Every person at least once in his life has thought about the cause of darkness at night. Mostly children ask such questions, although adults are probably interested in the answer too.
The simplest explanation
It is unlikely that a child will want to listen to a lengthy lecture about how our Universe consists of many stars that are distant from the Earth at varying degrees, etc. and so on. The best answer is short.
Due to the fact that the Earth constantly rotates around its axis, with a certain periodicity it is turned towards the sun on either one side or the other. The side that is facing the shining star will be illuminated. Accordingly, there will be a day on it. The other side, which at this moment is hidden from the rays of the sun, will be under the cover of night. It will be dark.
But what really?
Every moment the sun illuminates with its rays exactly that side of the planet Earth, which at that moment is facing it. In 24 hours (a day), our planet makes a full revolution around its axis. That is why it seems to everyone that the sun is moving across the sky, and gradually disappearing behind the horizon it causes the onset of evening, and then night.
In addition to the sun, there are many other luminaries in the universe. For example, the same stars. During the day they are not visible, because the sun shines and drowns out their glow. But what prevents them from illuminating the Earth at night, when the sun has hidden behind the horizon? Each star is a hot ball of impressive size. Yes, it is an undeniable fact that the stars are located much further than the sun, which is the closest shining star to our planet. Due to their great distance, a huge number of stars seem too small to people, or even not noticeable at all.
On the other hand, if we assume that our Universe is filled evenly with stars, then a person’s gaze (regardless of the point at which it is directed) must necessarily come across some star. It follows from this that there should be no dark places or voids in the sky. However, the light energy that comes from a star decreases with distance, and the area that each star directly occupies in the sky decreases in proportional amounts. Accordingly, the brightness of a star is maintained regardless of its distance from Earth.
Scientists' theories about the starry sky
The first person to draw attention to the problem of darkness at night was Johannes Kepler. He believed that darkness at night is direct evidence that the Universe is not infinite and has certain boundaries. As Kepler said, if the Universe were infinite, then the entire sky should be completely covered with a mass of shining stars, as bright as the sun itself.
In the modern world of astronomy and physics, this problem has come to be called “Olbers' paradox” in honor of Heinrich Olbers, a German astronomer. In 1823, he again raised the debate that had flared up around this issue earlier. The scientist put forward his theory, which was as follows. The light energy emitted by stars does not fully reach our planet for the reason that it is absorbed by cosmic dust. This idea, as Olbers believed, left the right for the existence of the theory of the infinity of the Universe. And yet the astronomer was mistaken. According to calculations, the light from the star should heat up the dust in space so much that it would glow no worse than the star itself.
At one time, Kepler so fiercely advocated the idea of the finiteness of the Universe, and it looked so convincing that astronomers, almost until the beginning of the 20th century, unshakably believed that the Milky Way was a kind of island of stars, which was surrounded by emptiness on all sides. Only in the 20s of the last century it was discovered that those faint nebulae that were previously observed among the stars in the Milky Way were nothing more than distant galaxies, and not accumulations of dust or gases. And these galaxies, like the Milky Way, consist of a colossal number of stars. This means that there is no emptiness on the outside of the Milky Way; it is filled with other cosmic bodies.
Since the Universe is unevenly filled with stars, it is obvious that in some intervals the stars will be denser and form galaxies. The latter, in turn, form galaxy clusters. But even if we imagine that all the stars are located with an average density over the entire distance of the Universe, then Kepler’s hypothesis will remain valid. That is, no matter where the human gaze is directed, it will in any case stumble upon some star.
Although in this case one key circumstance should be taken into account. The stars are located at a great distance from the Earth. And even if we take the average of this distance, it will be about 10 23 light years. This is exactly how long it will take for starlight to reach our planet. At the same time, according to modern astronomers, the Universe has existed for only 14 billion years. This is much less than 10 23 years. So, based on this, we can say that we observe only those stars in the sky that are located from the Earth no further than 14 billion light years, which are necessary for the light to reach us from them during the existence of the Universe. Other stars, much further away, are emitting light, but it is still in the “journey” stage.
So it turns out that that part of the Universe that is accessible to human observation is ten billion times less than what is necessary for it to fully illuminate the night sky of the Earth.
If someone asks whether it will be when the light from stars still invisible to us finally reaches the Earth, then the answer will also be negative. By the time this happens, the stars of our universe will already be extinguished. In other words, in order to completely illuminate the space in the sky, there is not enough matter in space.
Sometimes at night we have the opportunity to observe a phenomenon in which the sky seems not dark enough. And today we will look at questions about why the sky is bright at night.
Why is it light at night in winter?
In the winter season, we are accustomed not only to the fact that it begins to get dark much earlier than in summer, but also to the fact that the weather is usually such that even in the daytime the daylight hours seem less bright. Despite this, sometimes we have the opportunity to observe fairly bright nights, so we need to consider the question of why the sky is bright at night in winter.
There may be two reasons for a lighter sky at night:
- If you notice that the night is not as dark as always, and there is precipitation in the form of snow outside, you can be sure that the snow is the reason for such a bright sky. Snowflakes reflect the light of lanterns, as well as moonlight, thereby creating the illusion of a more illuminated night sky;
- If the sky is bright enough and there is no precipitation, then strong and low cloudiness can be considered the cause of this phenomenon. Pay attention to the clouds - they are lower than usual. For this reason, clouds act as reflectors of light from the earth, which leads to the illusion of a bright sky.
