Pierre Janssen

For the Belgian footballer, seePier Janssen.

Jules Janssen; photograph byNadar(date unknown)

Photo taken by Janssen, from theMeudonobservatory, ofRenardandKrebs'La Francedirigible (1885)

Pierre Jules César Janssen(22 February 1824 – 23 December 1907), usually known asJules Janssen,was a Frenchastronomerwho, along with English scientistJoseph Norman Lockyer,is credited with discovering the gaseous nature of the solarchromosphere,but there is no justification for the conclusion that he deserves credit for the co-discovery of the elementhelium.

Life, work, and interests

Janssen was born in Paris (DuringBourbon Restoration in France) into a cultivated family. His father,César Antoine Janssen(born in Paris, 1780 – 1860) was a well knownclarinettistfrom Dutch/Belgian descent (his father, Christianus Janssen, emigrated fromWalloon Brabantto Paris). His mother Pauline Marie Le Moyne (1789 – 1871) was a daughter of the architect Paul Guillaume Le Moyne.

Pierre Janssen studiedmathematicsandphysicsat the faculty of sciences. He taught at theLycée Charlemagnein 1853, and in the school ofarchitecture1865 – 1871, but his energies were mainly devoted to various scientific missions entrusted to him. Thus in 1857 he went to Peru in order to determine the magneticequator;in 1861–1862 and 1864, he studiedtelluric absorptionin the solar spectrum in Italy and Switzerland; in 1867 he carried out optical and magnetic experiments at the Azores; he successfully observed bothtransits of Venus,that of 1874 in Japan, that of 1882 atOranin Algeria; and he took part in a long series of solar eclipse-expeditions, e.g. toTrani,Italy (1867),Guntur,India (1868),Algiers(1870), Siam (1875), theCaroline Islands(1883), and toAlcossebrein Spain (1905). To see the eclipse of 1870, he escaped from theSiege of Parisin a balloon.Unfortunately the eclipse was obscured from him by cloud.

In the year 1874, Janssen invented theRevolver of Janssenor Photographic Revolver, instrument that originated thechronophotography.Later this invention was of great use for researchers likeEtienne Jules Mareyto carry out exhibitions and inventions.

Discovery of helium

In 1868 Janssen discovered how to observesolar prominenceswithout aneclipse.While observing thesolar eclipse of 18 August 1868,atGuntur,Madras State(now inAndhra Pradesh), British India, he noticed bright lines in the spectrum of the chromosphere, showing that the chromosphere is gaseous. From the brightness of the spectral lines, Janssen realized that the chromospheric spectrum could be observed even without an eclipse, and he proceeded to do so.But he never mentioned the emission line seen byJoseph Norman Lockyer,which later was shown to be due to the element helium.

On 20 October, Lockyer in England set up a new, relatively powerfulspectroscope.He also observed the emission spectrum of the chromosphere, including a new yellow line near the sodium D line, which he called "D3". Lockyer and the English chemistEdward Franklandspeculated that the new line could be due to a new element, which they named the element after the Greek word for the Sun, ἥλιος (helios).

Observatories

Passage de Venus(1874)

At the great Indian eclipse of 1868 that occurred inGuntur,Janssen also demonstrated the gaseous nature of the red prominences, and devised a method of observing them under ordinary daylight conditions.One main purpose of his spectroscopic inquiries was to answer the question whether theSuncontainsoxygenor not. An indispensable preliminary was the virtual elimination of oxygen-absorption in theEarth's atmosphere,and his bold project of establishing an observatory on the top ofMont Blancwas prompted by a perception of the advantages to be gained by reducing the thickness of air through which observations have to be made. This observatory, the foundations of which were fixed in the hard ice that appeared to cover the summit to a depth of over ten metres, was built in September 1893, and Janssen, in spite of his sixty-nine years, made the ascent and spent four days making observations.

In 1875, Janssen was appointed director of the new astrophysical observatory established by the French government atMeudon,and set on foot there in 1876 the remarkable series of solar photographs collected in his greatAtlas de photographies solaires(1904). The first volume of theAnnales de l'observatoire de Meudonwas published by him in 1896.(see alsoMeudon Great Refractor)

Janssen was the President of theSociété Astronomique de France (SAF),the French astronomical society, from 1895 to 1897.

International Meridian Conference

In 1884 he took part in theInternational Meridian Conference.

Death, honors, and legacy

Janssen's grave in Paris

Janssen died at Meudon on 23 December 1907 and was buried atPère Lachaise Cemeteryin Paris, with the name "J. Janssen" inscribed on his tomb. During his life he was made a Knight of theLegion of Honorand aForeign Memberof theRoyal Society of London.

Craterson bothMarsand theMoonare named in his honor. Thepublic squarein front ofMeudon Observatoryis namedPlace Jules Janssenafter him. Two major prizes carry his name: thePrix Jules Janssenof theFrench Astronomical Society,and theJanssen Medalof theFrench Academy of Sciences.

Janssen named minor planet225 Henriettadiscovered byJohann Palisa,after his wife, Henrietta.

Further reading

• "Pierre Jules Cesar Janssen".Monthly Notices of the Royal Astronomical Society.68(4): 245–249. 1908.Bibcode:1908MNRAS..68..245..doi:10.1093/mnras/68.4.245.
• Obituary,fromPopular Astronomy,1908, vol. 16, pp. 72–74
• Obituary,fromAstronomische Nachrichten,1908, vol. 177, p. 63 (in French)
• Obituary,fromThe Astrophysical Journal,1908, vol. 28, pp. 89–99 (in French)
• Janssen statue,description and black-and-white picture fromThe Observatory,1922, vol. 45, pp. 175–176
• Brief biography,from the High Altitude Observatory at Boulder, Colorado
• Pierre JanssenatWho's Who of Victorian Cinema
• Launay, Françoise (2012).The Astronomer Jules Janssen: A Globetrotter of Celestial Physics.Springer.
• Launay, Françoise (2008).Un Globe-Trotter de la Physique Céleste: L'astronome Jules Janssen.Vuibert.
• Nath, B. B. (2012).The Discovery of Helium and the Birth of Astrophysics.Springer.