On this day in 1868, the French astronomer Pierre Jules Cesar Janssen was the first to observe the element of helium in space, before it had ever been observed on Earth.
Janssen was brought up in Paris, where he studied mathematics and physics at the city’s Faculty of Sciences. Through the course of the 19th century he was entrusted with a series of exotic scientific adventures all over the world. In 1857 he travelled to Peru to discover the exact location of the magnetic equator; in the early 1860s he was in the Italian and Swiss Alps studying the solar spectrum; in 1867 he was in the Azores conducting experiments into optics and magnetics; in 1870 he escaped the Prussian siege of Paris by hot air balloon (although a helium balloon would have been even more appropriate) and soared away to Algiers where he watched the solar eclipse. And that’s not all.
The astronomer-adventurer also saw solar eclipses in Alocossebre in Spain, the Caroline Islands off of New Guinea, Guntur in India, Siam in present-day Thailand, and Trani in Italy; as well as observing the two transits of Venus in 1874 in Japan and 1882 in Algeria.
However, despite all of his globe-trotting expeditions, Janssen’s greatest discovery was outside of our atmosphere. On 18 August 1868, while observing the great Indian eclipse at Guntur, he spotted a bright yellow line in the spectrum of the Sun’s chromosphere (the second of the three layers of its atmosphere), with a unique wavelength of 587.49 nm; a couple of months afterwards, on 20 October 1868, Joseph Norman Lockyer independently spotted that same line. He concluded that because this particular spectral line had never been seen before, it must show an unknown element, and he called it helium after “helios,” the Greek word for “sun.”
This was the first time that an element had been discovered in space instead of on Earth, and although other scientists were skeptical, Janssen and Lockyer were soon shown to be correct. In 1895 a couple of Swedish chemists, Per Teodor Cleve and Nils Abraham Langlet, officially “discovered” helium gas coming out of the uranium ore cleveite, and then in 1903 vast reserves of the element were uncovered in the natural gas fields of the United States. And today the colourless, odorless, tasteless noble gas is known to be the second most abundant element in the entire observable universe.
Credit: © Alex’s Pictures / Alamy
Caption: Although used mostly in cryogenics, the most well-known use for helium is for balloons.