May 29, 1919 dawned cloudy in Sobral, a city in the state of Ceará. But the clouds gradually began to thin, and the sky cleared. People and equipment swarmed around the city’s Jockey Club, where astronomers from Rio de Janeiro and London were preparing to observe and record a phenomenon that would invert day and night for a few brief minutes. Shortly before 9:00 am, the moon’s disk began to slide over the sun’s, completely obscuring it within minutes. The total solar eclipse in Sobral entered the annals of science history because it provided the experimental evidence for a scientific assumption made by the general theory of relativity, published four years earlier by the German physicist Albert Einstein (1879-1955): matter and energy cause warps in space-time, which could also deflect the path of light traveling through it.
To record the eclipse and confirm whether the Sun’s gravitational field would bend starlight, the astronomers on the Brazilian and English expeditions used photographic glass plates, which have now been recovered and restored by researchers at the National Observatory (ON) in Rio de Janeiro. Stored in boxes in the library were 900 plates, of which 61 recorded the phenomenon and the moments that preceded it in the tranquil city in Ceará State. The plates, measuring 24 by 18 and 9 by 12 centimeters (cm), are coated with an emulsion consisting of light-sensitive silver salts. They were stored in a section of the ON library reserved for rare works, and had not yet been closely examined. Most are well preserved and clearly show an image of the Moon obscuring the Sun and the astrographic refracting telescope used to observe the eclipse.
In an article published several days prior to the event in a local newspaper, astronomer Henrique Charles Morize, then-director of the National Observatory, had requested that people viewing the eclipse do so in silence, refraining from shooting off fireworks in order not to interfere with the observation of the eclipse and compromise the quality of the photographs taken.
Morize oversaw the work of the English expedition and headed up the Brazilian team in Sobral. One of the mission’s objectives was to conduct spectroscopic observations of the Sun’s corona. During the eclipse, several photographic plates were taken in succession using cameras attached to telescopes, recording the position of the stars close to the Sun’s edge. The eclipse lasted exactly five minutes and 13 seconds. “Analysis of the images recorded on the plates subsequently helped to explain the laws of physics that govern the movement of celestial bodies,” says astronomer Carlos Veiga, a researcher at the Center for Astronomy and Astrophysics and head of the Division of Educational Activities at the Observatory. Veiga and other librarians worked for a month to restore the photographic glass plates on which the images of the eclipse are recorded. It is slow work, says Veiga, since the plates are fragile, and some have chipped edges. “The plates are now in special boxes, wrapped in paper that is designed for this type of material since it protects them from chemical reactions that can damage them,” he explains. Veiga and his team plan to digitize the material and make it available for on-site consultations at the Observatory.
Another team of English astronomers observed the 1919 eclipse simultaneously on the island of Principe in West Africa. Bad weather, however, compromised the quality of the images. On some plates, the stars appear clearer, while on others, they disappear in the cloudy sky. In Sobral, considered one of the best regions from which to observe an eclipse, the sky was clear during the event, and the plates recorded 12 stars, which were later used as a reference to measure the angle of deviation for the light beam trajectory. This effect, called light deflection, had been predicted by Einstein’s general theory of relativity: a beam of light from a star would have its trajectory curved or deflected as it passed through regions that had a very strong gravitational field. This deflection in the trajectory of light, according to astronomers, would make it seem as if the stars observed were in a position that appeared to be different from their actual position; according to Einstein, their light, as it passed near the Sun, would be deflected by 1.75 arcseconds.
The astronomers in Sobral planned to measure a small angle formed by these two positions. It was a unique opportunity, as the eclipse would make it possible for a few short minutes to photograph the stars deep in the sky and closest to the Sun’s edge at a distance of 150 light-years from Earth —each light-year equals about 9.5 trillion kilometers. One of the stars analyzed, Hip 20712, was a strong candidate to confirm the German physicist’s predictions. Measuring the angle of deviation in the light’s trajectory was not an easy task. The images of many stars were lost in the diffuse halo created by the Sun’s light or covered by the Moon’s disk, and atmospheric turbulence also slightly compromised the quality of the photographic plates. The English team stayed in Sobral until July 1919 to photograph the same star field at night without the influence of the Sun’s gravitational pull. The idea was to compare the positions of the stars closest to the Sun’s edge in the two images.
Confirmation of Einstein’s theory came months later. On November 6, 1919, astronomers at the Royal Astronomical Society in London concluded that the general theory of relativity was correct after analyzing the results obtained from the photographic plates created in Sobral and Principe. The final result of the observations made at Principe demonstrated an average deviation of 1.6 arcseconds, while the data gathered at Sobral had a deviation of 1.9 arcseconds, which was almost twice the estimated number in the gravitational theory devised by English physicist Isaac Newton (1643-1727), developed and announced 250 years earlier. In view of the margins of error, Einstein was right. “As a result, Newton’s universal theory of gravitation became a specific example of Einstein’s general relativity,” explains Veiga.
“We can view the photographic plates produced in Sobral and Principe as a decisive moment in the history of 20th century science, when ideas proposed by the theory of relativity had not yet been embraced,” says physicist and historian of science José Luiz Goldfarb of the Center Simão Mathias at the Pontifical Catholic University of São Paulo (PUC-SP). “Challenging the ideas derived from Newton related to classical mechanics met with strong opposition, given that proposals from Einstein and other scientists completely changed our understanding of the world.” Goldfarb believes that conserving the photographic plates keeps alive the memory of the transformations that occurred in science early in the 20th century and still affect how we see the world today.