(Astronomical) fall officially arrives at 8:05 p.m. Tuesday with the September equinox will occur as the Sun is directly over the equator. Nearly an hour after sunset here, that sub-solar point along the equator will be on the other side of the Earth in the Pacific Ocean at a point along the equator north of Tonga and Fiji.
The word “equinox” comes from Latin for “equal night.” But if you check actual sunrise and sunset times, you will find that the day is longer than 12 hours pretty much everywhere on Earth. That difference leads to another astronomical term, the equilux, or the day closest to 12 hours.
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For mid-Atlantic and most of the southern states, equilux occurs on Saturday September 27. Here in Raleigh the Sun will be (geometrically) above the horizon for 19 seconds shy of exactly 12 hours that day.
The equilux falls the day before north of a line extending from Manhattan to California's Napa Valley, and the day after for the tip of Texas and most of Florida.
Why does the equilux date change with latitude?
The equilux falls on different dates because the Sun’s path across the sky appears a bit different depending on where you are on Earth. Near the equator, the Sun rises and sets on a relatively steep path, while farther north or south it follows a shallower path.
That differencs changes how quickly daylight, or the amount of time the Sun spends above the horizon, lengthens or shortens.
Each equilux map is a little different
You might expect the March and September equilux maps to be mirror images. The Sun crosses the celestial equator in both cases, after all. But Earth’s orbit is slightly elliptical, so Earth does not travel around the Sun at exactly the same speed throughout the year. Earth’s axial tilt also means the Sun’s apparent north-south movement is not identical in March and September.
Earth’s changing distance from the Sun also slightly changes the apparent size of the solar disk, although that is a smaller effect. Combined with atmospheric refraction and the way sunrise and sunset are defined, these factors produce the different patterns seen on March and September equilux maps.
If you'd like to get deep into the geometry of calculating the date of the equilux, the python code used to generate these maps is available on my GitHub. It uses the same NASA datasets to calculate sun rise and set times across several thousands points that is used in planning navigation of NASA missions.