Fall will officially arrive Thursday morning at 9:21AM and the first day of fall is officially known as the Autumnal Equinox. You know the day where we get exactly 12 hours of day and night. Well, that isn’t exactly true and viewer Mark Armstrong from Erie, Illinois wanted to know why and I’m sure he is not the only one.
There are a couple of reasons why sunrise and sunset aren’t exactly 12 hours apart on the equinox. Number one has to do with our atmosphere. It refracts (bends) light so it actually makes the sun appear higher than it actually is creating more sunlight than if we had no atmosphere. This means we are able to see the sun before it actually rises. Number two has to deal with how we define sunrise and sunset. We define sunrise as when we first see light on the horizon from the sun’s disk and not the center of the sun. Obviously the tip of the sun will visible before the center of the sun creating more sunlight. Same goes for sunset. It is defined when the last part of the sun disk goes below the horizon and not the center. For these reasons above mid latitude areas, like here, don’t actually have 12 hours of day and night until a few days after the Autumnal equinox and a few days before the Vernal equinox. Happy fall!