Solar eclipse of May 29, 1919

• 1. Introduction

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  The solar eclipse of the 29th of May 1919 lasted just six minutes and fifty seconds at its maximum, but those seconds rewrote the laws of physics. On a Thursday morning, the Moon passed between the Earth and the Sun with a magnitude of 1.0719, plunging a narrow path across South America and Africa into total darkness. What made this particular eclipse unlike any before it was not its duration, though it was the longest since the 27th of May 1416. It was the fact that two teams of British astronomers were waiting for it, cameras ready, prepared to test one of the most radical ideas in the history of science. The question at the heart of their journey: does the Sun bend light? And if it does, does it bend it by the amount a young German physicist named Albert Einstein had predicted a few years earlier? The answer, captured on photographic plates in Brazil and on an island off the west coast of Africa, would shake the foundations of Newtonian physics and elevate Einstein to global fame.

• 2. The Geometry of Darkness

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  Only 19 hours before the eclipse reached its maximum, the Moon passed through perigee, its closest point to Earth, at 18:09 UTC on the 28th of May 1919. That timing was crucial. Because the Moon was near perigee, its apparent diameter in the sky was larger than usual, large enough to completely block the Sun's disc. The totality path cut across southeastern Peru, northern Chile, much of Bolivia, and central Brazil, then crossed the Atlantic and entered Africa, sweeping through southern Liberia, the southern Ivory Coast, the island of Príncipe, and on through what was then French Equatorial Africa, the Belgian Congo, northern Rhodesia, German East Africa, northern Nyasaland, northern Mozambique, and the western Comoros. For observers lucky enough to stand in that narrow band, day turned to darkness in an instant. The rest of South America and Africa saw only a partial eclipse, the Moon taking a bite from the Sun but never swallowing it whole. The greatest eclipse moment occurred at 13:08:54.5 UTC, and the greatest duration at 13:09:53.0 UTC.

• 3. Newton's World and Einstein's Wager

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  Isaac Newton's laws of physics rested on the idea of absolute time and three dimensions of space: time was universal, ticking the same for everyone everywhere. Einstein proposed something fundamentally different. He argued that space and time were not separate stages for the universe's drama but were woven together into a four-dimensional fabric. Extremely small matter particles could, in his framework, produce massive amounts of energy. One testable consequence followed from this: when light passed near a large mass like the Sun, gravity would bend its path. If Einstein was right, the apparent positions of stars seen close to the Sun's disc would shift slightly from their true positions, because the Sun's gravity would curve the starlight on its way to Earth. Stars cannot be compared to their true positions during normal daylight, since solar glare drowns them out entirely. A total solar eclipse was the only natural laboratory where that test was possible. Following an unsuccessful attempt to measure this bending during the solar eclipse of the 8th of June 1918, two expeditions were organized under the direction of Sir Dyson to try again in 1919.

• 4. Two Expeditions, Two Continents

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  Sir Arthur Eddington led one expedition to the island of Príncipe, off the west coast of Africa in Portuguese São Tomé and Príncipe. The second team, led by Andrew Claude de la Cherois Crommelin and Charles Rundle Davidson, sailed to Sobral in Brazil. Both teams were hunting the same stars: the Hyades, a star cluster in the constellation Taurus. On the morning of the eclipse, the Brazilian and African skies held their breath in the worst possible way. A thunderstorm had struck that morning, and the skies had been overcast for many of the preceding days. Thirty minutes before totality, the clouds finally began to dissipate, but even during the eclipse itself the astronomers were photographing stars through gaps in the clouds rather than through clear sky. The margin between success and failure was measured in minutes. When totality arrived, the two teams took their pictures as fast as they could, capturing the displaced positions of the Hyades against the darkened sky.

• 5. Stars Seen in New Places

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  The photographs came back from Príncipe and Sobral, and Sir Dyson led the careful analysis of the plates. His conclusion was unambiguous. He is quoted saying, "After a careful study of the plates, I am prepared to say that they confirm Einstein's prediction." He went on to explain that the results left little doubt about light deflection in the region around the Sun, and that the deflection matched the amount Einstein had demanded in his generalized theory of relativity. The Sun's mass had shifted the apparent path of starlight in exactly the way Einstein said it would. Those photographs provided evidence that the Sun's gravitational field bent starlight by a measurable amount. The eclipse of the 29th of May 1919 allowed Einstein to finalize his theory of relativity in the eyes of the scientific community. Physics, as it had been understood since Newton, would never look quite the same again.

• 6. Near Misses Before 1919

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  The 1919 eclipse was not the first attempt to test Einstein's idea. Before 1919, astronomers had fixed their hopes on two eclipses in 1912. The first, on the 17th of April 1912, became known as the Titanic Eclipse because it occurred just two days after the sinking of the RMS Titanic. Popular superstition connected the eclipse to the disaster, and what might have been a sober scientific expedition became something closer to a spectacle. Underfunding, poor preparation, and insufficient duration of totality compounded the problem. The second 1912 eclipse, on October 10, was simply rained out, photographs impossible due to persistent cloud cover. When the 1918 eclipse offered another chance, that too failed to produce usable results. Each near-miss pushed the decisive test forward. A longer total solar eclipse would eventually occur on the 8th of June 1937, but by then the 1919 plates had already closed the question.