Science of eclipses: from validating Einstein to studying the ionosphere

Total solar eclipses are natural laboratories that generate scientific knowledge, from the discovery of helium to the confirmation of the General Theory of Relativity, and now help us understand the behavior of living beings and Earth's atmosphere.

The oldest precisely dated record of a total solar eclipse dates back to 1223 BCE. This observation was inscribed on a clay tablet found in the ruins of Ugarit, Syria, according to solar physicist José Carlos del Toro. Science often takes its time; consider the eclipse of 133 BCE in Babylon, described on a tablet in the British Museum. This record has led scientists to question their models of Earth's rotation. If Earth had rotated at today's speed, the eclipse would have occurred in the Balearic Islands, but if it slowed at the expected rate, it would have been seen in Afghanistan. "We need to revise our calculations" because, in addition to tides, other causes must be influencing the braking rhythm of rotation.

Total eclipses allow for the observation and study of the solar corona, the outermost part of the sun's atmosphere, which holds a mystery: scientists are not certain why its temperature is vastly higher than the sun's surface. The first clearly interpretable reference to the corona came from Plutarch. In what is considered the first scientific description of an eclipse, that of 1560 in Coimbra, Portugal, by Jesuit Christopher Clavius, there is mention of a great light surrounding the sun. It took until the 19th century for scientists to determine if this halo was a solar, lunar, or terrestrial phenomenon, a century that saw significant discoveries.

The chemical element helium was discovered by British astronomer Joseph Lockyer while observing a solar eclipse. It would be years before helium was also found on Earth. However, coronium, an element believed to have been discovered by observing the corona's spectrum in 1869, turned out not to exist. More than 30 years later, it was determined to be iron ionized 13 times, explained Del Toro. But the most significant contribution of eclipses to science was providing one of the first experimental proofs of Einstein's Theory of Relativity. This was achieved by observing the May 1919 eclipse during a dual British expedition, one to the island of Príncipe and another to Sobral, Brazil.

The results confirmed that light bends when passing through an intense gravitational field like the sun's, and it did so within the predicted range. That demonstration significantly boosted Einstein's popularity and paved the way for numerous applications, including the relativistic gravitational corrections used by GPS, as cited by Del Toro. However, science is now refocusing on Earth, involving citizens more than ever in various projects, highlighted Dr. Montserrat Villar, an astrophysicist at the Astrobiology Center.