In 1868, an eclipse in India revealed an element found on Earth 27 years later

In August 1868, the Sun briefly became a laboratory for astronomers gathered in India to watch a total solar eclipse. Among them was the French astronomer Pierre Jules Janssen, who turned a spectroscope towards the bright prominences rising around the eclipsed solar disc. Their light contained a yellow spectral line that seemed to sit close to the familiar sodium lines, yet did not quite match them. At the time, nobody could say what produced it. The observation would eventually be linked to helium, an element that was then unknown on Earth. But the story is slightly different from the familiar version. Janssen saw the mysterious line, but he did not identify helium, and the substance itself would remain a solar mystery for decades.How the 1868 solar eclipse revealed a mysterious yellow spectral lineAccording to the study published in ResearchGate, titled ‘Observations of the total solar eclipse of 18 August 1868 carried out by Jules Janssen at Guntoor, India ’, the eclipse of 18 August 1868 gave astronomers an unusual opportunity to examine the Sun's prominences through spectroscopy. Instead of relying only on their visible appearance, they could break their light into its compone...