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Journal of Astronomical History and Heritage, 24(1), 114-124 (2021). 114

OBSERVATIONS OF THE TOTAL SOLAR ECLIPSE

OF 18 AUGUST 1868 CARRIED OUT BY JULES JANSSEN

AT GUNTOOR, INDIA

Françoise Launay,

Paris (France)

E-mail: francoise.launay4@wanadoo.fr

Abstract: Due to its exceptional length, the 18 August 1868 total solar eclipse gave the French physicist Jules

Janssen a very good oppor tunity to have spectroscopic ob servations put into practice to study the famous

prominences that could be seen only during total eclipses. He not only saw the hydrogen lines he was so

interested in, but he discovered too that it also would be possible to observe these lines at any time, without

waiting for the next e clipse. This 'prominence method' was independently discovered by Britain's Norman

Lockyer at the same time, and was deemed so important and promising that the French Académie des Sciences

struck a special medal in honour of both scientists.

Keywords: Jules Janssen, 1868, solar eclipse, Guntoor, prominences, hydrogen, helium, spectrohelioscope

1 INTRODUCTION fore he turned 17. He found a position as an

accountant in a bank and worked hard to com- plete his formal education. He first acquired a good literary and classical grounding and then began to learn mathematics and physics. He obtained the corresponding degrees in 1852 and 1855, and then became an assistant in a grammar school in Paris.

When he left Paris to conduct one of the two

French expeditions to India in 1868, Jules Jan-

ssen, who was to become the well-known Di- rector of the Meudon 'physical astronomy' Ob- servatory, which was founded for him in 1875, had no official position.

However, he was known and recognized

by the scientific and political authorities. Let us recall his backgroun d (Launay, 2008; 2012).

Pierre Jules César Janssen (1824-1907; Fig-

ure 1) was born in Paris on 22 February 1824 in a family where nothing seemed to suggest he would become an astr onomer: his father was a clar inettist and her mother was the daughter of an architect. Unfortunately, finan- cial difficulties forced him to look for a job be-

Thanks to the Minister of Public Instruct-

ion, in 1857 he was sent to Peru to determine the course of the magnetic equator. In 1860, he received a Doctorate of Science based on a thesis that he researched and wrote while serving as a tutor for a family of industrialists.

One year later, he began research on the solar

spectrum, observing from the roof of his Paris- ian house. After many observations and labo- ratory experiments, he was able to prove that some spectral lines, which were more notice- able at sunrise and sunset, were definitely of terrestrial origin. He wa s then appointed a teacher of physics in 1865, and again was sent abroad, thi s time to England, Italy, G reece,

Portugal and Spain.

In December 1867, following the success

of his previous travels and observations, among which the spectroscopic observation of an an- nular eclipse in Trani (Italy) ten months earlier,

Janssen had recei ved not only congratulat-

ions but also an offer of possible financial sup- port from the new Minister of Public Instruct- ion, Victor Duruy (1811-1894; Figure 2).

Janssen's request for help did not take

long to arrive at Duruy's office, because of the longest total solar ecl ipse which was due to occur on next August. Whereas an ordinary eclipse lasts between 2 and 3 minutes, the du- ration of this one would actually be 6 minutes

57 seconds. The conditions were thus except-

ionally favourable for achieving spectral analy- sis of the Sun's 'flames', the prominences: Figure 1: Jules Janssen (from L'Illustration, © Author's collection). Françoise Launay Jules Janssen and the 1868 Solar Eclipse 115

If the promin ences consist of solid mat-

erial, they will give a continuous spectrum.

Are they incandescent gas? The spectrum

will consist of brilliant lines, and the lo- cation of the lines will reveal, to an exper- ienced eye, the nature of the ga s. (Jan- ssen, 1903: 315). servatory, accompanied by his Paris Obser- vatory colleagues Georges Rayet (1839-1906) and Félix Tisserand (1845-1896), to Siam (now Thailand), on the Malayan Peninsula (Or- chiston and Orchiston 2017), where they were met and ac companied Siam's King Rama IV (Soonthornthum and Orchiston, 2021), who had a passion for as tronomy (Georgelin and

Arzano, 1999). The decisions were not taken

without bitter tensions. Duruy had to intervene once more, and signed the order for the ex- pedition on 29 May, that is, two months after

Janssen's. The French team for Siam had the

remarkable credit of 50,000 F, one third of the

Paris Observatory's annual budget.

