First observation of helium in the Sun’s spectrum
DOI: 10.1063/PT.5.031031
On this day in 1868 Jules Janssen (shown here) was observing the spectrum of the Sun’s chromosphere when he discovered a bright yellow line at a 587.49 nanometers. Norman Lockyer found the same line two months later and named it D3, because it was close to two sodium lines named D1 and D2. Lockyer concluded that D3 arose from an element in the Sun unknown on Earth. He and Edward Frankland named the element “helium,” after the Greek word for the Sun, ἥλιος (helios). Helium was indeed unknown on Earth -- until Luigi Palmieri detected the D3 spectral line in 1882 during his analysis of the lava of Mount Vesuvius. William Ramsey isolated the gaseous element 13 years later.
Date in History: 18 August 1868
