I have read in several books about the adventures of world travelers about the same phenomena observed at sunsets over the equator. They point out that at the equator "darkness descends almost immediately after sunset, where there is no twilight". The first few times I saw it, I ignored it, but after reading it over and over again, I began to wonder if it was true, and if so, why? Why exactly is dusk likely to get shorter or longer in a particular place? Is this true? Why?
This is a very good question, although the reasons for this may be somewhat complex and difficult to understand. The simple answer is that at low latitudes, the sun sets at a right angle toward the horizon; at high latitudes, the sunset may have a more oblique angle, causing the sun to stay near the horizon for longer.
The Sun rises and sets as the Earth rotates. Looking at long exposure photos at night, it's easy to see how celestial bodies move through the sky due to rotation. Each circular stripe has a star, and for an hour or any length of time taken in the photo, each star rotates around the north and north stars in the center of the illustration.
As you can see from the photos, stars closer to the North Star than to the equator never rise or fall, but continue to move around the North Star in the sky. Stars farther away from the North Star rise and fall because the circles they run cut below the horizon. The same idea applies to any celestial body in the sky, including the sun and moon.
Another thing you need to know about this rotation pattern is that the North Star is located at 1° north pole. If you're standing at the North Pole, you're right overhead and you're seeing the North Star, and all the celestial bodies are orbiting around that, and nothing in the sky is going to rise or fall.
This makes sense, especially considering that when standing at the pole, the rotation of the Earth is nothing more than happening around you. If you look in a different direction, nothing will leave or enter the sky above you. The closer you get to the North Pole, the higher the North Star is in the sky. At the equator, the North Star is on the horizon.
This means that in the Arctic, the sun does not rise and fall at all because of the rotation of the earth, although the sun does travel through the sky in annual cycles (from one circle on the film to another), and this is why there are 6 months of polar days and 6 months of polar nights there. So, above the Arctic Circle (or below the South Pole Circle), at the right time of year, you can see the midnight sun — the sun that doesn't set all day. At this time of year, the Sun is a celestial body orbiting the celestial poles, located on one of the star trajectories in the photograph that never descends below the horizon.
Now imagine being very far north, but not yet north of the Arctic Circle. The North Star is high in the sky, and as the Earth rotates, the sun may be on one of those low circles that you see in the photo skimming the horizon. The sun will set, but it will set at an angle. It will be close to the horizon for a long time, making the dusk longer.
The further south you go, the Lower the North Star appears in the sky, and the farther you go, the faster the sun will sink below the horizon at night. Eventually, when you reach the equator, the North Star will be on the horizon and the sun will fall directly below the horizon. Since the Sun does not move much parallel to the horizon, it quickly leaves the horizon behind after sunset, bringing a very brief twilight.
Then, as you travel south from the equator, this effect will be reversed, the twilight becomes longer and longer, you will see the southern celestial pole higher and higher in the sky, more and more celestial bodies near the celestial pole (that is, not rising and not falling), until you finally reach the south pole, the southern celestial pole is just above your head, and everything is once again around the celestial pole, and nothing rises and falls because of the rotation of the earth.
I should say one more thing again: there is nothing special about polaris in nature. It just happens to be very close to the North Celestial Pole, a geometric point just above the Earth's North Pole. There is no South Pole Star in the Southern Hemisphere because no star is as close to the celestial pole as the North Star, and there is no corresponding bright South Pole Star.
I hope this article has given you a better understanding of sunrise and sunset!
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