DENVER (KDVR) — Ask most people and they will tell you that the spring or fall equinox is the date when day and night are equal in length.
That, however, is not actually the case.
Day and night last for around the same time on the equinoxes, but it isn’t exact. There is a term for when the two are exactly the same: equilux.
What is the equinox, and when is it?
The equinox is a very specific thing astronomically and happens on the same date and time no matter where you are. It is, put most simply, when Earth’s axis is not tilted away or toward the sun.
Earth experiences two equinoxes a year: the spring equinox, which marks the start of astronomical spring, and the fall equinox, which marks the start of astronomical fall.
When the equinox happens is based on the equator, which is why it happens at the same time no matter where you are. This year, the fall equinox will occur Sept. 23 at 12:50 a.m. Mountain Daylight Time.
What is the equilux, and when is it?
Equilux is the date when day and night are equal lengths. Unlike the equinox, it depends on where you are or, more specifically, your latitude.
The equilux happens a few days before the spring equinox and a few days after the fall equinox. For example, in Denver, the sun will rise at 6:51 a.m. and set exactly 12 hours later at 6:51 p.m. on Sept. 26, three days after the equinox.
And in Hawaii, the equilux happens two days later on Sept. 28, when the sun rises at 6:22 a.m. and sets at 6:22 p.m.
One way to find out when the equilux is where you are is to use the National Oceanic and Atmospheric Administration’s Solar Calculator.
Depending on where you are, there might be a difference of around one or two minutes between the sunset and sunrise on this year’s equilux.