It's dark at night because... Different lengths of day and night

Indeed, why is the sky dark at night? Try to answer! It turns out that you can’t just say: it’s dark at night because there’s no light. This answer is incorrect. Even without the Sun, the sky, it would seem, “has no right” to be dark. Judge for yourself.

"BAN" ON DARKNESS

This is the sun. Some are close and therefore bright, others are unimaginably distant from us, almost invisible, or even completely inaccessible to our eyes. But each one radiates light. And if there are countless stars, then all together they give an infinite amount of light. It doesn’t matter that individually other stars are invisible. Since there are infinitely many of them above any, even the tiniest, corner of the sky, they must merge everywhere, creating a continuous background, sparkling with a dazzling brilliance, emitting an infinite amount of radiant energy. In short, the entire sky is prescribed to have a brightness no less than that of the surface of the Sun. And since the light of the stars is caused by their high temperature, the sky must pour out the same unbearable heat.

And if so, then there should be no place in the world either for or for people. Life is unthinkable in the unbearable light and heat. Any leaf, any insect would instantly be incinerated in such a thing. The substance of our usual states - solid, liquid - would become impossible. Everywhere would be as hot as in the depths of the Sun. An unexpected conclusion. And, of course, completely false.

Science has been trying to refute it for more than a hundred years. A lot of attempts have been made. Maybe the light is absorbed by the interstellar medium - gas, dark dust, cold planets, cooled “corpses” of stars? No, no matter how much light is absorbed, there will still be an infinite amount of it. The calculation is simple: no matter how large the proportion of non-luminous matter is, there will still be an infinite amount of luminous matter. Divide infinity in half, into ten, a hundred, a thousand parts - any part will still be infinitely large. In addition, the atoms of the interstellar medium do not “eat up” light without leaving a trace. They simply swallow it and then “spit it out” in another direction. They only dissipate, as if scattering energy throughout the world.

Then maybe there are giant opaque shells in our universe? One of Chekhov’s heroes philosophized: “Perhaps our entire universe is hidden in the hollow tooth of some huge monster.” And this “hypothesis,” no matter how seriously one puts it, does not solve the problem, but only postpones it. After all, any “hollow”, any dark shell in an infinitely light and hot world would inevitably melt and evaporate.

It remains to make one more, very risky assumption: to lift the ban on night darkness by abandoning its main reason - stellar infinity. Recognize that in the infinite space of the universe there are only a “handful” of stars and galaxies - and everything will fall into place, right?

No not like this. In such a conclusion there is philosophical cheating and physical thoughtlessness. Even the great classic of natural science, Isaac Newton, said that there cannot but be an infinite number of stars. If there were only a “handful” of them, albeit a huge one, thanks to them they would stick together into one single giant celestial body. Later, however, physics made an amendment: a “handful” of stars would not stick together, but, on the contrary, would scatter throughout infinite space. And then the Universe would be empty, virtually all matter would disappear from it.

But there is substance! No matter how far telescopes look, they find matter everywhere. So there are infinitely many stars? It turns out like this. Then why is it still dark at night? See how confused everything is! Trying to comprehend the appearance of the sky, we got lost in the jungle of contradictions, faced ideological questions, and touched upon the greatest problem of the structure of the entire Universe - a science called cosmology.

HIERARCHY OF SYSTEMS

You received a task on an exam and were horrified by its difficulty. But it is not customary to ask unsolvable problems in exam papers. That's why you stubbornly wrinkle your forehead, think painfully, and finally solve the problem! And then you look for another, and a third, and a general method of solution. Something similar happens in cosmology. Knowing that the riddle of night darkness is certainly solvable (the sky is dark!), astronomers, physicists, and philosophers are persistently looking for a solution. And little by little the darkness of the night begins to gain the right to exist.

Let's imagine this: the world is infinite, there are countless stars in it, but, nevertheless, the density of matter in the infinite volume of the Universe is zero. Impossible? It turns out it's possible. It is only necessary that as the volume of outer space increases, the density of matter in it decreases.

The density of a substance is mass divided by volume. Each star has a very high density, because its entire volume is filled with matter. But in a volume that includes two neighboring stars, the average density of matter is less (after all, cosmic matter between stars weighs practically nothing). The density of matter in the volume of the Galaxy is even much lower. So what is next? What if in the limiting case - for an infinitely large volume - we formally obtain zero matter density? After all, then the ban on darkness at night is lifted! Looking into the distance, we will see practically no substance - neither luminous nor dark. And this despite the fact that there will be countless stars and other celestial bodies.

This idea formed the basis for the curious star placement schemes that the Belgian cosmologist Charlier constructed at the beginning of the last century. Charlier borrowed their main feature from the 18th century astronomer Lambert. This feature is the hierarchical structure of the world. Celestial bodies form systems, the complexity of which increases with increasing size, and the average density of matter decreases. A world organized according to such rules is completely free not only from the prohibition of night darkness, but also from some other paradoxes that have arisen in cosmology.

