When is the first day of fall in 2024?
A carefully worded answer is that on Sunday, Sept. 22, at 8:44 a.m. Eastern daylight time (5:44 a.m. Pacific daylight time) autumn begins astronomically in the Northern Hemisphere, and spring in the Southern. At that moment, the sun would be shining directly overhead as seen from a point in the equatorial Atlantic Ocean, 461 miles (743 km) south-southwest of Monrovia, Liberia.
The definition of the equinox as being a time of equal day and night is a convenient oversimplification. For one thing, it treats night as simply the time the sun is beneath the horizon, and completely ignores twilight. If the sun were nothing more than a point of light in the sky and if the Earth lacked an atmosphere, then at the time of an equinox the sun would indeed spend one half of its path above the horizon and one half below. But in reality, atmospheric refraction raises the sun’s disk by more than its own apparent diameter while it is rising or setting. Thus, when we see the sun as a reddish-orange ball just sitting on the horizon, we’re looking at an optical illusion. It is actually completely below the horizon.
In addition to refraction hastening sunrise and delaying sunset, there is another factor that makes daylight longer than night at an equinox: sunrise and sunset are defined as the times when the first or last speck of the sun’s upper limb is visible above the horizon — not the center of the disk.
And this is why if you check your newspaper’s almanac or weather page on Wednesday and look up the times of local sunrise and sunset, you’ll notice that the duration of daylight, or the amount of time from sunrise to sunset, still lasts a bit more than 12 hours, and not exactly 12 as the term “equinox” suggests.
In Indianapolis, for instance, sunrise is at 7:32 a.m. and sunset comes at 7:40 p.m. So, the amount of daylight is not 12 hours, but rather 12 hours and 8 minutes. Not until Sept. 25, are days and nights truly equal (sunrise is at 7:35 a.m., sunset coming 12 hours later).
