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The view of the Earth from space shows which hemisphere gives the sunlight a part of the year. | Credit: NOAA
When is the first day of autumn 2025? The carefully formulated answer is that Monday, September 22, 14:19 Eastern Day Light (11:19 Pacific time) Autumn begins in the astronomically northern hemisphere and in the spring – in the south. At that moment, the sun shines directly above the head, as can be seen from the Equatorial Pacific point, 1.320 miles (2,124 km) southwest of Acapulk, Mexico.
Twice each year – around March 20 and September 22. – The sun It is said to “cross the equivalent or cross the line”, of course, that line is equator. These two dates are called an EquinoxFrom the Latin “equal night”, stating that day and night are equally length all over the world.
But that’s not necessarily that.
Not so smooth
Lychinox as an equal day and night time is comfortable with excessive simplification. On the one hand, at night, it is treated as a time when the sun is under the horizon and completely ignores twilight. If the sun was nothing more than the point of light in the sky and if the Earth lacked the atmosphere, the sun would actually miss half its knee above the horizon and half below. But in reality, the atmospheric refraction increases the sun’s drive more than the obvious diameter while it rises or sets.
So, when we see the sun as a pink orange ball just sitting on the horizon, we look at the optical illusion. It’s actually completely below Horizon.
In addition to the refraction that rushes sunrise and delaying the sunset, there is another factor for which daylight is longer than the night equivalent: sunrise and sunset are defined as a time when the first or last sun stain upper limb is visible above the horizon, not the center of the disk.
That is why on Monday you will check your newspaper almanac or weather page and look for a local sunrise and sunset, you will find that daylight or time from sunrise to sunset, still lasts just over 12 hours, not quite 12, as the term “equinox” shows.
For example, in Atlantic, sunrise is 7:25 am, and the sunset comes at 7:33 p.m. So, the number of daylight is not 12 hours, but rather 12 hours and 8 minutes. Only until September 26, day and nights are really smooth peaches (sunrise is 7:28, sunset comes after 12 hours).
And The North axis, Currently, the sun sets a 360 degree circle around the entire sky, it seems to be sinking just above the edge of the horizon. During this autumn equinox, it should theoretically disappear from the image, but its disc will still fluctuate directly above the horizon. Only after 52 hours the last point of the sunshine will finally disappear completely.
This strong refraction effect also causes the sun disc oval when it is near the horizon. The amount of refraction increases so quickly as the sun approaches the horizon that the lower limb is lifted more than the upper and noticeably distorts the sun drive.
The South Pole Telescope, Amundsen – Scott South Pole Station, is experiencing seasonal changes. | Credit: Keith Vanderlind
Not as dark as it seems
Certain astronomical myths die hard. One of them is that the entire Arctic region experiences six months and six months of darkness. Often the ‘night’ is just considered when the sun is under the horizon as if twilight did not exist. This mistake is repeated in countless geography textbooks, as well as in travel articles and guides. However, the twilight illuminates the sky to some extent, with less than 18 degrees below the horizon on the upper edge of the sun. It marks the boundary of the astronomical twilight, when the sky is actually completely dark from the horizon to the horizon.
There are two more Twilight types. Civil (light) twilight exists when the sun is less than 6 degrees under the horizon. It is freely defined as when most outdoor activities can be continued. Some newspapers give a time when you should turn on your car’s headlights. This time usually corresponds to the end of the civil twilight.
So even on the North Pole, and for six months from September 24th. The sun disappears from the image to say that Total Darkness immediately appears. The civilian twilight does not end until October 8th.
When the sun goes down to 12 degrees below the horizon, it marks the end of the maritime twilight, when the sea horizon is difficult to find out. In fact, at the end of the sea, most people will keep at night. We have to wait for the North Pole until October 24, when the marine twilight is over. Finally, astronomical twilight – when the sky is actually completely dark – ends on November 13. It remains forever dark until January 28, when the twilight cycles start again. Thus, in the North Pole 24 hours of darkness lasts almost 11 weeks, No Six months.
Joe Rao works as an instructor and invited lecturer in New York Hayden Planetarium; He writes about astronomy Natural History MagazineIs it Heaven and telescope and other publications.