DENVER-Many 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 10:03 p.m. Mountain Daylight Time today (Sept. 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 Sept. 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 6:57 p.m., with each time rounded to the nearest minute. This yields 12 hours and 9 minutes.

What's going on

In reality, no day has exactly 12 hours of sunshine. In Denver, the days that most closely match 12 hours of sunshine are the 25th and 26th. Rounded to the nearest minute, on Sept. 25 there are 12 hours and 2 minutes of sunshine in Denver, 11 hours and 59 minutes on the 26th. Much the same is true for other cities.

(If you are not in Denver, you can compute the rise and set times for your location from this U.S. Naval Observatory page.)

So are astronomers, who normally are very accurate, wrong on this one? Is the equinox really on the 25th or 26th this year?

No, the astronomers are correct. The equinox occurs at precisely 10:03 p.m. Mountain Daylight Time on the 22nd. 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."

Sunrise coincides with the instant a bit of the Sun peeks above the horizon. But 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, sat in the west.

Warped perspective

The reason is 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 sat.

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 actual period of sunshine should be perhaps 11 hours 50 minutes, the lengthening due to refraction brings it up to 12 hours even (or almost).

So the equinox occurs on the 22nd, but the days with 12 hours of sunshine come 3 or 4 days later. In the spring, the opposite effect occurs. As days get longer, the observed equinox comes a few days before the official equinox.


Larry Sessions, a former planetarium director, writes, teaches astronomy and serves as an educational resource through the JPL/NASA Solar System Ambassadors program. He writes SPACE.com's monthly Sky Calendar and also provides sky information here. And you can e-mail him.

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