Saturday is the 150th anniversary of a total eclipse of the Sun that was seen across a wide band of Asia on 18th August 1868. Any total eclipse is interesting, but this one is particularly historic for chemists, as it was during this eclipse that observations were made that, with hindsight, led to the discovery of helium, the first element to be discovered in space before it was found on Earth.

Image of total solar eclipse in 2017, photographed by Carla Thomas. Copyright NASA

However, the story often told in encyclopaedias, that Pierre Janssen and Norman Lockyer discovered helium by observing the 1868 eclipse, is far too simple. In fact, Janssen, who was in India and is often credited with the discovery, was interested in completely different things, and never claimed any credit during his lifetime, Norman Pogson, who was in India and was the first person to speculate that something unusual might be happening, was forgotten, and Norman Lockyer, who is often credited as the co-discoverer and made the biggest contribution, wasn’t in India and made his discoveries without needing the eclipse.

Helium is the second-most-common element in the universe after hydrogen, but is very rare on Earth, and odd in other ways. It is one of the so-called “noble gases”, that, because they have a particular number of electrons, are uniquely happy to exist as single atoms and reluctant to react with other elements. Helium only exists on Earth because it is given off when many radioactive elements naturally decay. Once produced, because it is so light and so non-reactive, it usually flies straight out of the atmosphere and vanishes into space. It only stays on Earth if it is produced deep underground and trapped within rocks. However, helium is very common in stars, including our Sun, because the energy of most stars comes from hydrogen atoms being fused into helium, and stars’ greater gravity than the Earth keeps it in.

So how was it possible to find helium in the Sun by looking at eclipse light?

For reasons too complicated to explain here, electrons in atoms and molecules can only have certain precise amounts of energy. They can climb from one amount to a higher one by absorbing a photon of light, or drop to a lower one by emitting a photon of light. The amount of energy contained in a photon varies according to the wavelength of the light, and so this means that atoms or molecules can only absorb or emit light of very specific wavelengths. As a result, if you shine a light through a particular substance, the light that comes out will have certain wavelengths and colours of light reduced or missing (an absorption spectrum), and if you heat up a substance to the point that it starts glowing, the light produced will be mainly or only of the same specific wavelengths and colours (an emission spectrum). By studying the light absorbed or emitted by a substance, we can derive a lot of information about what it is and what its structure might be.

The first step in the story of the discovery of helium happened in 1814, when the lens-maker turned physicist Joseph Fraunhofer split sunlight using a telescope, prism, and diffraction slit to create a spectrum broad enough to notice that there were dark lines, so-called "Fraunhofer" lines, where particular wavelengths of light were simply not present. In 1834, David Brewster suggested that the Fraunhofer lines were due to light of specific wavelength being absorbed by gas either within the Sun or in the Earth's atmosphere. James D Forbes suggested that the dark lines could be proved to originate from the Sun rather than the Earth's atmosphere by observing light from the edge of the Sun's disc during an eclipse - as this passes through more of the Sun's atmosphere on its path to the observer, the lines will be stronger if they are produced by the solar atmosphere.

Physicists and chemists began studying the absorption and emission spectra of known substances and found that their characteristic lines were constant. In 1857 William Swan showed that particularly strong dark lines in the yellow region of the Sun's spectrum, known as the D lines, corresponded to the emission spectrum of sodium - something we are all familiar with now given the yellow tinge of sodium-vapour streetlights.

In 1859, Gustav Kirchhoff and Robert Bunsen (of gas burner fame), at the University of Heidelberg, were among the scientists who were making systematic studies of the spectra of different elements. When a major fire broke out in the city of Mannheim, across the valley, they playfully turned their spectroscope on the light from the flames, and were able to identify the characteristic emission spectra of strontium and barium. This experience made them realise that, if they could discover trace elements in a burning building, the Fraunhofer lines might be the key to discovering the elements present in the Sun.

The following year, the two were studying the spectrum of mineral water from a major local spa, Bad Dürkheim. They spotted two blue lines that were found in the spectrum of no known substance, and guided by this managed to prepare and purify compounds of a previously unknown element, caesium. This was the first new element to be discovered using spectroscopic methods. Within the next few years, Kirchhoff and Bunsen would discover rubidium by a similar route, and William Crookes would discover thallium.

