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Why don’t we get equal day and night on the equinox?

We don’t get exactly equal daylight and night until a few days after the equinox

Autumnal equinox begins at 9:31 a.m. on 9/22/2020
Autumnal equinox begins at 9:31 a.m. on 9/22/2020

ROANOKE, Va. – For the longest time, we have all heard that the start to fall or spring is when we receive equal daylight and equal darkness. While we’re splitting hairs, that isn’t exactly the case. In mid-latitudes, like our region, we receive roughly 12 hours and 8 minutes of daylight on the autumnal equinox (first day of fall).

Why is that?

The sun isn’t like a laser pointer. It’s a disk. Sunset is defined as when the upper limb of the sun touches the western horizon. So it’s not when the ball crosses the plane. The entire player has to be in the end-zone for it to be a touchdown, if that makes sense. In our part of the world, this definition of sunset adds 2-3 extra minutes of daylight.

Another reason is because of how the earth bends, or refracts, light. When the sun nears the horizon, the earth bends the light upward from the sun’s original position. This refraction delays the sunset by a few more minutes.

Sunrise and sunset times at the start of any season in 2020
Sunrise and sunset times at the start of any season in 2020

When do we receive equal daylight?

This happens on the equilux. In 2020, that happens on September 25th. The equilux typically comes a few days before the first day of spring and a few days after the first day of fall. This happens on a different date for different parts of the earth, while the equinox is a world-wide occurrence happening at the same exact time.


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