Not so dark and rather flat: what really happens during the fall equinox

According to some calendars, the day of the beginning of fall is the day of the autumnal equinox. This year, it will occur on Saturday, September 23 at 9:49 a.m. Kyiv time. From this moment on, autumn will begin in the Northern Hemisphere, and spring will come in the Southern Hemisphere.

As Space.com explains, from an astronomical point of view, the sun will be directly over the head of an imaginary person in the equatorial Pacific Ocean, 500 km west of Yaren, a region of the Pacific country of Nauru. The popular belief that the length of day and night are equal at the equinox is not entirely accurate.

The length of day and night

The popular definition of the equinox as the moment when day and night become equal in length is nothing more than a convenient simplification. From the point of view of the average person, this logic views night as simply a part of the day when the sun is below the horizon. At the same time, such a period as twilight is completely overlooked.

If the Sun were just a point in the sky from which light emanates and the Earth had no atmosphere, then the light would indeed pass half of its way above the horizon and spend half of it out of sight at the moment of the equinox. But in reality, the phenomenon of atmospheric refraction at the time of sunrise and sunset raises the solar disk by an amount greater than its apparent diameter. This creates an optical illusion that makes the light look like a huge reddish-orange ball lying on the horizon. However, it is still beyond the line of sight.

In addition to refraction, which accelerates the sunrise and delays the sunset, there is another factor that makes the daylight hours longer than the night at the equinox. The moment of sunrise and sunset is considered to be the time when the boundary point of the upper edge of the Sun is visible above the horizon, not the center of its disk.

That's why when you go to any service that indicates the exact time of sunrise and sunset (for example, weather forecast sites do this) and look at the exact local time of these two phenomena, you can see for yourself that the duration of daylight hours is still slightly longer than 12 hours. That is, from the point of view of strict astronomy, the term "equinox" is not very accurate.

On September 23, 2023, the sun will rise at 6:45 a.m. and set at 6:55 p.m in Kyiv. That is, the day will be 10 minutes longer than 12 hours. This means that these two parts of the day will not be equal.

On the day of the equinox, the sun will make a full 360 degrees around the entire sky and will appear to move in a circle just above the horizon at the North Pole. Theoretically, it should completely disappear from view. But it will still be visible in the sky. Only 75 and a half hours later the last speck on the upper edge of the Sun will finally disappear from view.

The strong refractive effect that affects the visibility of the luminary also slightly distorts its shape, appearing oval when it is near the horizon. As the Sun approaches the horizon, the degree of refraction increases so rapidly that its lower edge rises more than its upper edge, and a visual distortion occurs.

What about darkness?

Even ancient religions that were based on the worship of the solar cycle believed that the autumnal equinox marked the beginning of the "dark" half of the year, which lasts exactly six months. However, ancient people, when calculating the length of day and night, did not take into account the phenomenon of twilight. This mistake is repeated even in geography textbooks, travel articles and guidebooks. Technically, twilight illuminates the sky to some extent whenever the upper edge of the Sun is less than 18 degrees below the horizon. But it is only when it falls below this conditional point that the sky becomes completely dark over the entire distance from horizon to horizon and night falls.

Science recognizes several types of twilight: civil, marine and astronomical. We've already talked about astronomical twilight. When it ends, night comes. Civil twilight is also called light twilight. It lasts until the Sun drops below six degrees below the horizon. At this time, it is still possible to continue most daytime outdoor activities.

Even North Pole, where the Sun disappears from view for as long as six months and the polar night begins, it cannot be said that complete darkness occurs immediately. The beginning of the polar night is around September 25, but the twilight of this night lasts until about October 8, which is two weeks.

The twilight at sea lasts until the sun falls 12 degrees below the horizon. At this point, the horizon in the sea becomes difficult to see. Most people call this moment the onset of night.

Earlier, OBOZREVATEL shared the significance of the autumnal equinox in different cultures of the world.

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