From Einstein to Columbus, passing through Halley: eclipses that have marked history

A sign from the gods, a bad omen, a cosmic wonder, an evidence that mathematics govern the visible world... Eclipses have fascinated humanity since the dawn of time. And at every historical stage they have been interpreted in different ways. Today we know that an eclipse is "the total or partial occultation of a celestial body by the interposition of another celestial body between it and the observer." But perhaps it was more romantic to believe that a dragon devoured the sun, as they thought in 8th century BC China.

In ancient Mesopotamia there was even the figure of the substitute king, who sat on the throne during an eclipse. We don't know if any of those human decoys finally received the wrath of the gods instead of their monarch. What we do know is that the Babylonians were the first to predict eclipses. It was a matter of simple arithmetic: "Solar eclipses only happen at new moon and lunar eclipses only at full moon, and there are only two times a year when they can occur," explains Miguel Querejeta, an astronomer at the National Astronomical Observatory (OAN). The Babylonians knew how to detect these patterns and managed to refine the various temporal cycles of the Moon and Sun's movement with extraordinary precision. In fact, they were the precursors of astronomy and horoscopes, and for this reason they had masters who advised kings and told them whether an eclipse would be favorable or unfavorable to them (and the substitute king would have to be used). In Ancient Greece, it was when for the first time the physical mechanism that causes an eclipse was detected. Philosophers like Anaxagoras were the first to propose that the Moon was covering the Sun, and Aristotle finally confirmed it thanks to the round shadow of the Earth during lunar eclipses. But the first scientific description of a total eclipse is considered to have been made on August 21, 1560 in Coimbra (Portugal) by the mathematician and astronomer Cristoph Clavius. And since then some eclipses have helped advance science. "For many years an eclipse was the only time when the Sun and the solar corona could be well observed. Now with the probes we have sent into space, it is no longer necessary," explains Eduard Masana, a researcher at the Institute of Space Studies of Catalonia (IEEC).

The first eclipse on record took place in Ugarit, Syria, in the year 1223 BC, according to the book Eclipses. The sun and its eclipses in science and the arts, coordinated by Rafael Bachiller for Geoplaneta. This book gathers scientific and historical data about a phenomenon that this year once again captures our attention: on August 12th, part of Catalonia will experience a total solar eclipse. From that first one until today, some eclipses have played a key role in history or science. We explain some of them to you.

The eclipse that enthroned Albert Einstein

May 29, 1919

Albert Einstein has earned a place of honor in the popular imagination as the scientist who revolutionized our view of the Universe. But what not everyone knows is that a large part of this immense reputation is owed to a solar eclipse.

Albert Einstein's theory of general relativity finally provided in 1915 an explanation for a fact that had been obsessing physicists for years: the movement of Mercury around the Sun did not exactly correspond to the calculations of Newtonian physics. But the explanation, although plausible, was only a theory on paper. According to Newtonian gravity theory, the deviation of light from distant stars passing near the Sun's mass was one, and with Einstein's gravity theory calculations, the deviation was double. Therefore, it was only necessary to be able to measure this deviation to check which of the two was right. "And this could be done during an eclipse, because in a total eclipse the light of the Sun disappears and it gets dark during the day, and this allows us to see the stars behind it: with an eclipse like the one that took place in 1919, astronomers could know the apparent position of certain stars that would be close enough to the darkened solar disk. By photographing them at that moment, it would be possible to determine whether the deviation of their light corresponded to Einstein's theory or Newton's," explains José Carlos del Toro Iniesta, solar physicist at the Institute of Astrophysics of Andalusia (IAA-CSIC).

And so it was done. An experiment was launched, which was driven by the British royal astronomer Frank Dyson, although it ended up being called the Eddington experiment. The Royal Astronomical Society and the Royal Society of the United Kingdom organized two expeditions, one to the island of Príncipe (in the Gulf of Guinea), led by Eddington and Edwin Cottingham, and another to Sobral, Brazil. In the first, the sky was so cloudy that the number of useful photographs was drastically reduced, and in the second, spare equipment also had to be used because the heat in the area distorted the mirrors of the main telescope. But with the few valid photographic plates they managed to rescue from both locations, they did the calculation and saw that the theory that had calculated it correctly was Einstein's. Newspapers around the world echoed this. "Revolution in science: new theory of the Universe", titled The Times. The theory of relativity had been proven and, with it, Albert Einstein was consecrated as a world celebrity.

A new element of the periodic table

August 18, 1868

A solar eclipse served to discover a new element in the periodic table, the only one discovered in space before it was known to exist on Earth as well. And this element is, of course, helium. It was all thanks to the spectroscope, the device invented in the 19th century that manages to break down light into its various colors and wavelengths and that made it possible to identify each chemical element by the characteristic lines left by its light in this device.