This is what spectral analysis would enable

one to determine. But Janssen had just enough time to prepare, because the path of visibility of the eclipse's totality crossed Asia, and it would be necessary to leave two months before the event.

2 PREPARATIONS

Whilst the British were gettin g organised to

observe from India (see Orchiston et al., 2017), the Germans from Aden, and the Dutch from

Celebes Island (Mumpuni et al., 2017), France,

which had initially thought to entrust observat- ions to seamen, rather than astronomers, vac- illated (Aubin, 1999). Being independent of any institutions and, in particular, avoiding the authoritarian influence of the Director of the 'Imperial' Paris Observatory, Urbain Le Verrier (1811-1877; Lequeux, 2013), Janssen dealt directly with the Minister and his Department that was responsible for expeditions. The or- der that concerned him, and which sent him to

British India, with a grant of 15,000 F, was

signed by Duruy (1868a) on 9 March.

So, when Le Verrier approached him, Jan-

ssen could take pleasure in telling him that it was too late:

I have received the letter in which you do

me the honour of asking me to take part in

Figure 2: The Minister of Public Instru ction,

Victor Du ruy (Album Félix Potin. © Au thor's collection). a com mittee relating to the Eclipse n ext

August. I my self, Monsieur, must leav e

immediately for India, most particularly for the plateaus of High Asia, where I intend to carry out research into both celestial and terrestrial Physics before obser vation of the eclipse. Being entirely involved in pre- parations for this major voyage, it would be extremely difficult for me to carry out the work of such a commission. I deeply re- gret not being able to give yet again this proof of my dev otion to our French

Science, but I hope that it will be con-

sidered that I am paying my debt in an- other form. (Janssen, 1868a).

Because the date of the eclipse fell in the

middle of the monsoon, ... stations that offered the most fa vour- able chance were those that proved to be sheltered by a high chain of mountains and the width of a major continent. (Janssen,

1903: 306).

Janssen decided quickly:

As soon as the Bureau des longitudes and

the Academy [of Sciences] had done me the honour of selecting me, I had no hes- itation in choosing th e east ern coast of

Hindustan. The station should be Masuli-

patam or nearby ...

A few days later, Duruy shook Le Verrier

... let us try, by actual activity, to make up for these really annoying delays. The lo- cation chosen will save time. Can you not find the means of adding spectroscopy and photography to the purely astronomical ob- servations ... (Duruy, 1868b). not far from Madras.

Leaving Paris on 16 June, Janssen took a

ship from Marseille on the 19 th , with the mem- bers of the other French expedition, from whom he parted in Ceylon. On his arrival in Madras, he was received with the greatest courtesy by

Finally, Le Verrier, who had suffered plenty of

other refusals, sent Édouard Stephan (1837 -1923), the young Director of Marseilles Ob- Françoise Launay Jules Janssen and the 1868 Solar Eclipse 116

Figure 3: Map of India showing Guntoor (Guntur) in the white ellipse (map modification: Françoise Launay).

the British authorities, partly thanks to the let- ters of recommendation from Warren De La

Rue (1815-1889), wi th whom he m aintained

extremely friendly relations. Despite the diffi- culties arising from the moist heat and from the considerable amount of material that had been imported (twice what was n ecessary, as a precautionary measure), Janssen arrived two weeks early at Guntoor (Figure 3), the obser- ving location finally selected, equidistant from the mountains and the sea. There he found a family of French merchants, who had long been established in India, the Lefauche urs, who offered him the whole of the first floor of their house, the hi ghest and best si tuated in the place, which was linked to a terrace. It was thus there that Janssen observed the eclipse: cleverly interlinked, and the overall motion was provided by a mechanism constructed by the brothers Émile (1834-1895) and Léon (1840-

1894) Brunner. Janssen, whose only assist-

ant was a cadet from the liner l'Impératrice, made the most of the abilities of three mem- bers of the Lefaucheur family, whom he intro- duced to the tasks of drawing and taking mea- surements!

It was known at that time that the lines that

Fraunhofer called C (in the red) and F (in the

blue) were part of the spectral signa ture of hydrogen, its 'identity card', whilst the yellow D line corresponded to sodium, and was in fact two close lines D 1 and D 2 , which had not been separated (by Fraunhofer) in 1814.

