Wednesday (September 22) marks the equinox, which, thanks to its Latin name meaning “equal night,” is often considered the day when darkness and light claim 12 hours each.
But that’s not the case, and twilight is to blame for the confusion surrounding the astronomy of an equinox.
Let’s back up. Astronomically speaking, fall begins in the northern hemisphere (and spring in the south) on September 22 at 3:21 pm EDT (1721 GMT. It is then that the sun will shine directly over our heads as seen from a point in the equatorial Pacific Ocean). 2,580 kilometers (1,600 miles) southwest of Mexico City.
But Northern Hemisphere locations will still see a little over 12 hours of daylight, despite the equinox designation.
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Not so equal
Defining the equinox as a day of equal day and night light is a convenient oversimplification.
For one thing, it treats night simply as the time when the sun is below 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 pass half its path above the horizon and half below it. But in reality, atmospheric refraction raises the sun’s disk by more than its own apparent diameter while it is rising or setting. Therefore, when we view the sun as a reddish-orange ball sitting on the horizon, we are seeing an optical illusion: the sun is actually completely below the horizon.
Furthermore, sunrise and sunset are defined as the times when the first or last speck of the upper limb of the sun is visible above the horizon, not the center of the disk. That’s why if you look up the local sunrise and sunset times on Wednesday, you’ll notice that the length of daylight, or the amount of time from sunrise to sunset, still lasts a little over 12 hours.
In Chicago, for example, sunrise is 6:38 am and sunset is 6:47 pm So the amount of daylight is not 12 hours, but 12 hours and 9 minutes. Only on Saturday (September 25) are day and night truly the same (sunrise is 6:41 am, sunset is 12 hours later).
On September 22 at the North Pole, the sun traces a 360-degree circle around the entire sky, which appears to glide just above the edge of the horizon. At the time of this year’s autumnal equinox, in theory, the sun should completely disappear from view, and yet its disk will still float just above the horizon. It will not be until 50 hours and 44 minutes later that the last speck of the upper extremity of the sun will finally disappear completely, more than two days later.
This strong refractive effect also makes the solar disk appear oval when close to the horizon. The amount of refraction increases so rapidly as the sun approaches the horizon that its lower extremity rises higher than its upper extremity, noticeably distorting the solar disk.
Not as dark as it seems
Certain astronomical myths take time to die. One of them is 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 below the horizon, as if twilight did not exist. This fallacy is repeated in countless geography textbooks, as well as articles and travel guides.
But twilight illuminates the sky to some extent when the upper edge of the sun is less than 18 degrees below the horizon. This marks the limit of astronomical twilight, when the sky is 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 below the horizon and is loosely defined as when most daytime outdoor activities can continue. (Some newspapers provide a time when you should turn on your car’s headlights, which usually corresponds to the end of civil twilight.)
So even at the North Pole, while the sun disappears from view for six months starting on September 24, saying that “total darkness” sets in immediately is not the case! Civil twilight doesn’t end there until October 8.
The last type of twilight is nautical twilight, which ends when the sea horizon becomes difficult to discern, usually when the sun falls 12 degrees below the horizon. At the end of nautical twilight, most people will consider that the night has begun. At the North Pole we have to wait until October 24 for the nautical twilight to end.
Finally, the astronomical twilight, when the sky actually turns completely dark, ends on November 13. It then remains perpetually dark until January 28, when the twilight cycles begin again. So at the North Pole, the 24-hour duration of darkness lasts for almost 11 weeks, not six months.
Joe Rao serves as an instructor and guest speaker at the Hayden Planetarium in New York. He writes about astronomy for Natural History magazine, Farmers’ Almanac, and other publications. Follow us on Twitter @Spacedotcom and on Facebook.