The last total solar eclipse in Earth's history

Since in the eclipse that will take place next week in North America, the Moon, even if only by getting in the way, in a certain way, will become the protagonist, let's talk about it for a moment. Let's analyze the fortuitous fact of having a natural satellite that is 400 times smaller than the Sun and that it is also 400 times closer. A cosmic coincidence that we have to thank, since it now allows the possibility that a solar eclipse could be total.

Let's clarify that now that we mentioned before. To do this, let's take a fact that we know very well thanks not only to the theory that explains the movement of bodies, but also to the laser retroreflectors installed on the surface of our satellite by the missions Apollo and Soviet Moon. The Moon is moving away from the Earth at a rate of 3.8 cm per year.

Let's do the math: moving away at 3.8 cm per year, there will come a time when, seen from our planet, the Moon will not be able to completely shadow the Sun. The last total solar eclipse in Earth's history will occur in 200 million of years and will last a brief second. However, you won't find a self-respecting astrophysics professional betting anything on this fierce prediction of geometry.

The key is that it is not possible to give a deterministic solution to the celestial dynamics of a system when there are more than two bodies involved on a time scale, we are talking about millions of years, so long. The solution to the problem is in chaos theory and it tells us, in a few words, that there are too many variables to predict the behavior of the Earth-Moon-Sun system within the Solar System in such a long term and that we would have to talk about probabilities.

Let us now travel to the past, to the dark. It is in the darkness of the night where we perceive the immensity of the cosmos. And this extends to solar eclipses, those few minutes where time seems to be suspended. Day turns into night and suddenly a revealing light appears: the solar corona. The solar corona is faint, hot, and tells us something we intuitively understand very well: we are connected to our star by more than just its light. The Sun evaporates and its particles in the form of wind They travel throughout the Solar System.

In the solar eclipses, In just a few minutes and in a long, narrow path, the shadow of our natural satellite travels across the surface of some fortunate places on the planet. In that moment where three celestial bodies (Sun, Moon and Earth) are perfectly aligned is when magic occurs in the human being who has the privilege of observing it.

And I affirm that it is a privilege because now, at this moment in the history of our civilization, there are people who can travel all over the planet to enjoy it, since total eclipses are rare at a given point on Earth in the brief interval of a human life. But, in addition, for a couple of centuries we have known how they work and if we go back to our most recent past, fear has been the most common emotion when observing the alteration of the natural order of things. . Science has freed us from fear and, in return, has given us fascination.

Various ancient cultures were able to predict the duration and dates of eclipses, although they did not understand their mechanism. But due to the doctrinal nature of the knowledge, it was not disclosed to the population and was used as a weapon of power. In a battle the fear they instilled could turn the tables towards the side whose astrologers had whispered more auspicious omens to their princes. Some eclipses, in fact, have somewhat changed the course of human history.

But what is fascinating is that thanks to the fact that historical records of the location of eclipses that occurred thousands of years ago are preserved, it is possible to measure the variation in the rotation speed of our planet. It may be surprising to learn that the length of the day has not been constant throughout Earth's history.

In predictions of the positions of the Sun, Earth and Moon over periods of thousands of years in the past and future, a variable factor is the speed of the Earth's rotation, where short-term variations are superimposed on the more regular ones of long period. Nowadays with atomic clocks and astronomical references it is easy to accurately measure the speed of rotation of our planet, but that measurement was not available in the past and that speed is not regular enough to be able to predict the exact location of the shadow of the planet. moon on the Earth's surface in periods of thousands of years. Therefore, by doing the exercise in reverse and using the historical records of well-dated eclipses in different places in the world in the last thousands of years, it is possible to determine what the variation in the speed of rotation of the Earth has been in periods of thousands of years. of years.

Thus it has been measured, thanks to historical eclipses, that the Earth rotates more slowly, which translates into the days becoming longer. How important is this? Well, it allows us to investigate the process(es) that contribute to the decrease in the rotation speed of our planet in the long term. It may be due to the melting of glaciers, due to magnetic interactions between the molten core and the mantle in the Earth's interior. Or also caused by heat dissipation due to tidal forces that we know increase the length of the day on average 2.3 milliseconds per 100 years, this last effect well measured thanks to artificial satellites.

In this journey through the future and the past of eclipses on Earth we only have to remember, since there is little left, that in Spain and visible from a large part of the territory we will be able to enjoy not one, not two but a trio of eclipses solar eclipses very soon: two total eclipses (2026 and 2027) and one annular eclipse (2028). And we will do it without fear because we know very well how they work.

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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