Background

Ethnicity: Janssen's father was of Belgian descent, his mother was French.

Pierre Janssen was born Pierre Jules César Janssen on February 22, 1824 in Paris, France, into a cultivated family. He was the son of Antoine César Janssen, a musician, and his wife Pauline Marie Lemoyne. Janssen's maternal grandfather was the architect Paul-Guillaume Lemoyne.

Education

Because of a serious injury Janssen was kept at home and never attended school. Financial difficulties obliged him to go to work at an early age. While working for a bank from 1840 to 1848, he devoted himself to completing his education and earned the baccalaureate at the age of twenty-five. Janssen later attended the University of Paris, receiving his licence ès sciences in 1852. In 1860 he earned his Doctor of Science degree.

Career

In 1852 Janssen obtained a post as substitute teacher in a lycée. In 1857 he was sent on an official mission to study the position of the magnetic equator in Peru. He contracted a severe case of dysentery and had to return to France, where he agreed to become a tutor for the Schneider family, who owned iron and steel mills in Le Creusot.

Janssen’s first scientific work was a study of the absorption of radiant heat in the mediums of the eye. He showed that the mediums are transparent only for visual rays and that the focalization of the thermal radiation has no harmful effect on the retina because nine-tenths of the radiation is absorbed. The work actually had no real scientific significance; the conclusion could be expected because the absorbing mediums, being aqueous, have precisely the properties of water. Yet it is of interest because of the way in which Janssen was led to undertake it.

In October 1859, in a celebrated report, Kirchhoff demonstrated the presence of terrestrial elements in the constitution of the sun. Janssen immediately realized that the study of the radiation from the Le Creusot furnaces would never allow him to make discoveries so splendid as that of solar radiation, and he decided to direct his career toward physical astronomy.

In Paris, where in 1862 he had come to work with E. Follin of the Faculty of Medicine on the construction of an ophthalmoscope, Janssen mounted a small observatory on the flat roof of the house that his wife owned north of Montmartre. Here he began work on a problem posed by Brewster in 1833, that of the nature of certain dark bands in the solar spectrum, bands irregular in presence and most noticeable at sunrise and sunset. For this purpose, Janssen constructed a spectroscope possessing a high dispersive power and furnished with a device for regulating the luminous intensity. He was able to establish in 1862 that these spectral bands resolve into rays and that their presence is permanent. On a mission to Italy in the following year, he demonstrated with precision that the intensity of the rays varies in the course of the day as a function of the density of the terrestrial atmosphere traversed. The terrestrial origin of the phenomenon was demonstrated - thus Janssen proposed for them the name “telluric rays.”

In 1864 Janssen moved to the Bernese Alps, at an altitude of 2,700 meters, to verify that the intensity of the telluric rays is lessened in the mountains. The weakening exceeded that expected on the basis of the decrease in density of atmosphere traversed, and Janssen attributed it partly to the dryness of the air. In order to verify his supposition he studied, at Geneva, the telluric rays of the spectrum of an artificial source situated twenty-one kilometers away on the shores of Lake Leman. His results confirmed the effect of humidity on the intensity of the rays. Janssen then turned to the direct study of the absorption rays of water vapor, carrying out his experiment in a gasworks near his home and using an iron tube thirty-seven meters long to hold the water vapor under a pressure of seven atmospheres. The source, illuminating gas, which ordinarily yields a continuous spectrum, furnished a spectroscopic image displaying most of the telluric rays. Janssen had stated at the beginning of his research that the existence of telluric rays entailed the possibility of making a chemical analysis of the atmospheres of the planets. His determination of the water vapor spectrum was a major step in this direction. As early as 1867 he was able to announce the presence of water vapor in the atmosphere of Mars.

In the following year, Janssen made another important contribution to the knowledge of solar structure. In order to observe the total eclipse of August 18, 1868, which was visible in India, Janssen went to the city of Guntur, near the Bay of Bengal. His aim was to study the solar prominences. Keeping the slit of the spectroscope on the lunar limb, he was able to observe highly luminous spectra while the sun was in eclipse. Visual observation in a finder showed that these spectra came from two great prominences. Janssen measured the position of the brightest rays: they corresponded to rays C and F of the solar spectrum, which are produced by hydrogen.

The brightness of the rays led Janssen to suspect the possibility of observing the prominences even when there was no eclipse. The next day he resumed his observation of the solar limb, admitting only the red portion of the spectrum. He ascertained first that a bright line appeared in the exact extension of a dark line of the solar spectrum, the C-ray. Exploring the contour of the sun, Janssen observed the variations in the intensity of the line and the modifications in its structure. He also made other bright lines appear, all of them corresponding exactly to the dark lines of the absorption spectrum.

From August 18 to September 4, Janssen worked on establishing maps of prominences. He continued his observations at Simla, in the Himalayas. Several other astronomers worked in these areas during this period, notably Lockyer, who arrived at the same method of analyzing the prominences as Janssen did; but it was the latter who, in the course of his stay at Simla, created the first spectrohelioscope.

