the horizon, then it counts as being daylight,” Brettschneider said.

That’s one reason we see more than 12 hours of daylight on the autumn equinox — the Earth’s atmosphere plays tricks with the light.

“We can actually see a little bit below the horizon, because the atmosphere kind of bends light rays a little bit,” Brettschneider said. Even after the sun is entirely below the horizon, this bending allows us to continue seeing the sun’s light.

Equilux is coming

It won’t be until Monday that Californians see equal hours of day and night — well, almost. The equilux is the date when the hours of daylight is closest to the number of hours of night.

The specific date varies by latitude. In the Northern Hemisphere, northern locales experience more dramatic changes in their amount of daylight each day compared with places at the equator.

“Here in Alaska, where I’m at, the daylight changes by six minutes a day,” Brettschneider said.

The more substantial time shifts in regions like Alaska mean day lengths drop to 12 hours more quickly. That’s why at higher latitudes, the gap between the autumn equinox and the equilux is smaller. Closer to the equator, daylight changes by fewer minutes a day after the equinox — it takes more days to make up the difference.

In Anchorage, the equilux is on September 25. But further south, like in Miami, the equilux isn’t until September 27.

In the Bay Area, the equilux will be on Sept. 26 — so you can rest easy knowing that you have more than 12 hours of daylight to enjoy this first weekend of fall.

Jack Lee (he/him) is a San Francisco Chronicle staff writer. Email: jack.lee@sfchronicle.com Twitter: @jacklee