Why is it dark when the sun sets?

Billions of stars surround us in the fringes of galaxies. That is, the light must be sufficient to turn the sky into daylight even if our Sun disappears behind the horizon.

This question has already occupied the most famous astronomers in history. Johannes Kepler In the 17th century, he still believed that darkness was evidence of the relatively limited size of the universe: in his view, there were not enough stars or points of light to cover the entire sky.

Today we know that the number and distribution of stars would certainly be enough to fill our eyes with light even at night. That's why they found a new explanation for the darkness at night: since the Big Bang 14 billion years ago, the universe has been expanding and galaxies have been moving away from each other.

The result is that we have Our Milky Way Its average distance from other galaxies is 10²³ (Ten on the twenty-third, a lot, even more) kilometer. Thus, light would need many years longer than the age of the universe to reach us from distant stars.

It is also surprising to many people that the tropics become pitch black in no time, while the Northern and Southern Hemispheres are relatively light long after sunset. But there is an explanation for that too.

Astronomers define sunset as starting when the center of the sun's disk reaches the horizon, and ending when the sun is six degrees below the horizon. Since the Sun crosses the horizon line almost vertically near the equator, it disappears behind it relatively quickly.

In Europe, for example, it crosses the horizon at a good angle of 40 degrees, making the journey noticeably slower. Thanks to this, the place remains bright for such a long time that one can read a newspaper outdoors even without artificial light.