# The eclipse that changed the world

## Today is truly an auspicious day for science

Exactly one hundred years ago, on this day, there was an eclipse of the sun. It could only be seen in the southern hemisphere — in Africa and South America. A number of scientists made the arduous, expensive journey, for a very specific reason. Which leads us to the slightly long-winded story I want to tell you today. To make it especially auspicious I will publish it at exactly 14:14h UTC.

In the 17th century people had finally discovered that the sun was not circling a flat earth, and that the moon, planets and stars, and in fact the earth itself, were all spheres orbiting around the sun. Nicolaus Copernicus and Johannes Kepler, assisted by the observational astronomers Tycho Brahe and Galileo Galilei, did the pioneering work, and in the later part of the century one of the smartest people who ever lived, defined the elliptical motion of the planets.

It was Isaac Newton who worked everything out. To do this he had to discover and formulate the laws of gravity, and to invent calculus, an entire new branch of mathematics that was needed to work out the details. He also invented and constructed the first reflective telescope to check the validity of his calculations. *And after that he turned 26!*

Later Pierre-Simon Laplace worked out why the planetary orbits were so incredibly stable. Spoiler: it wasn’t God — the laws of gravity and Newton’s maths explained it perfectly.

So for a couple of hundred years everything was hunky-dory, planetary orbits could be calculated more and more precisely, and astonishingly exact predictions of astronomical events became possible. Newton’s laws and mathematics were all that was needed to explain everything that transpired in the Solar System. *Except in the case of Mercury.*

Newtonian physics told us how the closest point in the elliptical orbit of a planet, its perihelion, would “precess” (slowly rotate around the sun). But the precession of Mercury, the planet nearest the sun, deviated slightly from Newtonian predictions, and no amount of mathematical refinement could fully explain its behaviour.

The 19th Century saw another interesting development. Newton, like everyone else, used traditional geometry. But some mathematicians, starting with Carl Friedrich Gauss (who, incidentally, was deeply interested in celestial orbits), began contemplating a new form of mathematics. While the original founder of geometry, Greek mathematician Euclid of Alexandria, had postulated that parallel lines will never converge, these mathematicians devised the axioms and equations for an alternative system where they could actually meet. This non-Euclidean geometry was quite anti-intuitive, but it was logically consistent and fun to play around with. Of course it had nothing to do with the real world and the universe we lived in. So people thought.

Along came a German-born assistant examiner at a Swiss patent office, who was an imaginative mathematician and a theoretical physicist. Between 1905 and 1915 Albert Einstein published a number of papers on his Theory of Relativity, postulating that gravity is not a force but the effect of a curvature of spacetime. That implied that the universe cannot be adequately described in the traditional way — we must use non-Euclidean geometry.

Sounded interesting, yes, but where was the proof? Well, it worked for Mercury. Its orbit had been meticulously tracked for three centuries. Newton’s laws predicted that the gravitational tug of other planets should have caused a precession of 5557 arc-seconds per century on Mercury’s orbit, but astronomers had measured a precession of 5600 arc-secs. Newton’s laws provided no explanation for the discrepancy. Einstein’s physics, on the other hand, explained the 43 arc-sec difference perfectly, without any additional assumptions (like dust between Mercury and the sun, or an additional planet even closer to our star, which scientists had been considering in order to save the Newtonian equations). Clearly something was wrong with the older theory of gravity, something that the General Theory of Relativity corrected perfectly.

Einstein finished his description of General Relativity in 1915, in the middle of a vicious World War. He was a socialist, now trapped (and starving) in Berlin, under constant government surveillance. Hardly anyone outside Germany knew about him, and his theory was so counter-intuitive and enigmatic that only a handful of people understood it. One of them was British astronomer Arthur Stanley Eddington, and he made it his business to spread the news to the scientific world. Eddington was convinced that Einstein’s theory would revolutionize the foundations of physics.

It was not easy to convince scientists in Britain and non-German countries that this new theory, put forward by an “enemy”, had any validity. The correct prediction of the precession of Mercury could, many believed, be the result of *ad-hoc* adjustment of Einstein’s theorems to fit the observations. So Eddington used a classical strategy for empirical verification: draw a daring, non-obvious prediction from the theory, and test it against reality. And the opportunity to do this was fast approaching.

While Newton’s laws worked flawlessly in space when no substantial gravitational forces were involved, Einstein’s Relativity predicted that light passing near a large object would not travel in a straight line but be bent. Actually the ray would not bend — it was the warping of space-time next to the body that would cause the light to appear to deviate.

How could this be tested? Light (from a star) passing close to the sun could not be directly observed — the sun is so bright that seeing stars close to it was impossible. Unless…

During a total eclipse you could actually see (or photograph) stars close to the sun, and due to the curvature of space they should appear to be closer to each other than normal. Eddington and other scientists decided to undertake an arduous journey to the next solar eclipse to test Einstein’s theory.

The next eclipse would occur on May 29, 1919, and be visible in Africa and South America. The team decided to travel to the island Príncipe, off the coast of Africa in the Gulf of Guinea. There Eddington could record this truly historic image:

Six months later Eddington returned to Príncipe and photographed the same area of the sky, this time in the middle of the night. The results were compelling: without the sun, the stars were in specific places, with the sun between them they had moved to exactly where Einstein’s theory predicted. This was clearly due to the bending of the rays — actually of space — by the sun. Final confirmation was announced in November 1919.

For a while Eddington’s data and analysis remained controversial: some accused him of doctoring it to confirm Einstein’s predictions. But subsequent eclipses, conducted with more precise instruments, showed that General Relativity really worked: it could generally explain things just as well as Newton’s laws and mathematics (an important criterium for the success of a new theory); but it could, in addition, explain things where Newton’s model failed. So Relativity soon became accepted science.

The confirmation of Relativity through the 1919 eclipse was celebrated in newspapers around the world. In the above New York Times article Sir Joseph Thomson, President of the Royal Society, is quoted as follows: “Einstein predicted the deflection of starlight when it passed the sun, and the recent eclipse has demonstrated the correctness of the prediction.”

The confirmation made Einstein one of the most celebrated personalities in scientific history. Today nobody doubts Relativity — our entire understanding of physics has been shaped by it. Cosmological theory is based on Relativity, even near-earth technology (like GPS) takes the effects of Einstein’s theory into account, and would not work without it.

I wrote this article with the anniversary of the Príncipe eclipse in mind. That took place on May 29, 1919, and I am publishing this article in Medium.com on May 29, 2019. Today I saw that a number of news organisations have done likewise, though none with my fanatical adherence to timing: the eclipse in Príncipe reached its maximum at 14:14:22, and as I type these lines I am waiting for the UTC clock to reach 14h 14m and 22 seconds. That is when I will switch the article on — exactly 100 years, 0 hours and 0 minutes and 0 seconds after the Eddington images were recorded.

Now how auspicious is that?