In 1868, a total eclipse of the Sun was predicted to occur in India. The eclipse ws expected to have six minutes of totality, an extremely long time by the usual standards in which to perform observations. Spectroscopists were particularly interested in the eclipse, as with the main part of the Sun obscured from the Earth it would be possible to study the light from the Sun's outer atmosphere, potentially helping to investigate both the Sun's chemical composition and its internal structure.

The French astronomer Pierre Janssen had already made his name in the field of the solar spectrum. He had invented a much-improved astronomical spectroscope with the instrument maker Ignazio Hofmann, although the two men quickly fell out bitterly about whose contribution was greatest. In 1866 he had captured the absorption spectrum of water vapour, by a logistically challenging experiment in which he viewed the light given off by sixteen gas burners through long iron pipes filled with high-pressure steam, and verified which of the Fraunhofer lines were produced by it as sunlight passed through the Earth's atmosphere. He was selected by the French Bureau of Longitude to make a government-funded trip to India.

1880 automatic spectroscope by John Browning. Image by Science Museum, released under a CC-BY-NC-SA licence

Meanwhile, the government of the British Empire, rulers of India at the time, were making their own plans for scientific observations of the eclipse. The main expedition, led by Major James F Tennant, headed for the town of Guntur in Andhra Pradesh, in Southeastern India. Meanwhile, Norman Pogson, director of the Madras Observatory, headed to Machilipatnam (then known to English-speakers as Masulipatam), closer to the coast. When Janssen arrived in India, he also considered Machilipatnam, but decided that on the coast there was too much risk of fog and cloud. He decided to go to Guntur as well, possibly because it had at one time been ruled by the French and there were still some wealthy French merchants living there. Tennant's team moved into the British government compound, while Janssen set up at the home of one Jules Lefaucheur. Janssen generously helped Tennant to set up his spectroscope and telescope.

When the eclipse occurred, all the investigators paid attention to the spectrum. Janssen did not mention anything unexpected. Tennant saw an orange line which he thought was the normal sodium D line. Only Pogson saw something unusual - a third line close to the sodium D line, but not identical with it.

Pogson's eclipse observations, from his printed report.

It was not until the following days that Janssen made the realisation that would be his real breakthrough of the event, and the one that popular history would later confuse with the discovery of helium. He realised that the emission spectrum of the solar atmosphere and prominences was so strong that, if one could focus the spectroscope on the precise edge of the Sun, they might be visible even without an eclipse. He experimented and found that it was entirely possible, but was easiest if you moved the spectroscope to try to find the spectrum, rather than trying to focus visually on the edge of the Sun. He excitedly wrote to his wife in a letter, "They sent me to observe the eclipse for five minutes, and I am bringing back a perpetual eclipse from India." Finally, he sent a letter to the Academy of Sciences, announcing his discoveries for the first time.

Back in London, Norman Lockyer, a civil servant and prominent amateur astronomer, with a great interest in studying the Sun, was independently realising that the spectrum of the outer atmosphere of the Sun could be viewed by accurately focussing a spectroscope, without any need for an eclipse. He also seems to have somehow got a copy of Pogson's report with its reference to a previously unidentified line in the spectrum. In October, he received a new spectroscope and managed to focus on the solar atmosphere and obtain its emission spectrum. He also noticed a new line near the D line. Among the organisations he sent preliminary reports to was the French Academy of Sciences, his letter arriving within a few days of Janssen's report from India, both being read out at the same meeting on 26th October. In 1872, to avoid a potentially ugly interpersonal and international row, the French government issued a medal featuring both Janssen and Lockyer to commemorate their solar discoveries.

By the end of the year, both Janssen and Lockyer were convinced that the yellow line near the sodium D line was new. Lockyer and the chemist Edward Frankland spent some time experimenting with the spectrum of hydrogen under different conditions, and by the end of it were convinced that the Sun consisted mostly of hydrogen, but the the yellow line could not be produced by that element. By 1871 Lockyer was convinced that the yellow line was produced by a new element never found on Earth which he named "helium", but did not make such an extreme speculation in public, only in private communications with other scientists. The first public statement of it is believed to have been in Sir William Thompson's presidential address to the British Association for the Advancement of Science in 1871. This concluded the series of events that led, in later years, to Janssen and Lockyer wrongly being jointly credited with the discovery of helium in 1868.

