(Image credit: David Clapp/Getty Images)
When is the first day of fall in 2022?
A carefully worded answer is that on Thursday (Sept. 22) at 9: 04 p.m. EDT (0104 GMT on Sept. 23), autumn begins astronomically in the Northern Hemisphere, with spring kicking off in the Southern Hemisphere. At that moment, the sun would be shining directly overhead as seen from a point in the equatorial Pacific Ocean, 300 miles (500 km) west of Yaren, a district of the Pacific nation of Nauru.
This date is called an equinox, from the Latin for “equal night,” alluding to the fact that day and night are then of equal length worldwide. But this is not necessarily so.
Not so equal equinox
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 (Sept. 21) 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 St. Louis, Missouri, for instance, sunrise is at 6: 48 a.m. local time with sunset coming at 6: 57 p.m. local time. So, the amount of daylight is not 12 hours, but rather 12 hours and 9 minutes. It won’t be until Sept. 25 that the day and night are truly equal (sunrise is at 6: 52 a.m., with sunset coming 12 hours later).
And at the North Pole, the sun currently is tracing 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, it should theoretically disappear completely from view. And yet its disk will still be hovering just above the horizon. It’s not until 75 hours and 29 minutes later that the last speck of the sun’s upper limb will finally drop completely out of sight.
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.
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Not as dark as it seems
Certain astronomical myths die hard. One of these 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 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 exists when the sun is less than six degrees beneath the horizon. (Your closed fist held out at arm’s length covers 10 degrees of the sky.) It 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. That time usually corresponds to the end of civil twilight.
So even at the North Pole, while the sun disappears from view for six months beginning on Sept. 25, to state that “total darkness” immediately sets in is hardly the case! Civil twilight does not end there until Oct. 8.
When the sun drops down to 12 degrees below the horizon, it marks the end of nautical twilight, when a sea horizon becomes difficult to discern. In fact, at the end of nautical twilight most people will regard night as having begun.
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At the North Pole we have to wait until Oct. 24 for nautical twilight to end in 2022. 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 (opens in new tab). He writes about astronomy for Natural History magazine (opens in new tab), the Farmers’ Almanac (opens in new tab) and other publications. Follow us on Twitter @Spacedotcom (opens in new tab) and on Facebook (opens in new tab).