Equinox: Why the Length of Day & Night are Not Equal

The autumnal equinox occurs Wednesday morning at 3:20 am CDT, but the days and nights are not considered equal until Saturday, September 26th. This is because of the atmospheric refraction of sunlight.  When a wave encounters a medium whose density differs from that through which the wave has been propagating, conservation of momentum and energy dictate that the direction must change to account for the change in speed.  All waves can be refracted including sound, radio, and light, to name a few. Obviously, some are visible; others aren't.  One common example that we've all encountered is a pencil in a glass of water; the pencil appears to bend when viewed from certain angles.

Example of refraction

Example of refraction

The density of the medium that the light encounters will determine the extent of refraction. Although the density of Earth's atmosphere decreases rapidly with height, it is fairly constant globally (temporally/spatially) which makes our atmosphere's ability to refract sunlight quite predictable.  The sun's position relative to the Equator will be at full Equinox when autumn begins, but the atmosphere will bend the sunlight and create an optical illusion to our eyes. When you see the sun setting in west or rising in the east, it is not really where you think it is. Astronomically, the sun has already set or has yet to rise, but the atmosphere bends the light upward about 0.5 degrees, making the sun appear to be in a different spot above the horizon.  Collectively, this phenomenon adds 8 to 10 minutes of sunlight to the day here in the mid-latitudes.

The sunrise/sunset tables take refraction into consideration, so you will notice the sunrise/sunset being roughly 12 hours apart on Saturday. It takes about 3 days after the astronomical equinox before the duration of daylight and darkness become equal as the sun advances southward (i.e., as Earth tilts away from the sun here in the Northern Hemisphere).

Illustration of Light Refraction ("Bending" of Sunlight)

Optical illusion created by the bending of sunlight through the atmosphere (known as refraction). This physical process provides an extra 8 to 12 minutes of sunlight through the Equinox. The duration of day and night do not become equal for 3 days after the official Equinox.