At one time, Charlier's schemes seemed like a revelation. They seemed to contain a clue to the structure of the Universe. However, then the enthusiasm cooled. There was no direct evidence for Charlier's ideas. His systems remained just a guess. And yet, Charlier’s work was very useful. He showed that there are ways out of the dead ends of cosmology.

Meanwhile, the famous theory of relativity entered the scientific arena. But read about this in our next article.

Why is it dark at night

Every day the Earth makes a complete rotation around its axis. During this period, which takes 24 hours, all 4 times of day change. First comes morning, then day, evening and night. It is dark at night, the temperature drops at this time of day, because the Sun remains below the horizon. When morning comes, the Sun rises again, becoming brighter and warmer. Why does this happen, and for what reason is the night longer or shorter? Does night everywhere change in its length as the seasons change?

These are interesting questions that require answers. Modern science can analyze them in detail and provide answers to all points of interest to a person.

Rotation of the Earth around its axis and around the Sun

The Earth never stands still; it constantly rotates, both around the Sun and around its own axis. A complete revolution around its axis takes the planet approximately 23 hours, 56 minutes and 3 seconds. But this is not an absolutely accurate value, since the length of a day can vary within a few seconds - in this regard, the Earth can either “slow down” or “speed up” a little. These are subtle variations, but for convenience it is generally accepted that a day is exactly 24 hours.

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What are the Northern Lights?

As the planet rotates, the zone that is illuminated by the star closest to us gradually moves from one point to another. So, the sun's rays fall first in the area of Japan, then move to Vladivostok, and then the illuminated point moves in a westerly direction until it returns to its original position. At the moment when it moves from its position, for example, leaves the territory of Japan further to the west, evening, twilight begins at this place, and then night falls. The darkness will continue until sunlight hits the area again, making a full circle around the rotating planet.

Interesting fact : the sun's rays move across the planet from east to west. This is where the word east comes from - the place where the Sun rises. The West is the territory where it falls below the horizon and disappears. Japan is called the “Land of the Rising Sun”, as local residents are the first to observe the appearance of the luminary every new day.

But why are the days longer in summer and short in winter? This phenomenon is observed almost throughout the entire territory of Russia, since the country has a northern location. It should be noted that in addition to the rotation of the planet, another factor plays a role - the tilt of the earth's axis. The closer the latitudes are to the poles, the stronger the presence of this factor is felt. The tilt of the earth's axis is approximately 66 degrees, which creates the effect of the sun rising low in winter and shortened daylight hours. Also thanks to this, in the summer in Russia the nights are short, and it is possible to observe such a phenomenon as “white nights”, when complete darkness actually does not occur.

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Why do the Moon and Sun change color?

In the polar territories this phenomenon is even more pronounced - during 3 summer months the Sun does not set at all. It makes a certain path across the sky, barely touching the horizon, and then rising again. But in the winter months the sun does not rise, and the polar territories are plunged into complete darkness.

Day at the equator

At the equator, the opposite phenomenon is observed. Here, the tilt of the earth's axis actually does not play any role; at any time of the year, night and day are equal, amounting to 12 hours each. These areas are illuminated by sunlight at an angle of 90 degrees, so the lighting here is more intense and the daylight hours are more stable.

Why is it dark at night? This seemingly simple childish question has interested both famous astronomy researchers and ordinary people for several centuries in a row.

There are an infinite number of stars in the sky and each of them is much larger than the Sun. The powerful starlight should have incinerated everything around, but, strangely enough, this does not happen and every night it becomes dark again.

Common hypotheses about darkness at night

Astronomer Johannes Kepler refuted the infinity of the Universe, erroneously claiming that the stars do not completely cover the sky. He believed that it becomes dark at night because of the empty places in the sky, where there are absolutely no stars.

In fact, numerous stars are simply distributed unevenly throughout the Universe and are located at different distances from the Earth. Therefore, we do not see all existing stars in the sky, but only those located closer to our planet.

There were other opinions. At night, everyone saw many of the brightest stars overhead, but despite this, the night sky always remained dark. German astronomer Heinrich Olbers called this phenomenon a paradox and in 1823 put forward a theory about the absorption of stellar light flux by cosmic dust. And only about a hundred years ago, scientists proved that interstellar nebulae are clusters of galaxies, and not cosmic dust.

The Universe is infinite, and the sky is densely dotted with luminaries of different sizes. There are no empty or dark places in the sky, just many stars are infinitely far away and therefore invisible, some of them cannot be seen even with a very strong telescope.

Scientific explanation for darkness at night

The time of day changes for several reasons:

The Sun has a strong influence due to its close location to the Earth;

- The light of distant stars cannot reach our planet.

Earth Rotation

Now no one disputes the fact that the Earth has the shape of a huge ball and rotates around its axis at breakneck speed in the direction from west to east. This rotation is called diurnal; it is repeated with a period of one sidereal day.

The sun is the closest star to us

Of all the heavenly bodies, the Sun is located closest to the Earth. The sun's rays are a natural source of heat and light for several planets at the same time. When the time of day changes, the Sun illuminates and warms opposite corners of the globe.