All the ob servers whose spectroscopes

were directed at the assumed position of the prominences, observed bright lines, of greater or lesser i ntensity, and character istic of gas- eous emi ssion. Whereas among the Bri tish,

Lieutenant John Herschel (1837-1921), son of

Sir John (1792-1871) and grandson of Sir

William (1738-1822), saw three only (one of

which was in the red, closer to B than C; an orange one exactly on D; and a blue line al- most on F); Norman Pogson (1829-1891) saw five (one of which was yellow, on or near D, but no red); and Colonel James Tennant (1829-1915), who was also at Guntoor, again five (one red, one yellow, and one blue corres-

I am ha ving a screen of bamb oos an d

mats made against the wind. We have the whole of an immense room for our instru- ments. These families are proud and happy to receive us ... he wr ote to his wife Henriette on 4 August (Janssen, 1868b).

3 OBSERVATION OF THE ECLIPSE

Janssen had four large 16-cm refractors at his

disposal, together with a 21-cm Foucault re- flector, the mirror of w hich had been para- bolised, free of charge, by the optician Ad- olphe Martin (1824-1896). The refractors were Françoise Launay Jules Janssen and the 1868 Solar Eclipse 117

Figure 4: A prominence followed by Janssen on 4 September 1868, from original drawings preserved at the Institut de France

Library (OEuvres Scientifiques © Author's collection). ponding exactly with C, D and F), but with considerable difficulty ; the Frenchm an Rayet beat all records despite the absence of the red line corres ponding to C, observing nine (of which one yellow line corres ponded to D).

Janssen himself saw only "five or six", but he

only cites five colours, without specifying their exact positions: (Janssen, 1868h). It was during the following night that he perfected this. Here is how Jan- ssen, as a lecturer, explained it later during the course of a meeting at the Sorbonne in 1870:

For a long ti me people had tried t o see

prominences outside eclipses. Stops at the focus o f refractors, colou red g lasses, etc., have all been tried without success.

The intense light from the photosphere has

always swamped the weak light from the prominences, above all be cause of the illumination of our own atmosphere around the disk of the Sun ... But let us interpose a prism ... The light from the Sun is approxi- mately white; it is rich in all the rays [of the spectrum], it is thus spread out by the prism.

The prominences, by contrast, co ntribute

just a few sim ple [monochromatic] lines, their light is n ot weakened ... (Janssen,

1903: 316-317).

Two spectra, each consisting of five very

brilliant lines (red, yellow, green, blue, and violet) filled the spectral field, and replaced the prismatic image of the Sun, which had just disappear ed ... These two spect ra were caused b y two magnificent promin- ences ... One of them ... resembled the flame from the fire of a forge ... The right- hand prominence ... presented the appear- ance of a range of snow-covered mount- ains. (Janssen, 1869e).

The very next day he implemented 'the

method' and was able to follow the evolution of prominences with time, his eyes being fixed on the C line of hy drogen, which we nowada ys know as the Hα line:

4 THE DISCOVERY OF THE

'PROMINENCE METHOD'

What in fact struck Janssen was, above all, the

exceptional intensity of t he lines, their 'bri- lliance' in the blue and in the red, and the analysis of the light

I was able to witness phenomena revealed

by a new eclipse that lasted all day. The prominences seen the day before w ere greatly altered ... From that day until 4

September, I constantly studied the Sun by

this method. I have drawn charts of the prominences ... This period ... was like an eclipse that lasted seventeen days. (Jan- ssen, 1869e). ... immediately showed him that it consist- ed of an immense column of incandescent gas, primarily consisting of hydrogen. (Jan- ssen, 1868h).

He soon noticed that these lines were in the

exact position of the C and F lines, and it was only those lines and, in particular, the red C line, that interested him. He also noted the similarities in the chemical composition of the two prominences observed during the eclipse: their spectra corresponded line for line. At the moment when the weather clouded over, just after the eclipse, he exclaimed "I shall see these lines outsi de eclipses ..." (Janssen,

1869a): he had just discovered a method of

observing prominences and the region immed- iately around the Sun "... at any time, without it being necess ary to have recourse to inte r- posing an opaque body in f ront of t he Sun."quotesdbs_dbs43.pdfusesText_43

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