Fall arrives this morning. This is a shot from a big Fall Festival happening today in the Smoky Mountains. (I do video weather for their tourism site. Clip follows this post.)
The Autumnal Equinox happens at 10:49 Saturday morning. On the equinox, the earth’s tilt is perfectly perpendicular to the sun’s radiation. If you were on the sun it would appear Earth was tilted neither toward or away from you. At the equator, the sun will be directly overhead at solar noon today.
This is the day every point on the earth will have 12 hours of daylight and 12 hours of darkness. Sorta, kinda. Your mileage may vary with latitude, for a couple of reasons.
Sunrise is defined as the moment light becomes visible from the sun’s disk. Since the edge of the sun’s disk is visible before the center point of the sun, this produces a little more daylight time. Sunset is defined as the moment the sun’s disk is no longer visible. Sunset occurs after the sun’s center point has set. This also produces a little more daylight.
There is also refraction within the atmosphere. Refraction makes the sun appear higher than it really is. This produces more daylight time than would occur if the earth had no atmosphere. (Handy that it does.)
For people living in the mid-latitudes, the period of exactly 12-hours of light occurs a few days after the fall equinox and a few days before the spring equinox. In Indy the sun will rise at 7:35 and set at 7:36 on Tuesday, the 25th. (AM, and PM, respectively. This isn’t Alaska!)