John Wheeler: Tomorrow's equinox is not all that equal
For a number of reasons, the day is longer than the night on the equinox.
FARGO — Tomorrow (Sept. 22) is the autumnal equinox, often referred to as the day in autumn when the night and the day are both exactly 12 hours. Actually, this is not precise. Sunrise is when the top tip of the sun shows above the horizon, but sundown is when the entire sun has gone below the horizon. The time it takes for the disk of the sun to go below the horizon makes the day slightly longer than the night.
Another issue is light refraction. The bending of light at the horizon adds a little bit more time to the day at both sunrise and sunset. A third complicating factor is the fact that Earth rotates while traveling an elliptical orbit around the sun. The time it takes Earth to spin around with respect to the sun (solar day) is slightly different than the 24 hours by which we measure a day. The elliptical orbit of Earth keeps that difference changing slightly throughout the year.