For carrying out his investigations Janssen received official subsidies at the times of his missions. In addition, in 1865 he was appointed professor of physics at the Ecole Spéciale d’Architecture. Yet, in France at least, he had little more than his home to use for his technical and experimental work. In 1869 the minister of education, Victor Duruy, tried to find him an observatory. Janssen had had the opportunity to make several measurements at the Paris observatory but was not able to install himself there permanently, since the director, Le Verrier, considered the establishment to be his personal property. Janssen was offered the pavilion of Breteuil in Sèvres, but the Franco-Prussian War prevented his using it.

In 1874 the French government decided to establish an observatory for physical astronomy. Janssen had the choice of two sites: Malmaison or Meudon, better located in terms of climate. Janssen chose Meudon and it was granted to him in 1876. The estate at Meudon was in ruins when Janssen moved there in October 1876. He commenced repairs on the buildings and began to prepare the astronomical equipment. The offices and laboratories were lodged in the principal part of the estate, which formerly consisted of a modest chateau, stables, and other outbuildings. A separate building, the Chateau Neuf, built by Mansart in 1706, was restored and topped by an astronomical dome 18.5 meters in diameter.

Janssen had hoped he would rapidly acquire the means necessary to extend his investigations, which until then he had conducted with small instruments and improvised devices. Soon, however, there were financial difficulties. It became necessary to use funds budgeted for research for the completion of the buildings, a task that required twenty years. The staff was insufficient moreover - until 1906 there were only two astronomers. Nevertheless, Janssen was able to endow the observatory with two large instruments: a double refractor of sixteen meters with a visual objective of eighty-three centimeters and a photographic objective of sixty-two centimeters; and a telescope with an aperture of one meter and a focal length of three meters. For the spectral investigation of gases, he also set up a large laboratory with a steel tube sixty meters long, closed by thin transparent plates and capable of supporting a pressure of 200 atmospheres.

The most famous of Janssen’s projects at Meudon during this period was the atlas of solar photographs. Composed of a selection of exposures made between 1876 and 1903, it summarized the history of the surface of the sun during these years. Janssen employed a photoheliograph of his own design. Its telescope was achromatized for violet radiations; and its shutter, which had a movable and variable slit, permitted exposures on the order of 1/3,000 of a second.

It was not possible to make all the solar observations at Meudon. Janssen was well aware of the advantages of observing at high altitudes. Wishing to know whether the dark rays of oxygen are entirely telluric or whether certain of them are present before the radiation reaches the earth’s atmosphere, he went in October 1888 to the Mont Blanc massif, to the refuge of the Grands-Mulets at an altitude of 3,000 meters. His age and his lameness did not allow him to make the climb on foot, especially at that season. Thus, he invented a conveyance to be borne by porters. It consisted of a seat fixed under a horizontal ladder: the upper part of his body emerged between two rungs in such a way that his arms were supported by the uprights. The ascent, which lasted thirteen hours, was as exhausting for Janssen as for his porters. But the instruments were installed immediately; and the observations, which he was able to make during the whole of the third day, were sufficient to provide a solution to the problem under study. The dark rays were either nonexistent or so weak that it could be deduced that they would not exist for an observer at the limit of the terrestrial atmosphere.

Encouraged by this experiment, Janssen repeated it in 1890, this time at the summit of Mont Blanc. The measurements confirmed the earlier results. Despite the difficulties encountered, Janssen decided to erect an observatory there for conducting studies in physical astronomy, terrestrial physics, and meteorology. By July 1891 he had gathered the necessary funds and equipment, and two years later the observatory was completed. The initial stages were completed at Meudon, where a fifteen-ton building had been set up, which was then transported to Mont Blanc in pieces. Each piece had to weigh less than thirty kilograms so that it could be carried by porter to the summit.

Although his observatory did not withstand the rigors of the weather, Janssen had set a splendid example by his energy and unfailing courage. In 1897 the annual Mont Blanc expedition set out to determine the solar constant. Janssen had broken a leg on the staircase of the large dome at Meudon and was unable to manage the climb. He nevertheless arranged to be carried on a stretcher to Chamonix in order to organize the work of his collaborators. He was then seventy-three.

The most famous instance of Janssen’s adventurous spirit occurred during the Franco-Prussian War. He had planned to observe the eclipse of December 22, 1870, in Algeria. A balloon, the Volta, was placed at his disposal, and Janssen left Paris with an assistant on December 2. He headed west at an altitude of 2,000 meters and descended in sight of the Atlantic coast. In spite of a violent wind he succeeded in making a good landing, and his instrument cases remained intact. The weather proved to be unfavorable for the observation of the eclipse. Janssen later profited from his experience as a balloonist to think out an aeronautic compass designed to furnish instantly direction and speed of flight by observation of the apparent movement of the ground. He never ceased to be interested in aeronautical problems, the importance of which he foresaw.

