August 18, 1868. Jules Janssen “invents” helium. (At least, according to principal Skinner. “Curse the man who invented helium! Curse Pierre Jules César Janssen!)

Janssen was observing an eclipse and measured an emission line with a wavelength of 587.49 nm, which didn’t correspond to any known element. Norman Lockyer also observed the line later that year, and as it could not be reproduced in the lab, proposed that it was a new element, which was named after helios, the sun.