King Mongkut of Siam, also known as Rama IV of what is now Thailand, accurately predicted the total solar eclipse of August 18, 1868 that passed through India and southeast Asia, to the general acclaim of European astronomers. It was already known by 1860 that different chemical elements gave off distinctly coloured flames when subjected to a Bunsen burner. So Gustav Kirchhoff, in 1859, decided to study the gases surrounding the Sun during an eclipse, using the same logic.

A new instrument would be aimed for the first time upon the eclipse — a spectroscope, which would break down the peripheral light of the Sun into its constituent colours, revealing the composition of the sun’s atmosphere. For this purpose, several teams of British and French astronomers were dispatched across the path of totality, the journey that the eclipse’s shadow makes on the Earth’s surface. The first total eclipse since Kirchhoff’s theory, this would also be the first eclipse to see a widespread use of photography.

James Tennant, a soldier who had fought in the 1857 battle for independence with the Bengal Engineers, had already worked with the Great Trigonometrical Survey (one of the greatest scientific expeditions of its time, primarily to map India, but an ulterior motive was to measure the curvature of the Earth at the tropics), and was now an astronomer. Tennant too began alerting the community to the celestial event, which would last only six minutes. The French astronomer Pierre Jules Janssen stationed himself in Guntur, less than a kilometre away from Tennant . Norman Pogson positioned himself at the Madras Observatory. A priest with Pogson called John Sharp described the scene thus, “About a dozen cows and calves feeding in the compound stopped grazing as the totality came on, and hurried to the gate as at evening; but as the light returned they again dispersed over the field and began to graze as before.”

The events that unfolded after the eclipse were completely unexpected. An entirely new chemical element was being observed in the light from the Sun. Biman B Nath describes it in his excellent book The Story of Helium and the Birth of Astrophysics . Nath argues that Pogson and not Janssen (as commonly stated) was the first to discern a signature that had never been observed from an element on earth. He writes: “The drama around the discovery of helium did not end with the events of the 1868 solar eclipse; this was only the beginning. Afterwards, for three decades, scientists wondered if astronomers had got it all wrong because they could never find helium on Earth. In fact, Dmitri Mendeleev, the most famous chemist at that time and the person behind the periodic table of elements, dismissed the idea of this elusive element called helium.”

In the ensuing decades, Janssen became more involved with photography. As Samuel Colt was inspired for his revolver’s design while watching a ship’s wheel during a voyage from Boston to Calcutta in 1830, Janssen was now inspired by the widely used Colt revolver. His new astronomical camera or “photographic revolver” was intended to put an end to the debate surrounding the size of the solar system.

One method of determining the distance to the Sun was the rare event of Venus passing in front of the Sun. Depending on where you are on Earth — the Antarctic or Siberia — the planet’s apparent position against the background solar disc changes. This is known as solar parallax. It had been troublesome in centuries prior for astronomers scattered across the globe to agree on the exact moment the planet Venus began and ended its transit across the solar disc. Janssen decided to remove the subjective human observer from this equation and replace it with a rapid, clockwork camera that could “shoot” almost one photograph per second. “It is the Sun itself that records its transit… [and] …the stars shall henceforth register themselves,” emerged as slogans of a new astronomy.

The reception of Janssen’s method was far from cordial, and many debates simmered around it. Historian of science Jimena Canales writes, “For most scientists Janssen’s successes with his revolver — although spectacular — were illusory. The debate over chronophotography’s claims to truth and the value of the solar parallax quickly moved beyond the confines of astronomy and into the domain of physics. In light of astronomy’s failure to establish a single, reputable value for the solar parallax, a new role for physics emerged with respect to precision measurements — in particular, regarding the determination of the speed of light.”

Improvements to the Janssen camera continued in the hands of other innovators across the world, such as Eadweard James Muybridge, Thomas Edison, Étienne-Jules Marey and others. One of the first films by The Lumière brothers (1895) contains footage of Janssen himself disembarking from a boat to attend an event in their honour.

Helium would later be used in a different kind of photography — magnetic resonance imaging (MRI) to cool the superconducting magnets. There is now widespread concern that all the terrestrial helium will eventually escape from the earth’s atmosphere, just as Janssen flew from the siege of Paris in 1870 in a hot air balloon to catch an eclipse of the Sun.

( Rohit Guptaexplores the history of science as Compasswallah )

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