Why was Pogson forgotten, even though Lockyer credited him in his own brief memoir of the discovery of helium, in Nature in 1896? Although he is now remembered for his development, earlier in his career, of a scale for the apparent magnitude, or brightness of astronomical objects, his career in India was not a success. He seems to have suffered from social snobbery due to his middle-class background and lack of a university degree, but he was also a somewhat abrasive personality, as can be seen from the negative comments in his report on the "needless and lavish expenditure" on the various expeditions to view the eclipse, and the even more offensive remarks about the local Indian people in general, which I will not quote in detail here. Another item in the India Office records shows his conflict with the government and the Dutch astronomer Jean Oudemans over longitude measurements that he did not consider particularly important and delayed in analysing. Pogson's report on the eclipse was not published in a peer-reviewed journal, but in a low-profile government publication - Pogson himself complained in a letter in 1882 that it had been treated as "waste paper".

Helium was subsequently shown not just to exist in the Sun, when in 1876 the French astronmer Alfred Cornu observed it in the spectrum of a star in the Cygnus constellation. In the meantime, however, speculation on new elements in the stars had become somewhat wild and uncontrolled, developing a bad name due to multiple announcements of "new elements" that proved too frequent to be credible. (One of the most notorious was "coronium", assigned to a spectral line from sunlight at 5303 angstroms wavelength, which was eventually discovered to come from very highly-ionised iron atoms.)

In 1887, William Hillebrand discovered a mysterious gas while treating uranium ore with acid, that he suspected to be nitrogen. He noticed that its spectrum did not match that known for nitrogen, but did not realise that it was a new element, as at the time it was known that the spectrum of nitrogen could vary considerably with the conditions. In 1895, Baron Rayleigh found that nitrogen extracted from the atmosphere had a different molecular weight to chemically-produced pure nitrogen, and suspected that another element was present. He investigated further, and managed to purify a completely new element, which he named argon. William Ramsey, who was working with Rayleigh on argon, was shown Hillebrand's paper by another colleague who thought Hillebrand's gas might have been argon as well. He repeated Hillebrand's experiment with a different type of uranium ore, and discovered that the gas he produced was much lighter than argon, and had a spectrum that included the D3 line of the mysterious solar element helium. Helium had finally been discovered on Earth.

But scientific research on the Sun continues - this week NASA launched its Parker Solar Probe, to become the first human-created object to enter the Sun's outer atmosphere and observe it.

Sources and further reading:

Janssen, P J, The total solar eclipse of August 1868. Part I, Astronomical Register, 1869, 7(77), pp. 107–110. Shelfmark PP.1556 or 1755.800000
Janssen, P J, The total solar eclipse of August 1868. Part II, Astronomical Register, 1869, 7(78), pp. 131-133 Shelfmark PP.1556 or 1755.800000
Lockyer, J. N. The story of helium, Nature, 1896, 53(1371), pp.319-22. Shelfmark P.P.2011c or (P) BX 80-E(3). Also available online in BL Reading Rooms
Nath, B B. The story of helium and the birth of astrophysics. New York City: Springer, 2013. Available online in British Library Reading Rooms.
Pogson, N R. Report of the Government Astronomer upon the proceedings of the Observatory in connexion with the total eclipse of the Sun on August 18th, 1868, as observed at Masulipatam, Vunpurthy, Madras and other stations in Southern India. Madras: Madras Observatory, 1875. Shelfmark IOR/V/27/430/8.
Pogson, N. R. Letter to Captain Awdry, 10th June 1882, in Grant Duff Collection, Miscellaneous English Correspondence, pp. 96-98. Shelfmark Mss Eur F/234/67
Ramsay, W. Helium, a gaseous constituent of certain minerals, Part I Proceedings of the Royal Society, 1895, 58 pp. 80-89. Shelfmark Ac.3025/21 or (P) JA 00-E(12). Also available free online at https://www.jstor.org/stable/115763
Reddy, V., Snedegar, K.. Balasubramanian, R. K. Scaling the magnitude: the fall and rise of N. R. Pogson, Journal of the British Astronomical Association, 2007, 117(5), pp. 237-245. Shelfmark Ac.4176, (P) OT 00-E(34), or 4713.000000

Posted by Philip Eagle. Thanks to Margaret Makepeace for help in researching India Office records.