AUSTIN (KXAN) — You’ve heard of the term “Equinox” when we refer to the beginning of astronomical spring or fall.
Equinox in Latin means “equal-night,” but that meaning is not exactly true for the spring and fall equinoxes.
The fall equinox was on Sept. 22, but our day length in Central Texas DID NOT equal the length of our night. In fact, in Austin we had 12 hours and seven minutes of daylight on the day of the autumnal equinox. Depending on your latitude, you may have to wait a few additional days after the fall equinox until you get the 12 hours of daylight and 12 hours of night.
Why the difference? Two reasons:
- The sun appears as a disk and not a single point: The geometric center-point of the sun is only above the horizon for 12 hours on the first day of autumn (or spring), but the light from the sun extends all the way to its edge. The edge of the sun comes up over the horizon before the center does, and the edge of the sun goes down over the horizon after the center does. This adds up to additional daylight before and after the center of the sun appears or disappears.
- Refraction: The atmosphere causes refraction, a bending of light around the earth. That means that you’ll see the edge of the sun starting to appear over the horizon before it actually does. You’ll also see the light of the sun appear above the horizon even after the sun has actually disappeared below it.
These two factors add additional daylight and mean that we don’t see the 12 hour day/night until a little before the spring equinox and a little after the autumnal equinox.
The day you achieve the perfect balance — which happens twice a year — is known as the ‘Equilux’ which means “equal-light” in Latin.
Sunday, Sept. 26, was our autumnal ‘Equilux’ in Central Texas. Until the next Equilux we’ll have longer nights than days.