The first day of fall in 2026 arrives Tuesday, Sept. 22, at 8:05 p.m. EDT (1205 GMT on Sept. 23), when autumn begins astronomically in the Northern Hemisphere and spring begins in the Southern Hemisphere.
At that moment, the sun will shine directly overhead from a point in the equatorial Pacific Ocean, 250 miles (400 kilometers) southeast of Tarawa, a coral atoll and the capital of the Republic of Kiribati in the central Pacific Ocean.
This date, like March 20, is called an equinox, derived from the Latin for "equal night," a reference to the idea that day and night are then of equal length worldwide. In practice, however, that is not exactly true.
Not so equal
The definition of the equinox as a time of equal day and night is a convenient oversimplification. It treats night simply as the period when the sun is below the horizon and ignores twilight. If the sun were only a point of light and Earth had no 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. In reality, atmospheric refraction raises the sun's disk by more than its own apparent diameter as it rises or sets. When we see the sun as a reddish-orange ball on the horizon, we observe an optical illusion: It is actually completely below the horizon.
In addition to refraction hastening sunrise and delaying sunset, another factor makes daylight longer than night at an equinox: Sunrise and sunset are defined as the moments when the first or last speck of the sun's upper limb is visible above the horizon — not the center of the disk.
That is why, if you check your newspaper's almanac or weather page on Tuesday and look up local sunrise and sunset times, you will see that daylight still lasts a bit more than 12 hours, not exactly 12 as the term "equinox" suggests.
In Pittsburgh, for instance, sunrise is at 7:07 a.m. and sunset comes at 7:16 p.m., giving the city 12 hours and 9 minutes of daylight. Days and nights are not truly equal there until Sept. 25, when sunrise is at 7:10 a.m. and sunset follows 12 hours later.
At the North Pole, the sun is now tracing a 360-degree circle around the sky, appearing to skim just above the horizon. At the moment of this year's autumnal equinox, it should theoretically disappear from view, yet its disk will still hover just above the horizon. Only 50 hours and 44 minutes later will the last speck of the sun's upper limb finally drop 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.
Not as dark as it seems
Certain astronomical myths die hard. One is the claim that the entire Arctic region experiences six months of daylight and six months of darkness. This misconception often defines "night" only as the period when the sun is below the horizon, ignoring twilight. It appears in many geography textbooks, travel articles and guidebooks. Yet twilight illuminates the sky whenever the sun's upper rim is less than 18 degrees below the horizon; beyond that point, astronomical twilight ends and the sky becomes fully dark from horizon to horizon.
There are two other types of twilight. Civil, or bright, twilight occurs when the sun is less than 6 degrees below the horizon. It is generally defined as the time when most outdoor daytime activities can continue. Some daily newspapers provide a time when drivers should turn on their headlights; that time usually corresponds to the end of civil twilight.
Even at the North Pole, where the sun disappears from view for six months beginning Sept. 24, "total darkness" does not set in immediately. Civil twilight does not end there until Oct. 8.
When the sun drops 12 degrees below the horizon, nautical twilight ends and the sea horizon becomes difficult to discern. At that point, most people would consider night to have begun. At the North Pole, nautical twilight does not end until Oct. 24. Astronomical twilight — the point at which the sky becomes completely dark — ends Nov. 13 and remains in effect until Jan. 28, when the twilight cycle begins anew. As a result, 24-hour darkness at the North Pole 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, Sky and Telescope, The Old Farmer's Almanac and other publications.
