Wednesday (Sept. 22) marks the equinox, which, thanks to its Latin name meaning "equal night," is often thought of as the day when dark and light each claim 12 hours.
But that isn't the case, and twilight is to blame for the confusion surrounding the astronomy of an equinox.
Let's backtrack. Astronomically speaking, fall begins in the Northern Hemisphere (and spring in the south) on Sept. 22 at 3:21 p.m. EDT (1721 GMT. That's when the sun will be shining directly overhead as seen from a point in the equatorial Pacific Ocean, 1,600 miles (2,580 kilometers) southwest of Mexico City.
But Northern Hemisphere locations will still see slightly more than 12 hours of daylight, despite the equinox designation.
Not so equal
The definition of the equinox as being a day of equal daylight and night is a convenient oversimplification.
For one thing, it treats night as simply the time the sun is beneath the horizon, completely ignoring 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 — the sun is actually completely below the horizon.
Moreover, 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. This is why if you look up the times of local sunrise and sunset on Wednesday, 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.
In Chicago, for instance, sunrise is at 6:38 a.m. and sunset comes at 6:47 p.m. So, the amount of daylight is not 12 hours, but rather 12 hours and 9 minutes. Not until Saturday (Sept. 25) are the day and night truly equal (sunrise is at 6:41 a.m., sunset comes 12 hours later).
On Sept. 22 at the North Pole, the sun traces out a 360-degree circle around the entire sky, appearing to skim just above the edge of the horizon. At the moment of this year's autumnal equinox, the sun should theoretically disappear completely from view, and yet its disk will still be hovering just above the horizon. Not until 50 hours and 44 minutes later will the last speck of the sun's upper limb finally drop completely out of sight, more than two days later.
This strong refraction effect also causes the sun's disk to appear oval when it is near the horizon. The amount of refraction increases so rapidly as the sun approaches the horizon, that its lower limb is lifted more than the upper, distorting the sun's disk noticeably.
Not as dark as it seems
Certain astronomical myths die hard. One of these is that that the entire arctic region experiences six months of daylight and six months of darkness. Often, "night" is simply considered to be when the sun is beneath the horizon, as if twilight didn't exist. This fallacy is repeated in innumerable geography textbooks, as well as travel articles and guides.
But twilight illuminates the sky to some extent whenever the sun's upper rim is less than 18 degrees below the horizon. This marks the limit of astronomical twilight, when the sky is indeed totally dark from horizon to horizon.
There are two other types of twilight. Civil (bright) twilight occurs when the sun is less than 6 degrees beneath the horizon and is loosely defined as when most outdoor daytime activities can be continued. (Some daily newspapers provide a time when you should turn on your car's headlights, usually corresponding to the end of civil twilight.)
So even at the North Pole, while the sun disappears from view for six months beginning on Sept. 24, to state that "total darkness" immediately sets in is hardly the case! Civil twilight does not end there until Oct. 8.
The last type of twilight is nautical twilight, which ends when a sea horizon becomes difficult to discern, typically when the sun drops down to 12 degrees below the horizon. At the end of nautical twilight, most people will regard night as having begun. At the North Pole we have to wait until Oct. 24 for nautical twilight to end.
Finally, astronomical twilight — when the sky indeed becomes completely dark — ends on Nov. 13. It then remains perpetually dark until Jan. 28, when the twilight cycles begin anew. So, at the North Pole the duration of 24-hour darkness lasts almost 11 weeks, not six months.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications. Follow us on Twitter @Spacedotcom and on Facebook.