Wednesday (September 22nd) shows the equinox. This is often regarded as a day when darkness and light each claim 12 hours, as it means “equal night” in Latin.
But that’s not the case, and Twilight is due to the turmoil surrounding astronomy. Equinox..
Let’s go back. Astronomically speaking, autumn begins in the Northern Hemisphere at 3:21 pm EDT (1721 GMT) on September 22 (and spring in the south, at that time. Sun It illuminates directly above when viewed from a point in the equatorial Pacific Ocean, 1,600 miles (2,580 km) southwest of Mexico City.
However, in the northern hemisphere, more than 12 hours of sunlight can be seen despite the designation of the equinox.
Not so equal
The definition of the equinox that day and night are equal days is a convenient oversimplification.
For one thing, it treats the night simply as the time when the sun is below the horizon, ignoring the twilight altogether.The sun is just a point of light in the sky, to the earth atmosphereThen, at the equinox, the sun actually spends half of its path above the horizon and half below. But in reality, the atmospheric refraction rises more than its own apparent diameter while the Sun’s disk rises or sinks.Therefore, when we see Sun As a reddish-orange ball just sitting on the horizon, we see the optical illusion — the sun is actually Completely below horizon.
In addition, sunrise and sunset are defined as the time when the first or last spots of the sun occur. Upper limbs It appears above the horizon, not in the center of the disc. So if you look at the local sunrise and sunset times on Wednesday, you’ll see that the duration of the sun, that is, the time between sunrise and sunset, is still more than 12 hours.
In Chicago, for example, the sunrise is 6:38 am and the sunset is 6:47 pm. Therefore, the amount of sunlight is 12 hours and 9 minutes instead of 12 hours. Until Saturday (September 25th), day and night are not truly equal (sunrise is 6:41 am and sunset is 12 hours later).
September 22 Arctic, The sun draws a 360-degree circle around the entire sky and appears to be scooping just above the edge of the horizon. At the moment of the equinox this fall, in theory the sun should be completely invisible, but the disk is still floating just above the horizon. Until 50 hours and 44 minutes, the last spots on the sun’s upper limbs are finally completely invisible after more than two days.
Due to this strong refraction effect, the sun’s disk appears elliptical as it approaches the horizon. As the sun approaches the horizon, the amount of refraction increases sharply, causing the lower limbs to be lifted more than the upper limbs and the sun’s disk to be significantly distorted.
Not as dark as it looks
Certain astronomical myths die hard. One of these is six months of sunlight and six months of darkness throughout the Arctic Circle. Often, “night” is simply considered when the sun is below the horizon, as if dusk did not exist. This fallacy is repeated not only in travel articles and guides, but also in countless geography textbooks.
However, whenever the upper edge of the sun is less than 18 degrees above the horizon, twilight illuminates the sky to some extent. This shows the limits of astronomical twilight when the sky is completely dark from horizon to horizon.
There are two other types of twilight. Citizen (bright) twilight occurs roughly when the sun is less than 6 degrees below the horizon and is roughly defined as the time when most outdoor daytime activities can continue. (Some daily newspapers provide time to turn on car headlights, which usually corresponds to the end of dusk for citizens.)
So even in the Arctic, the sun will disappear for six months from September 24th, but it’s unlikely that the “black” will set soon! Citizens’ twilight does not end there until October 8.
The last type of twilight is nautical twilight, which usually ends when the sun falls 12 degrees below the horizon when the sea horizon becomes difficult to identify. At the end of the voyage twilight, most people consider the night to begin. In the North Pole, we have to wait for the voyage twilight to end until October 24th.
Finally, the twilight of the celestial body-when the sky is actually completely dark-ends on November 13. After that, it will be forever dark until January 28, when the twilight cycle begins anew. Therefore, in the Arctic, the duration of 24-hour darkness lasts almost 11 weeks. No 6 months.
Joe Rao is a New York instructor and guest lecturer. Hayden Planetarium..He is writing about astronomy Natural history magazine, NS Farmers Almanac And other publications. Follow us on twitter @Spacedotcom When Facebook..
Twilight and Equinox Myths and 6 Months Polar Night
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