Why does night exist? | Cosmic Void

Questions that seem too simple, naive or childish, such as why night exists, often lead us to complex answers containing many physical concepts and curious derivatives. The existence of day and night is directly related to the fact that the Earth rotates around its axis and does not always face the same side to the Sun. Therefore, the question of why night exists becomes the question of why the Earth rotates on its axis and how it does so.

The explanation is based on fundamental physics and is therefore extremely important for understanding how the Universe works. We’re not talking about popular physics like the law of gravity or the equivalence between mass and energy E=mc², but we are talking about the conservation of angular momentum, which is just as important as those equations. There are already many explanations for this, I will only say that during the formation of planetary systems, conservation of angular momentum makes the planets rotate.

Let’s continue our journey towards understanding why night exists by adding more variables that influence this rotation. Do all planets rotate on their own axis? Well, they should if it was just for the sake of conservation of angular momentum since their formation, but other things happen to planets that can stop them from spinning. In fact, Venus is thought to have stopped its rotation and even reversed it: it is spinning in the opposite direction to the rest of the solar system’s planets and its orbit around the Sun. The result is one Venusian year – a Venusian year. time of revolution around the Sun: about 224 Earth days – it lasts slightly less than a Venusian day – 243 Earth days. We can conclude that the concepts of day and night are not the same in other worlds; at least if we take into account the duration of both, which we earthlings have in mind.

There is day and night on Venus, they just last a long time. Is it possible for the planet to have no day and night at all? That is, to be more precise, is it possible that in some area of ​​the planet it is always night? The answer is yes, and it relates to other physics concepts. Rotation is not the only thing responsible for the existence of day and night: if the Earth stopped rotating tomorrow, day and night would not end up on opposite sides of the planet.

Eternal days on exoplanets

Since the Earth revolves around the Sun, if it did not rotate and always faced the same side, at some point it would leave the Sun aside and finally behind the planet. Let’s move to the center circle of the football field and imagine that the Sun is the ball at the starting point. If a walker moves along the edge of a circle without rotating on its axis (that is, always looking at the same field background, which implies that sometimes we will move sideways and sometimes backwards), in the middle there is a path equivalent to the orbit of a planet , he would lose sight of the ball, and it would be like the night of the planet. But after walking half a circle again, I saw the central point again, and it was like daylight.

In order for any area of ​​a planet to always be night or day, during its orbital journey around a star, the planet’s rotation around its axis and its orbit around the star must be synchronized. In our football field example, a person moving around the center circle must turn as he walks accordingly so as to never see the center point (always at night) or always see it (eternal day). This degree of synchronicity seems incredible, but it is possible, and very common in the Universe: not by chance, of course, but by physics: the Moon itself is synchronized with the Earth, in the sense that it rotates around its axis more strongly. or less with the same period of revolution around the Earth. Therefore, there is a hidden side of the Moon that we never see, and it never sees us. If the Earth were its star, and the Moon its planet, part of its surface would be in eternal night.

Are there planets that are synchronized with their star, similar to what happens with the Moon and Earth? Answer: yes. We believed this was possible given our selenite experience, and we now know of rocky planets like Earth that have their rotation and orbit locked. They are called tidally locked planets, a name that comes from the fact that they are the same thing that causes tides on Earth. Obviously, this connection has a significant impact on the likelihood of the existence of life. This may be the case with planet LHS3844b, named Kua’qua (hummingbird in the indigenous language of Central America): it is the first planet discovered around the star LHS3844, a star about 3000 degrees cooler than the Sun, which makes it redder, so the days there are not very bright.

Is there another way to go without night (or day) for an extended period of time? Well, yes: the length of the night also depends on how the axis of rotation is oriented relative to the orbit around the star. On the Earth itself, the axis of rotation is not perpendicular to the plane along which the planet moves around the Sun, which is called the ecliptic plane and is common to approximately all planets. As if it is a slightly inclined peak, there is an angle of 23 degrees between the axis and the ecliptic. This angle of the ecliptic, in addition to determining the change of seasons in some areas of the globe, makes the night very long or non-existent for several months in the polar regions.

Nights of 42 years on Uranus

Each planet has its own ecliptic angle, it depends on the process of its formation and, of course, on very specific events. Perhaps this angle was so great that the rotation of the planet did not allow day and night to exist? Well, Uranus, without going any further, rotates around an axis that forms almost 90 degrees with the ecliptic. It was as if our walker on the football field was always moving around a central point, looking at the background but doing sideways pirouettes so that he was spinning around his belly button. As a result, for this planet in our solar system, there is a point on the surface of Uranus where in the course of a year there are 42 years of night and 42 years of day, which is equivalent to 84 Earth years.

Is there another way to make sure that it is always night on the entire planet? And again, yes: it does not have a star, and it is a wandering planet. We have already talked about these lone wolves, so this concludes today’s article. We concluded, of course, earlier when we said that something as everyday to us as day and night should not exist. Although we certainly wouldn’t exist the way we do without those nights and those days, which might lead to the claim that we wouldn’t be who we are without the conservation of angular momentum, the law of gravity, or those tidal forces (and other effects that we have not described in detail), that physics that ultimately explains the reason for the existence of the Universe.

Cosmic Void This is a section in which our knowledge of the Universe is presented qualitatively and quantitatively. Its purpose is to explain the importance of understanding space not only from a scientific point of view, but also from a philosophical, social and economic point of view. The name “cosmic vacuum” refers to the fact that the Universe was and remains largely empty, with less than one atom per cubic meter, despite the fact that our environment, paradoxically, contains quintillions of atoms per meter. cubic, which invites us to think about our existence and the presence of life in the Universe. The section has been compiled. Pablo J. Perez Gonzalezresearcher at the Astrobiology Center, and Eva VillaverDirector of the Space and Society Directorate of the Spanish Space Agency and Research Professor at the Institute of Astrophysics of the Canary Islands.

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