How eclipses taught us to be free

Spanish version here.

A total solar eclipse is one of those phenomena capable of disrupting our view of the world, at least for a few minutes. Day turns into night, temperatures drop, the landscape takes on an eerie hue, and animals alter their behaviour. The Sun, which had seemed the grandest presence in the sky, disappears.

For a society without astronomy, physics or the ability to calculate such events, the disappearance of the Sun was much more than a mere geometrical accident. It was a cosmic threat. Many cultures interpreted eclipses as the attack of a mythological creature or the manifestation of a supernatural will. In Hindu tradition, Rahu devours the Sun. Other cultures tell of dragons, wolves, serpents or celestial jaguars. The stories vary, but they nearly always contain the same element. There is someone behind the phenomenon.

This tendency to look for agents does not belong solely to the past. It is a deep-seated feature of our brains. We are wired to detect intentions, even where none exist. We see faces in clouds or in the textured plaster on a wall, we hear howling voices in the wind, and attribute purpose to events that may be entirely accidental. From an evolutionary point of view, this makes sense. For our ancestors, it was safer to mistake the wind for a predator than to make the opposite mistake. Natural selection favoured a highly sensitive system for detecting agents—whether human or animal—even if it produced false positives.

The anthropologist Stewart Guthrie developed this idea in his book Faces in the Clouds. Our minds interpret the world in terms of the presence of agents because agents are the most important elements in our environment. A moving branch, a storm that destroys the harvest or a lightning bolt that sets a tree on fire can seem to be acting under the influence of some will. From there, it is a relatively small step to the god of the storm, the spirit of the forest or the creature that eats the Sun. Superstition did not arise because our ancestors were less intelligent. It arose because they were human.

There is a fundamental difference between the two ways of interpreting nature: magical and scientific thinking. Magic conceives of the world as the result of acts of will. Something happens because someone desires it, permits it or causes it. Scientific thinking seeks underlying causes that are predictable and repeatable. It replaces the question “Who wanted this to happen?” with a very different one: “What pattern does it follow?”

Eclipses played a privileged role in that transition. They were the first truly powerful phenomenon that we learned to predict. The Sun can disappear in an utterly overwhelming way and yet do so on precisely the predicted day and at precisely the predicted time.

One of the most famous stories from antiquity concerns the eclipse of Thales of Miletus. Herodotus recounts that the Lydians and the Medes were fighting a battle when day suddenly turned into night. The combatants interpreted the phenomenon as a sign, stopped fighting and agreed to make peace. According to Herodotus, Thales had previously announced that the eclipse would occur.

The episode is usually identified with the eclipse of 28 May 585 BCE, although the story should be treated with caution. We do not know to what extent Thales could have predicted it or what method he might have used. The account was written long after the event and may contain a considerable amount of later elaboration. Its symbolic importance, however, is enormous.

In a single scene, two ways of looking at the sky—and of thinking about the world—intersect. For the combatants, the eclipse is a message from the gods. For Thales, at least in the image of him constructed by tradition, it is a predictable natural phenomenon.

Prediction does not necessarily require a complete theory. Babylonian astronomers discovered cycles by observing the sky over generations. One of the most important is the Saros, an interval of 223 synodic months—slightly more than 18 years—after which the geometry of the Sun, Earth and Moon becomes approximately similar again. Eclipses do not recur in the same place because the cycle includes an additional fraction of a day, and they therefore appear over a different part of the Earth. With enough observations, however, the pattern becomes recognisable.

The Babylonians did not know about universal gravitation. They had no modern theory of orbits. But they could recognise patterns, and on that basis they learned to make predictions. This contains an important lesson about the history of science: we often learn to predict before we attain a deep understanding.

The Antikythera mechanism represents one of the most extraordinary moments in this process. It was recovered in the early twentieth century from the remains of a shipwreck and, for years, appeared to be little more than a corroded mass of bronze. Modern imaging techniques later revealed a complex network of gears. It was an astronomical machine built in the Hellenistic world, probably between the second and first centuries BCE.

