How a Solar Eclipse Revealed Helium

For six minutes and 47 seconds, the Sun was darkened by a solar eclipse on August 18, 1868. In the Indian city of Guntur, French astronomer Jules Janssen pointed his spectroscope at the chromosphere, the outer layer of the Sun’s atmosphere. A striking yellow line at 587.49 nanometers did not match any known element. Janssen sent his report to the French Academy of Sciences.

At almost the same time, English astronomer Norman Lockyer was investigating the same clue. His instrument was not completed in time for the trip overseas, so in October 1868 he examined photographs of the solar spectrum instead. Lockyer also recognized unknown lines and attributed them to a new element. He called it helium, derived from the Greek hélios, meaning sun.

Spectral analysis — the process of breaking light down into individual wavelengths — made the element visible before anyone had held it on Earth. Each chemical element leaves behind a characteristic pattern of spectral lines, an optical fingerprint. The Academy considered the reports on October 26, 1868, but many scientists doubted the existence of an element identified from just one line and outside Earth.

The skepticism persisted for a long time. Researchers spent around 27 years searching for helium in metals and salts, until chemist William Ramsay detected it alongside argon in minerals in 1894. Today, the noble gas is used in MRI scanners — devices for magnetic resonance imaging — as well as in the manufacture of fiber optics and flat-panel displays.

And then? A yellow line became a raw material for research, medicine and industry. According to heise online, demand is rising while helium is running out on Earth: Russia, Algeria and Qatar do supply it on a large scale, but deliveries from Russia and Qatar are currently restricted, the report says. Earth therefore has less available helium than a look into space might suggest; plans to mine it on the Moon already exist.