This Day In Tech: August 18th, 1868

Helium Discovered During Total Solar Eclipse

By

Hadley Leggett

Wired.com Aug. 18th, 2009

1868: A French astronomer spots an unknown element, now known as helium, in the spectrum of the sun during a much-anticipated total eclipse. The event marks the first discovery of an “extraterrestrial” element, as helium had not yet been found on Earth.

Astronomers had been eagerly awaiting a total solar eclipse since 1859, when German physicist Gustav Kirchhoff figured out how to use the analysis of light to deduce the chemical composition of the sun and the stars. Scientists wanted to study the bright red flames that appeared to shoot out from the sun, now known to be dense clouds of gas called solar prominences. But until 1868, they thought the sun’s spectrum could only be observed during an eclipse.

French astronomer Pierre Jules César Janssen camped out in Guntoor, India, to watch as the moon passed in front of the sun and revealed the solar prominences. Like other sun-gazers that morning, Janssen discovered that the prominences were mostly made of super-hot hydrogen gas. But he also noticed something extra: Using a special prism instrument called a spectroscope, he determined that the line of yellow light everyone had assumed to be sodium didn’t match up to the wavelength of any known element.

Janssen wanted to keep studying the mysterious line, and he was so impressed by the brightness of the sun’s emission lines that he felt sure they could be seen without an eclipse, if he could just figure out how to block other wavelengths of visible light. Working feverishly over the next few weeks, Janssen built the first “spectrohelioscope,” a device specifically designed to examine the spectrum of the sun.

Unbeknownst to Janssen, a second scientist was also working on the same problem 5,000 miles away. English astronomer Joseph Norman Lockyer succeeded in viewing the solar prominences in regular daylight in October 1868. In stunning scientific synchronicity, the two scientists’ papers arrived at the French Academy of Sciences on the same day, and today both men are credited with the first sighting of helium.

At the time, however, Lockyer and Janssen got ridicule rather than accolades for their discovery. Other scientists didn’t believe the astronomers’ account of a new element … until 30 years later, when Scottish chemist William Ramsay discovered a perplexing earthly gas hidden inside a chunk of uranium ore.

Ramsay sent the sample to Lockyer for confirmation. The scientist was thrilled by the element’s “glorious yellow effulgence,” which he described in the Proceedings of the Royal Society of London in 1895. Finally vindicated, Janssen and Lockyer were honored by the French government with a gold medal bearing both their faces.