The mechanism could represent various calendrical and astronomical cycles. Among its functions was a dial based on the Saros, capable of indicating the possible occurrence of solar and lunar eclipses. The sky was no longer merely something to be observed. Its cycles had become a machine.

It is difficult to imagine a better symbol of the transition from magic to calculation. By turning a crank, the user set the cosmos in motion. It has been suggested that the mechanism may have been connected to the technical tradition of Archimedes. There is no evidence that he built it, and any direct connection remains speculative. Nevertheless, ancient authors such as Cicero described devices attributed to Archimedes that mechanically reproduced the movements of the celestial bodies. At the very least, the Antikythera mechanism belongs to that same intellectual universe, in which geometry, astronomy and precision engineering began to intertwine.

Of course, the ability to predict eclipses did not completely eliminate superstition. Since antiquity, magical and scientific thinking have coexisted to varying degrees. And this coexistence contains a fundamental asymmetry. Just as knowledge is power, magical thinking makes us vulnerable and easy to manipulate. In Mark Twain’s novel A Connecticut Yankee in King Arthur’s Court, the protagonist, a time traveller, uses his knowledge of an imminent eclipse to present himself to the inhabitants of the Middle Ages as a superior being capable of extinguishing the Sun.

Although fictional, this story has a rather troubling historical precedent. In 1503, during his fourth voyage, Christopher Columbus became stranded in Jamaica with two damaged ships. The expedition depended on food supplied by the local population, but relations deteriorated and provisions began to run short. Columbus had astronomical tables and knew that a lunar eclipse would occur on 29 February 1504. According to the traditional account, Columbus told the native inhabitants that his God was angry and, as proof, would make the Moon disappear. When the Moon entered the Earth’s shadow and took on a reddish hue, the phenomenon had the intended effect, terrifying the local population and prompting them to reopen negotiations. Columbus pretended to intercede on their behalf and, when the eclipse ended, presented the Moon’s recovery as the result of his mediation with his powerful God.

Rational thought makes us free. Not because it automatically eliminates fear or superstition, but because it turns knowledge into something verifiable. Authority no longer depends on a priest or an intermediary with the gods. In principle, anyone can learn the method, repeat the calculation and test the prediction. Historically, eclipses stand at a crossroads between magical and scientific thinking. They constitute the first great victory of human predictive power over the portents of nature, demonstrating that we can look beyond the faces in the clouds and recognise the natural patterns that shape the world and its circumstances. The universe did not become any less astonishing as a result. But we have learned to become freer—among other things, freer to appreciate it.

The geometry of a total eclipse is, in fact, extraordinary. The diameter of the Sun is approximately four hundred times greater than that of the Moon, but the Sun is also approximately four hundred times farther away. As a result, the two have almost the same apparent size in the sky.

This coincidence has not always existed, nor will it last forever. The Moon is slowly moving away from the Earth as a consequence of the interaction between the tides and the Earth’s rotation. In the past, it appeared larger in the sky and covered the Sun more completely. Several hundred million years from now, it will be too small to conceal the Sun entirely. Total eclipses will then cease to occur, leaving only annular eclipses. We therefore live at an exceptional time in the history of the Earth–Moon system. Let us enjoy total solar eclipses, because they will not be here forever.

Today, we continue to study the Sun and attempt to understand its regularities and cycles, such as its 11-year magnetic cycle. We are still trying to identify patterns that will allow us to predict and understand its cycles and solar storms: what configuration of the magnetic field prepares them, how energy accumulates and at what point the system loses stability and erupts. We have learned to observe these magnetic structures, track the clouds of plasma travelling through the Solar System and estimate whether they will reach the Earth. Our predictive ability, however, remains limited. We can identify potentially dangerous regions, but we are still unable to anticipate with sufficient precision when an eruption will occur, how intense it will be or what its consequences will be.

This is one of the objectives of modern solar physics and of telescopes such as the European Solar Telescope (EST). EST will study the magnetic fields and dynamics of the solar plasma in enormous detail, in the regions where the energy that drives solar activity is stored and begins to be released. The goal is to continue along the path that began thousands of years ago with eclipses: to turn phenomena that appear capricious into processes that can be understood and, one day, predicted.

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