The autumnal equinox officially occurred on the 22nd.
However, it isn’t until today that our sunrise and sunset times reflect equal day and night length.
It all has to do with two things:
1.) Our location here on Planet Earth.
2.) How the Sun’s light interacts with the atmosphere.
Because our Earth is tilted at about 23.5 degrees, sunlight hits it at different angles.
Because of this, apparent sunrise and sunset times will vary according to latitude.
It isn’t by much, but it does vary.
For example, the Sun will appear to rise today in Chicago about one minute later than it does in Terre Haute.
One way to think about this is to remember that in the winter, the Sun barely rises at all in the higher latitudes of Canada and Alaska.
Our latitude, therefore, slightly impacts when we see the Sunrise and set.
The bigger impact on sunrise and sunset is how light travels through our atmosphere.
When the Sun is low on the horizon, its light is refracted or bent.
An example of bent or refracted light is when you look at the pavement on a hot summer day You’ll see wavy lines because light is being refracted.
The same thing happens when the Sun is low on the horizon.
In fact, when the Sun appears to rise or set, it is actually below the horizon! Because of refraction, sunrise and sunset times don’t exactly coincide with the astronomical equinox.
So, today almanacs and most sunrise/sunset websites will show that day and night are equal, even though astronomically the equinox was on Tuesday.