Earth “falls” into a new season today – astronomically speaking. It’s the September equinox, when the Sun crosses the equator from north to south. It marks the start of autumn in the northern hemisphere, and spring in the southern hemisphere.

On the equinoxes, neither the north pole nor the south pole tips toward the Sun, so night and day are roughly the same length in both hemispheres – about 12 hours between sunrise and sunset.

We say “roughly” because there are a couple of caveats.

One is the way we calculate the times of sunrise and sunset. For the days to be truly equal, we’d have to mark the times when the Sun is bisected by the horizon – half in view, half still hidden. But we don’t. Instead, sunrise is the moment when the Sun first peeks into view, and sunset is the moment when the limb of the Sun drops from view. That adds a couple of minutes to the day.

The other correction factor is Earth’s atmosphere. It “bends” the sunlight above the horizon. So when we see the Sun standing just atop the horizon, it’s actually a little below it.

That combination adds a few minutes to the equinox “day.” So at the equator, daylight lasts for 12 hours plus six and a half minutes. At 30 degrees north – the latitude of Austin – it’s 12 hours and eight minutes. And at 60 degrees – roughly the latitude of Anchorage – it’s 12 hours and 16 minutes – an extra dose of sunlight as we fall into autumn.

Script by Damond Benningfield