OTD In Space - Aug. 18: Total Solar Eclipse Leads to Discovery of Helium

On Aug. 18, 1868, a French astronomer named Pierre Jules César Janssen discovered helium while observing a total solar eclipse in India.
During the eclipse, Janssen was using a spectroscope to look at the spectrum of light emitted by the sun. This tool allows scientists to determine what elements they're looking at. While observing solar prominences during the eclipse, Janssen noticed a bright yellow line that didn't correspond to any elements that was not yet discovered on Earth. Thirteen years later, the Italian physicist Luigi Palmieri found the first evidence of helium on Earth in the lava of Mount Vesuvius.