Well, it was by analyzing the solar prominences that remain visible during a solar eclipse (the immense flares that emerge from the sun's corona) that the French astronomer Pierre Janssen detected, during the eclipse of August 18, 1868, which he went to see in India, that there was a bright and very intense yellow line next to the known lines of hydrogen. It did not match the fingerprint of any element known on Earth, so it had to be a new element. "The discovery is attributed to Janssen, but Joseph Norman Lockyer had already predicted it two or three years earlier, not with an eclipse but with a new method that allowed seeing the sun's prominences without an eclipse, by placing the spectrograph in a certain way," explains Del Toro. He had seen a new element and, together with the chemist Edward Frankland, named it helium, taking the name that Greek mythology gave to the sun god: Helios.It wasn't until 1895, almost 30 years later, that helium was detected on Earth, specifically in a laboratory. The Scottish chemist William Ramsay managed to isolate it -explains Del Toro- "by working on a uranium mineral called cleveite": upon burning it, he realized it produced the same yellow line that Janssen and Lockyer had seen on the Sun.

A chronological record of history

June 15, 763 BC and others

The science of eclipses is very precise. We can know with great accuracy on which day of which year – of our calendar, obviously – each of the total eclipses that have occurred throughout history took place. And it has been a very useful tool for historiography, as it has allowed very ancient chronologies to be dated. For example, in Assyrian culture, records on clay tablets speak of a solar eclipse in the 10th year of the reign of Assurdan III. Modern science has been able to identify that phenomenon as the eclipse that took place on June 15, 763 BC according to our calendar, and from there it was possible to accurately date the entire Assyrian chronology, the Mesopotamian civilization that lived in present-day Iraq, part of Syria, and southern Turkey from the end of the third millennium BC to 609 BC. And this is just one example of the chronologies that we have been able to decipher thanks to the eclipses recorded by civilizations such as the Hittites or the Zhou dynasty of China.

Halley, the salvation of sailors

May 3, 1715

A solar eclipse led to one of the first citizen science experiments in history. It was May 3, 1715, when astronomer Edmund Halley, who gave his name to the famous comet, asked all citizens of England to note when the totality of the eclipse began and ended for them, to have a record of the various points in geography. Halley observed it from the Royal Society, in London, where totality lasted three minutes and twenty-two seconds.

The astronomer was able to conduct a detailed study of that phenomenon, and with this data and others he later collected, he was able to propose a method for calculating the distance between the Sun and the Earth, a fundamental piece of data so that ships could calculate their longitude at sea. "Latitude was easy to calculate with a quadrant or an astrolabe, but geographic longitude was difficult to measure if the size of the solar disk and the distance between the Earth and the Sun were not accurately known," explains Del Toro. Halley proposed this by suggesting a careful observation of a transit of Venus: when this planet passes in front of the Sun. It involved observing this transit from different geographical points to calculate the angle formed between the different lines of sight of Venus on the solar disk from Earth and, with trigonometric formulas, to extract the distance in kilometers between the Sun and the Earth.

This data was fundamental for calculating the longitude of geographical coordinates. Halley could not see the definitive calculations because he was not alive when the transits of Venus in 1761 and 1769, which proved his theory, occurred. It was another scientist, Simon Newcomb, who in 1890 refined the definitive value. "It took more than a century to have the final result, but it was a very important milestone, because an error of one arcsecond in parallax means a deviation of four kilometers at sea," summarizes del Toro.

Geophysical calculations

April 15, 136 BC

A Babylonian tablet preserved at the British Museum describes "with almost scientific accuracy" an eclipse that occurred on April 15, 136 BC. "It details the time, duration, and other variables. With our current knowledge, we know that if the Earth's rotation speed at that time were the same as it is now, the eclipse would not have been seen in Babylon but in the Balearic Islands and Italy. And if the rotation speed responded to our calculations of braking caused by tides, it would have been seen in Afghanistan," explains the solar physicist. But since from Babylonian records we know that it was seen from Babylon, "we have to modify our geophysical models of Earth's rotation braking and adjust several factors. After all, it is an error of half a millisecond per century, but in the long run, it is an important difference," he points out. Therefore, science has been able to refine its calculations on the Earth's rotation speed – says Del Toro – "thanks to an eclipse that took place in the year 136 BC."

An eclipse to bring peace

May 28, 585 BC

"And suddenly day turned into night." This is how the Greek historian Herodotus (484 BC - 425 BC) recounts this mythical episode of ancient history and celestial history. He explained it a century after it happened, because the events had taken place on May 28, 585 BC, as computer astronomical calculations have been able to determine. The Medes and Lydians, two border civilizations disputing territory in present-day Turkish Anatolia, had accumulated five years of bloody and intermittent war. On that day, the two armies met again on the battlefield on the banks of the Halis River. And as the soldiers, on foot and on horseback, crossed arrows and spears in a violent and fierce combat, suddenly the sky darkened. The men dropped their swords and shields. The combat stopped. Everyone interpreted it as a sign from heaven: the gods were very angry about that war. The leaders of both sides understood that they had to sign peace, and so they did. They set the Halis River as the official border between the two empires and, to seal the alliance, they married the daughter of the Lydian king to the son of the Median king. What five years of deaths, weapons, and diplomatic attempts had not achieved, the Moon achieved by interposing itself between the Earth and the Sun.

