When is the first day of fall in 2024?
A carefully worded answer is that on Sunday, September 22, at 8:44 a.m. Eastern Daylight Time (5:44 a.m. Pacific Daylight Time), astronomically, fall begins in the Northern Hemisphere and spring begins in the Southern Hemisphere. At that moment the sun would shine directly overhead as seen from a point in the equatorial Atlantic, 743 km south-southwest of Monrovia, Liberia.
This date (like March 20th last year) is called the equinox, which comes from Latin and means “equal night” and alludes to the fact that day and night are then the same length around the world. However, this is not necessarily the case.
Not so equal
The definition of the equinox as Time of equal days and nights is a convenient simplification. Firstly, it treats night simply as the time when the sun is below the horizon and ignores twilight completely. If the sun were nothing more than a point of light in the sky and if the Earth had no atmosphere, then at the time of the equinox the Sun would actually spend half its journey above the horizon and the other half below it. But in reality, atmospheric refraction raises the Sun’s disk by more than its own apparent diameter as it rises or sets. So when we see the Sun as a reddish-orange ball just sitting on the horizon, we are seeing an optical illusion. It is actually at the bottom the horizon.
In addition to refraction, which speeds up sunrise and delays sunset, there is another factor that causes daylight to be longer than night. equinox: Sunrise and sunset are defined as the times when the first or last spot of the sun upper extremity is visible above the horizon – not above the center of the disk.
Therefore, if you look in the almanac or on the weather page of your newspaper on Wednesday to see the local sunrise and sunset times, you will find that the length of daylight, or the period between sunrise and sunset, is still just over 12 hours, rather than exactly 12, as the term “equinox” suggests.
In Indianapolis, for example, the sun rises at 7:32 a.m. and sets at 7:40 p.m. The duration of daylight is therefore not 12 hours, but 12 hours and 8 minutes. Only from September 25th will day and night really be the same length (sunrise at 7:35 a.m., sunset 12 hours later).
And at the North Pole, the sun is currently making a 360-degree circle around the entire sky and appears to be gliding just above the horizon. By the time of this year’s autumn equinox, it should theoretically disappear from view completely, and yet its disk is still hovering just above the horizon. It will not be until almost 51 hours later that the last spot at the top of the sun will finally disappear from view.
This strong refraction effect also causes the solar disk to appear oval as it approaches the horizon. The refraction effect increases so rapidly as the sun approaches the horizon that its lower edge is raised more than the upper one, noticeably distorting the solar disk.
Not as dark as it seems
Certain astronomical myths persist. One of them is that there are six months of daylight and six months of darkness throughout the Arctic. Often it is simply assumed that it is night when the sun is below the horizon, as if there is no twilight. This fallacy is repeated in countless geography textbooks as well as travel articles and guidebooks. But twilight illuminates the sky to a certain degree when the top of the sun is less than 18 degrees below the horizon. This marks the limit of astronomical twilightwhen the sky is actually completely 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. It is roughly defined as the time when most outdoor activities can continue during the day. Some newspapers give a time when you should turn on your car’s headlights. This time usually corresponds to the end of civil twilight.
Even though the sun disappears from view at the North Pole for six months starting on September 24, it can hardly be said that “total darkness” sets in immediately! Civil twilight there only ends on October 8.
When the sun drops 12 degrees below the horizon, it marks the end of nautical twilight, at which point the ocean horizon becomes difficult to see. In fact, most people consider the end of nautical twilight to be the beginning of night. At the North Pole, we have to wait until October 24 for nautical twilight to end. Astronomical twilight – when the sky actually goes completely dark – finally ends on November 13. Then it remains permanently dark until January 28, when the twilight cycles begin again. So at the North Pole, the 24-hour darkness lasts almost 11 weeks, not six months.
Joe Rao is a lecturer and visiting professor at the New York Hayden PlanetariumHe writes about astronomy for Journal “Natural History”The Farmers’ Almanac and other publications.