Due to the constant rotation of the Earth, we create the illusion of the Sun moving across the sky. In reality, the Sun is always in one place, and our planet gradually turns towards it with its different sides. Each hemisphere in turn falls into shadow, the amount of light decreases and night falls.

Such distant stars

The stars do not disappear anywhere; at night and during the day they are directly above our heads. During the day they are not visible because they fall within the range of hot solar radiation. At night, the Sun is covered by the shadow of the Earth, and the stars are very far away, their light simply does not have time to reach us.

Thus, even the celestial bodies visible to the human eye are located at a distance of billions of light years. For this reason, it becomes dark at night.

What will happen to the stars in billions of years?

If in the future the light of invisible stars finally reaches the Earth, the night will still not become brighter. By this time, the stars of our Universe will have time to go out, and it will take a long time for other, more distant ones to approach.

The Universe has no boundaries - some stars constantly fly towards the Earth, others go out. Therefore, even after billions of years, nothing will change; daylight will always be replaced by night darkness.

Everywhere on earth there is a time when it is light and a time when it is dark. This is primarily due to our main luminary - the Sun. It moves across the sky, changing the level of illumination. Simply put, it is dark at night and the sun goes below the horizon.

In ancient times, people assumed that this Sun was circling around our planet and hiding behind the horizon. For a long time, no one could even imagine that it was our planet that revolved in the vast space around the star. The same goes for the Moon. The Sun and Moon were given a divine origin: they were worshiped, given gifts, and praised in songs and rituals. But the era of science has come, which has proven that everything is happening just the opposite. Planet Earth is not but just a tiny component of it, and why it is dark at night is not connected with any divine manifestations.

What is the rotation of the Earth and what does it affect?

There are two simultaneous movements and movements of the planet in space: movement in orbit around the Sun, and also around its own axis, like a children's top. That is, at the same time, when the planet flies in outer space, it revolves around itself, and the combination of these factors is the reason why it is dark at night and light during the day. Moving in orbit in space, coupled with the fact that the Earth's axis is inclined to this same orbit at an angle of approximately 66 degrees, is the reason for the change of seasons and their “unevenness.”

In different parts of the Earth, depending on the degree of heating by the rays of the sun, autumn, winter, spring and summer change in due time. Thus, middle latitudes are most often visited by all four seasons, to varying degrees of intensity (for example, in Italy, summer, like winter, is much warmer than in Moscow). On the Equator, which for the most part is exposed to direct rays of the sun, close to an angle of 90 degrees during midday, the day lasts somewhat longer than 12 hours.

Poles: why is it cold, even though it’s daylight there for six months?

At the poles, the picture is completely special - the sun's rays fall in such a way that they glide and are almost completely reflected from the surface, without lingering and leaving no heat in it, although day and night here last almost half a year each. Why are the north and south poles the coldest regions of our planet?

Different lengths of day and night

The rotation of the planet around the Sun, the main star for us, sets the change of seasons, as well as the change of day and night. The spherical shape of the planet, the heterogeneity of the surface and the ability of light rays to be reflected complement and make the climate in regions similar in location more diverse. But there are days when, in all latitudes up to the polar zone, days begin that have the same distribution of time between the clear part of the day and the dark part - the days of the spring and autumn equinox. At this time, at the equator, any object will give the smallest shadow, because the sun sends its rays at an angle of 90 degrees to its surface.

In principle, everything is clear with the question of why it is dark at night. But the interesting thing is that it can be dark for a long time, and sometimes it can be a little dark. Our Northern Hemisphere is characterized by a decrease in the length of the night from March 21 to September 23 (autumn equinox), and vice versa - long nights are observed in the winter. In the southern hemisphere the opposite is true.

What about the children?

Explaining to children the phenomenon that it is dark at night because the sun does not shine is not entirely correct. After all, in fact, the sun always shines. It does not turn on and off at someone else's discretion, like a table lamp. But there is no need to talk about the position of the Earth in space, about the angles of incidence of rays and about other abstruse things that can already be understood by school-age children. To do this, it is best for parents to be smart and demonstrate clearly how this happens. In order to explain why it is dark at night, you need to familiarize the child with two concepts: what the Sun is and what planet Earth is. This is quite simple to do: draw two balls, one yellow and with rays (the Sun itself), and the other blue with a semblance of the outlines of the continent. Next, without going into complex terminology, talk about the form and clearly demonstrate it with the help of. A yellow ball and a small globe will be enough, and if possible, it is better to buy a full-fledged model or make a similar one yourself, and even together with your child.

Show that the sun stands still, but we are spinning, which is why its rays do not always fall on different parts of the globe. Then the child will understand that it is dark at night, because at this time we turn away from him, expose his back to the sun, so to speak. For complete clarity, you can demonstrate this phenomenon in the dark using the same globe and a flashlight acting as the Sun.

Why is it dark at night? This seemingly simple childish question has interested both famous astronomy researchers and ordinary people for several centuries in a row.

In the sky, each of them is much larger than the Sun. The powerful starlight should have incinerated everything around, but, strangely enough, this does not happen and every night it becomes dark again.