Why is it as bright as day at night?
If, when wondering about the night illumination of the Earth’s surface, you were directly interested in information about the so-called “White Nights”, which are observed, for example, in St. Petersburg, then in this situation the answer will be completely different.
To begin with, it is worth noting that such white nights are observed not only in St. Petersburg, but also in many other parts of our planet. For example, it is quite possible that someone will be interested in the question of why it is light at night in Greenland, since a similar phenomenon is also present there.
Events on a planetary scale are considered to be to blame for the occurrence of such a phenomenon. The fact is that at a certain point in time, due to the fact that the Earth rotates around the Sun along a certain trajectory, and also rotates around its own axis, our planet is on such a trajectory that even at night the Sun is in the territory, for example, St. -Petersburg or Greenland does not set much below the horizon. Accordingly, even at night, sunlight is scattered over the surface of the Earth and in the above-mentioned territories a kind of twilight is observed instead of the usual night.
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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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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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.
Earth is a planet that rotates around the Sun. It also moves around its own axis. These turns are complex and not fully explored. Despite the difficulties in determining rotations, scientists were able to establish why it is dark at night.
From Earth, the worlds appear to revolve around us. If you get up in the same place at the same time every day, you can see how the stars move across the sky, how the Sun passes across it. At such moments, it seems that our planet is the center of the universe.
Around the Sun
As already mentioned, the Earth always rotates around the brightest star of our solar system, and at the same time rotates around its own axis.
The planet completes a revolution around the star in three hundred sixty-five days and six hours. To make it more convenient to count dates, a calendar year of 365 days was introduced. And once every four years, when twenty-four out of six hours have accumulated, another day is added. This year is called a “leap year”, and a new day is added to February.
As the Earth rotates around the star, the seasons change. Due to the fact that the planet has a certain angle of inclination - sixty-six and a half degrees, it moves in outer space. Due to the inclined position, sunlight illuminates one side or the other of the planet. When it is day in the Western Hemisphere, it is night in the Eastern Hemisphere.
When the rays of the Sun fall at right angles on the planet, equinoxes are observed - day and night are the same length. This event happens twice a year: on the days of the spring (in March) and autumn (in September) equinox. The beginning of summer and winter are considered to be the dates when the Sun shines on the planet from the greatest height (in June and December).
The rotation of the earth's axis to the Sun
During the winter solstice, the earth's axis turns toward the luminary with its southern end. And, accordingly, the sun's rays fall at southern latitudes. From this day on, on parallels south of the equator, the days become long and the nights short. Polar day begins to reign in the Antarctic Circle.
In all parts north of the equator, days are shorter than nights, and in the Arctic Circle there is deep night.
The polar circles are the boundaries of polar day and night, which last from one to 178 days. During the entire polar night, the Sun sets below the horizon. During the polar day, the Sun shines and does not hide behind the horizon.
Spinning around yourself
The earth rotates around itself, making a full revolution every day. The planet rotates from east to west, so the Sun rises in the east and sets in the west.
The rotations of the planet around itself make it possible to understand why it is dark at night and the light hides behind the horizon. It is the appearance of the Sun and its descent that causes the change of day and night.
So why is it dark at night, and where is the light at this time? One part of the planet is constantly facing the Sun. And on that part of it where the sun's rays fall, daylight is observed. On the opposite (dark side), to which the light does not reach, night is observed. During rotation, sunlight gradually penetrates into those areas that were dark until that moment, and from those where it was light, the Sun leaves. From Earth, this phenomenon is observed in the form of sunset and dawn.
The rotation of the planet around itself, during which it alternately exposes different areas to the sun's rays, counts the earth's day and changes day to night. At a time when the Sun does not shine on the western part of the planet, it illuminates the eastern. Based on this, in the first part there will be night, and in the other - day. That's why it's dark at night.
Rotating around the galaxy
Earth is located in the solar system, which is located in the Milky Way galaxy. It got its name for its special appearance: in the night sky it looks like spilled milk. In fact, the white stripe is a cluster of millions of stars.
The galaxy has a spiral shape. According to modern calculations, our Solar system is located closer to the edge of the galaxy, in one of its arms. It moves in a spiral around the center of the galaxy. It is estimated that the spiral of the Milky Way completes a complete rotation around the center in approximately 225 million years.
Scientists about galactic rotation
Scientists believe that this rotation around the center of the galaxy must somehow affect our existence and all the planets of the solar system. However, there is no data yet on what specific events entail full revolutions. This is due to the small age of humanity, it is estimated at only tens of thousands of years, and scientists have been making serious observations of space and the phenomena occurring in it for only a few centuries.
Conclusion
The Earth moves with our solar system around the center of our galaxy. One complete rotation takes approximately 225 million years. At the same time, the Earth always rotates around our star. Moreover, during rotation the planet either moves away or approaches it. Because of this phenomenon, people observe the change of seasons. While rotating around a star, the Earth simultaneously rotates around its axis from west to east: because of this phenomenon, day gives way to 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.