Change of regime in Ancient Egypt

May 14, 1338 BC

In the 14th century BC, a change occurred in Ancient Egypt that has fascinated historians for centuries. The son of Amenhotep III, destined to reign as Amenhotep IV, imposed a radical change in state policy: he banished the cult of Amun, which dominated the polytheistic tradition until then, and established the monotheistic cult of the sun god, Ra. He had his name changed to Akhenaten, introducing into his title the name that referred to the solar disk: Aten. "We believe that this decision was the result of a total solar eclipse that we know took place on May 14, 1338 BC, and which also led to the displacement of the capital from Thebes (modern Luxor) to Amarna, hundreds of kilometers to the north, in a place in the middle of the desert," where it is believed that eclipse could be seen, explains Miguel Querejeta, from OAN. In Thebes, the eclipse covered the Sun by 90%, but in Amarna it was seen completely. "There are no written records of this – Querejeta admits – but he builds a city in the middle of nowhere, which is very strange, and it just so happens to be in the path of totality of the eclipse".

Everything points to the eclipse being the trigger for all these political changes, or at least that is the theory defended by experts such as the archaeoastronomer Juan Antonio Belmonte from the Institute of Astrophysics of the Canary Islands (IAC). However, this theory implies accepting a chronology of Akhenaten's reign that is not universally known. According to traditional chronology, 1338 BC does not mark the beginning of Akhenaten's reign, but rather the penultimate year of his reign. For proponents of this other version, the solar eclipse would, on the other hand, be a bad omen that was interpreted as the beginning of the end for Akhenaten and a sign that it was necessary to put an end to the experiment of the sun cult and recover the previous orthodoxy, as was later done by Akhenaten's nephew, Tutankhaten, who changed his name to Tutankhamun.

It is very difficult to know which is the real version, because in more than 2,000 years of pharaonic history there are no written records of any solar eclipse. The first record made was in the 6th century BC, after two millennia of hieroglyphs and Egyptian writing. How can this be? "The indirect deduction from this fact is that in Ancient Egypt solar eclipses were considered a taboo – says Querejeta – and that merely mentioning them brought bad luck".

The deception of Christopher Columbus

February 29, 1504

At times the ability to predict eclipses has been very useful, unfortunately, as a tool for manipulation and collective deception. The most famous case is Christopher Columbus's trickery of the indigenous people of the island of Jamaica. The episode took place in 1504, during Columbus's fourth and final voyage to America. An epidemic of marine borers had devoured the structure of the four ships of his expedition, to the point that they had to abandon two of the ships. With the other two, they docked on the north coast of present-day Jamaica and settled there as a floating camp, unable to navigate them. For months, the native population of the island, the Tainos, fed Columbus's crew and exchanged provisions for mirrors, knives, whistles, and other interesting items. But after six months of forced exile, Columbus's sailors mutinied and even robbed and murdered some of the indigenous people, while they were tired of having to share food with a bunch of ungrateful parasites.

Fearing for the survival of his men and himself, Columbus, who was confined to his cabin with a bout of gout, devised a stratagem. He carried with him a copy of the Almanac of Regiomontanus, a German astronomer, which contained detailed astronomical predictions. There he saw that on the night of February 29, 1504, there would be a total lunar eclipse. Three days before the date, he requested an audience with the indigenous chief. He explained that they served a very powerful God who was angry with the island's natives because they had stopped providing them with food. He told them that, to show this anger, within three days he would hide the Moon and dye it blood-red, as a sign of the punishments he would inflict upon them. The indigenous leaders laughed at the outlandish prediction. But three days later, as the full moon rose, they saw it all come true: little by little the Earth's shadow began to cover the Moon, which took on the reddish color typical of a lunar eclipse. The indigenous people panicked and hurried to bring provisions to the ship, and they prayed to Columbus to intercede for them before his God. To add to the theatricality – according to the account given by his son Hernando – Columbus told them that he had to retire to his cabin to pray, and there he stayed for 50 minutes, waiting for the eclipse to end. When he had only a few minutes left, he emerged and told them that their God was withdrawing the punishment because they had pledged to continue bringing food to the crew. Shortly thereafter, the Moon was seen whole again. The poor Taínos, deceived, continued to feed Columbus's men until they left, months later, on a rescue ship.