A lot of people know that on the Equinox, every location on Earth is supposed to get 12 hours of daylight. In fact, the term “Equinox” means “equal night,” signifying that the period of night should equal the period of daytime, and since the day is 24 hours long, we should have 12 hours of each. This year, the September Equinox occurs at exactly at 8:49 a.m. Mountain Daylight Time on Saturday, September 22. At that moment, Autumn or Fall begins for the Earth’s Northern Hemisphere. [Note that I avoid calling it the “Autumnal Equinox” simply because that is true only for half of the Earth. In the other half, the Southern Hemisphere, the same event marks the transition from Winter to Spring.]
So it might be expected that September 22 would have exactly (or almost) 12 hours of possible sunshine and 12 hours of night. But it doesn’t. A simple check of sunrise and sunset times will show that there are more than exactly 12 hours on that date. In Denver, the Sun rises at 6:48 a.m. and sets at 6:56 p.m., with each time rounded to the nearest minute. This yields 12 hours and 8 minutes.
In reality, no day has exactly 12 hours of sunshine. In Denver, the day that most closely matches this is September 25 this year. (It changes a bit from year to year, due mostly to the Leap Year cycle). Rounded to the nearest minute, the Sun rises at 6:51 a.m. and sets at 6:51 p.m. on September 25th in Denver. Much the same is true for other cities. (See link below.)
So are astronomers, who normally are very accurate, wrong on this one? Is the Equinox really on the 25th this year?
No, the astronomers are correct. The Equinox occurs at 8:49 a.m. Mountain Daylight Time on the 22nd (this year). Astronomy is an observational science, but in this case, observations can be misleading. The times of sunrise and sunset are given for the apparent rise and set times. In other words, given clear skies and an unobstructed horizon, the times given are those that the Sun would be seen to rise and set. The key word here is “apparent.”
The fact is that the Sun appears to rise a few minutes before it actually crosses the eastern horizon, and stays in the sky a few minutes after it has, in fact, disappeared below the western horizon.
The reason lies with the Earth’s atmosphere, which bends the sunlight around the horizon. It is somewhat like peering around the corner with a periscope. The atmosphere bends the Sun’s light around the horizon slightly, such that the Sun appears in the East a couple of minutes before it actually rises. In the evening the reverse happens – the Sun’s light is bent around the horizon to cause the Sun to remain visible for a few minutes after it has really dipped below the western horizon.
This is known as refraction, a fancy way of saying bending, due to the atmosphere. It is the same effect that causes a pencil stuck in a glass of water to appear offset slightly. Truth is, when you sight along the horizon, you are looking through much more atmosphere then when you look high in the sky. Because of this, the refractive effect is much greater near the horizon, even allowing objects that are technically just beyond the horizon to appear.
Thus on the day of the Equinox the theoretical period of sunlight is 12 hours, but because of this refraction or bending of sunlight around the horizon, the apparent day is longer.
However, at this time of year the days are getting shorter. So a few days later, when the theoretical period of sunshine should be significantly less than 12 hours, the lengthening due to refraction brings it up to 12 hours even (or almost).
So the Equinox occurs on the 22nd, but the day with 12 hours of sunshine comes 3 days later. (Again, this can vary a bit with geographic location…. for example, in Houston it is on the 26th). In the Spring, the opposite effect occurs. As days get longer, the observed equinox comes a few days before the official Equinox.
(If you are not lucky enough to live in Denver, you can compute the rise and set times for your location from this US Naval Observatory page: http://aa.usno.navy.mil/data/docs/RS_OneDay.php)