A prophet of aviation, Janssen was perhaps also the precursor of observations from outside the atmosphere. He was interested, for example, in the meteor shower of the Leonids, which appears about mid-November, a period generally unfavorable for observations in Europe. The determination of its intensity, which attains a maximum every thirty-three years, was of great interest to celestial physics. Janssen had the idea of undertaking the observations in a balloon above the cloud layer. He obtained a balloon for this exploit in 1898 and for several others in the following years. In 1900 he wrote, “The application of balloons to astronomical observations is destined to render to this science services whose extent is difficult to measure today.”

Janssen, who thought that “the photographic plate is the retina of the scientist,” was one of the first to understand that a photograph can do more than record what the eye perceives. He subsequently made photographs and, through the brevity of the exposures and a special method of developing, he obtained images which disclosed a new and scientifically true aspect of the solar surface. The technique of short exposures led Janssen in 1873 to conceive of a device of historical interest, the photographic revolver.

In planning for the observation of the transit of Venus, which he was to observe in Japan on December 9, 1874, Janssen decided to substitute for visual observation at the time of transit a series of photographs taken in rapid succession, which would permit him to measure the successive positions of the planet in relation to the solar limb. He ordered the construction of an apparatus consisting of three circular disks with the same axis: the first, pierced by twelve slits, served as the shutter; the second contained a window; the photographic plate, which was circular, was fixed to the third. The first two disks turned with a synchronized movement, the shutter disk continuously and the other irregularly in the intervals of time in which the window was not swept by a slit. A series of separate images laid out on a circle was thus obtained on the plate. In a general manner the apparatus provided an analysis of a motion on the basis of the sequence of its elemental aspects. Here Janssen realized one of the operations necessary for cinematography, which was invented twenty years later, and which required, besides analysis, the synthesis of images.

Even in fields in which he was not a specialist, Janssen displayed astonishing insight. In 1865, in a course designed for architects, he set forth the principles of effective illumination.

Achievements

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  Pierre Janssen acquired an international reputation from the start of his scientific work, and his energies were mainly devoted to various scientific missions entrusted to him. Thus, he went to Peru in order to determine the magnetic equator; studied telluric absorption in the solar spectrum in Italy and Switzerland; carried out optical and magnetic experiments at the Azores; successfully observed both transits of Venus, that of 1874 in Japan, that of 1882 at Oran in Algeria; and took part in a long series of solar eclipse-expeditions. To see the eclipse of 1870 he escaped from besieged Paris in a balloon. At the great Indian eclipse of 1868, he demonstrated the gaseous nature of the red prominences and devised a method of observing them under ordinary daylight conditions. He was also the first to regularly use photographs to study the Sun, and he published his great Atlas de photographies solaires, containing more than 6,000 solar pictures.
  
  During his life, he was made a Knight of the Legion of Honor. He also received the Lalande Prize in 1868 and the Rumford Medal in 1876.
  
  Craters on both Mars and the Moon are named in his honor. The public square in front of Meudon Observatory is named Place Jules Janssen after him. Two major prizes carry his name: the Prix Jules Janssen of the French Astronomical Society, and the Janssen Medal of the French Academy of Sciences.
  
  Janssen named minor planet 225 Henrietta discovered by Johann Palisa, after his wife, Henrietta.

Works

More photos

• book

  • Solar Radiation Researches 1909
  • Annales De L'observatoire D'astronomie Physique De Paris, Sis Parc De Meudon 1906
  • Oeuvres Scientifiques, Tome 1 1929
  • Oeuvres Scientifiques, Tome 2 1930
  • Annales De L'observatoire D'astronomie Physique De Paris, Sis Parc De Meudon, Volume 1 1896

Views

Throughout his life, when a phenomenon aroused Janssen’s curiosity, he immediately studied it. In the case of the blast furnace, he perfected a delicate experimental device that permitted, in particular, the measurement of weak radiations by suitable adaptation of Melloni’s thermopile; and in less than six months he carried out the essential portion of his study.

Membership

Janssen was elected to the Academy of Sciences in 1873 and to the Bureau of Longitudes in 1875, and was also a member of the academies of Rome, Brussels, St. Petersburg, Edinburgh, and the United States.

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  Member

  Royal Society , United Kingdom

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  Member

  French Academy of Sciences , France

  1873 - 1907

• Member

  Société Philomathique de Paris , France

• Member

  Bureau des Longitudes , France

  1875 - 1907

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  Member

  Société française de photographie , France

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  Member

  Fédération photographique de France , France

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  Member

  Lincean Academy , Italy

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  Member

  Russian Academy of Sciences , Russia

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  Member

  Royal Society of Edinburgh , United Kingdom

Personality

Physical Characteristics: An accident in his early childhood left Janssen permanently lame.

Connections

Janssen was married to Philiberte Henriette Forestier; the couple had a daughter, Antoinette Marie Janssen.

Father:

Antoine César Janssen

Mother:

Pauline Marie Lemoyne

Spouse:

Philiberte Henriette Forestier

Grandfather:

Paul-Guillaume Lemoyne

Daughter:

Antoinette Marie Janssen

Acquaintance:

Victor Duruy

References

• Dictionary of Scientific Biography, Volume 7
  1973
• The Astronomer Jules Janssen
  2014