Questions that seem too simple, naive or childish, such as why night exists, often lead us to complex answers that contain a multitude of physical concepts and curious derivatives. That day and night exist derives directly from the fact that the Earth rotates on its axis and does not always face the same face towards the Sun. So the question of why night exists becomes why the Earth rotates on its axis and as it does.
The explanation is based on fundamental physics and, as such, is extremely important for understanding how the universe works. We are not talking about physics as popular as the law of universal gravitation or the equivalence between mass and energy E=mc², but we are talking about the conservation of angular momentum, which is as important as those equations. There are already many explanations for this, I will only say that in the formation of planetary systems the conservation of angular momentum forces the planets to rotate.
Let's continue the path towards understanding why the night exists, adding more variables that affect that rotation. Do all planets rotate on their axis? Well, they should do it, if it were only for conservation of angular momentum since their formation, but other things happen to planets that can stop their rotation. In fact, Venus is thought to have stopped its rotation and even reversed it: it rotates in the opposite direction to the rest of the planets in the solar system and to its own orbit around the Sun. The result is that one Venusian year—the time it takes to go around the Sun: about 224 Earth days—it lasts slightly less than the Venusian day—243 Earth days. We can conclude that the concept of day and night is not the same in other worlds; at least, if we consider the duration of both that we earthlings have in mind.
Venus does have day and night, they just last a long time. Is it possible for a planet to have no day and night at all? That is, to be more precise, is it possible that in an area of a planet it is always night? The answer is yes, and it is related to other physical concepts. Rotation is not the only thing responsible for there being day and night: if the Earth stopped its rotation tomorrow, day and night would not end on opposite sides of the planet.
Eternal days on exoplanets
As 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 to one side and finally behind the planet. Let's move to the central circle of a soccer field, and imagine that the Sun were a ball that is at the kickoff point. If a walker moves along the edge of the circle without rotating around its axis (that is, always looking at the same background of the field, which implies that sometimes we would move sideways and sometimes walking backwards), in the middle that path, equivalent to the orbit of a planet, he would lose sight of the ball and that would be like the night of the planet. But after traveling half a circle again, I would see the central point again and that would be like daylight.
So that it is always night or day in an area of a planet, in its orbital journey around a star the rotation of the planet on its own axis and the orbit around the star must be synchronized. In our example of the soccer field, the person moving through the central circle should turn as they walk, in an appropriate way so as to never see the center point (always at night) or always see it (eternal day). Such a degree of synchrony seems unbelievable, 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 on its axis more or less with the same period that revolves around the Earth. There is, therefore, 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, the Moon would have eternal night on part of its surface.
Are there planets synchronized with their star, in a similar way to what happens to the Moon with the Earth? The answer is yes. We believed that was possible Given our selenite experience, and we now know rocky planets like Earth in which their rotation and orbit have been coupled, they are called tidally coupled planets—the name comes from the fact that it is the same thing that causes the tides on Earth— . Obviously, this coupling has a dramatic effect on the probabilities of life. This may be the case of the planet LHS3844b, named Kua'kua (hummingbird, in a Central American indigenous language): it is the first planet discovered around the star LHS3844, which is a star about 3000 degrees cooler than the Sun, which It makes it redder, so the days are not very bright there.
Is there any other way to go without night—or day—for long periods of time? Well yes: the length of the night also depends on how the axis of rotation is oriented with that of the orbit around the star. On the Earth itself, the axis of rotation is not perpendicular to the plane through which the planet moves around the Sun, which is called the ecliptic plane and is common to approximately all planets. As if it were a slightly tilted top, there is an angle of 23 degrees between the axis and the ecliptic. This angle of the ecliptic, in addition to causing the seasons in some areas of the globe, makes the night very long or non-existent for months in the polar areas.
Nights of 42 years in Uranus
Each planet has its ecliptic angle, it depends on its formation process and surely on very specific events. Could it be that this angle was so large that the planet's rotation would not allow night and day to exist? Well, Uranus, without going any further, rotates around an axis that forms almost 90 degrees with the ecliptic. It is as if our walker on the football field always moved around the central point looking towards the background, but doing lateral pirouettes, so that he rotated around his navel. The result for that planet in our solar system is that there is a point on the surface of Uranus that has 42 years of night and 42 years of day, throughout its year, which is equivalent to 84 Earth years.
Is there any other way to make it always night on an entire planet? Again, yes: it has no star and is a wandering planet. We already talked about these lone wolves, so we close today's article here. We concluded before, of course, saying that something as everyday for us as day and night should not exist. Although we would surely not exist as we do without those nights and those days, which could lead to saying that we would not be as we are without that conservation of angular momentum, that law of universal gravitation or those tidal forces (and other effects that we have not detailed) , that physics that ultimately explains the why of the universe.
Cosmic Void It is a section in which our knowledge about the universe is presented in a qualitative and quantitative way. It aims to explain the importance of understanding the cosmos 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 is and is, for the most part, empty, with less than one atom per cubic meter, despite the fact that in our environment, paradoxically, there are quintillions of atoms per meter cubic, which invites us to reflect on our existence and the presence of life in the universe. The section is made up Pablo G. Pérez Gonzálezresearcher at the Astrobiology Center, and Eva VillaverDirector of the Space and Society Office of the Spanish Space Agency, and Research Professor at the Institute of Astrophysics of the Canary Islands.
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