NASHVILLE, Tenn. (WKRN) — The fall equinox is Saturday, Sept. 23, and many people might think that means the day and night will be equal in length, but that’s not exactly the case.
While they are close to the same during an equinox, they are not quite equal. Instead, it’s called an equilux when the day and night are exactly the same. The term “equilux” is drawn from the Latin terms for equal and light.
When you’ll experience an equilux varies depending on where you are, but an equinox happens at the same time and date no matter the latitude. That’s because an equinox is based on when a point at the center of the sun crosses the equator, according to timeanddate.com.
To calculate an equilux, the sun is thought of more as a disk, meaning the top edge of the sun appears a little earlier and sets a little bit later than the center point. That creates a few extra minutes of daylight at different latitudes.
In Nashville, for example, the sun will rise at 6:38 a.m. and set at 6:38 p.m. on Tuesday, Sept. 26. But Hawaii’s equilux is two days later on Thursday, Sept. 28, with the sun rising at 6:22 a.m. and setting exactly 12 hours later.
In most latitudes, there are two days out of every year where the length of the day and night are exactly the same. Those days happen a few days before the spring equinox and a few days after the fall equinox in the Northern Hemisphere.
It’s the other way around for people living south of the equator. However, those living on or very close to the equator will never experience equal day and night.