The Astronomer Jules Janssen Astrophysics and Space Science Library EDITORIAL BOARD Chairman W. B. BURTON, National Radio Astronomy Observatory, Charlottesville, Virginia, U.S.A. ([email protected]); University of Leiden, The Netherlands ([email protected]) F. BERTOLA, University of Padua, Italy J. P. CASSINELLI, University of Wisconsin, Madison, U.S.A. C. J. CESARSKY, European Southern Observatory, Garching bei M¨unchen, Germany P. EHRENFREUND, Leiden University, The Netherlands O. ENGVOLD, University of Oslo, Norway A. HECK, Strasbourg Astronomical Observatory, France E. P. J. VAN DEN HEUVEL, University of Amsterdam, The Netherlands V. M. KASPI, McGill University, Montreal, Canada J. M. E. KUIJPERS, University of Nijmegen, The Netherlands H. VAN DER LAAN, University of Utrecht, The Netherlands P. G. MURDIN, Institute of Astronomy, Cambridge, UK F. PACINI, Istituto Astronomia Arcetri, Firenze, Italy V. RADHAKRISHNAN, Raman Research Institute, Bangalore, India B. V. SOMOV, Astronomical Institute, Moscow State University, Russia R. A. SUNYAEV, Space Research Institute, Moscow, Russia For further volumes: http://www.springer.com/series/5664 Françoise Launay The Astronomer Jules Janssen A Globetrotter of Celestial Physics Translated by Storm Dunlop Françoise Launay Observatoire de Paris Paris, France [email protected] ISBN 978-1-4614-0696-9 e-ISBN 978-1-4614-0697-6 DOI 10.1007/978-1-4614-0697-6 Springer New York Dordrecht Heidelberg London Library of Congress Control Number: 2011936386 Translation from the French language edition: Un globe-trotter de la physique céleste: L’astronome Jules Janssen by Françoise Launay, © Vuibert 2008. All rights reserved © Springer Science+Business Media, LLC 2012 All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street, New York, NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identifi ed as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights. Cover photo: “Statue of Janssen at Meudon” courtesy of the author Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com) Fig. 1 Antoinette Janssen in 1907 © Paris Observatory Library This book is dedicated to Antoinette Janssen, who did so much for her father’s memory. A senior nursing offi cer in the Association des dames françaises [Association of French Ladies] during the First World War, the sole daughter of Jules Janssen did, in effect, take the initiative in ensuring the publication of her father’s Œuvres scien- tifi ques. 1 This work collects together the scientifi c articles that she herself and Henri Dehérain, the curator of the Library of the Institut de France, judged to be the most signifi cant. Henri Dehérain was the son of Pierre-Paul Dehérain, a lecturer at the Muséum, the friend that Janssen knew in 1852 when they were students together at the Sorbonne, and who was elected to the Académie des sciences in 1888, during his presidency. The two volumes were published, respectively, in 1929 and 1930, after Antoinette’s death. 1 Janssen, Jules, Œuvres scientifi ques, recueillies et publiées par Henri Dehérain , Paris, Société d’Éditions Géographiques, Maritimes et Coloniales, vol. I (1929) and vol. II (1930). v vi Dedication Antoinette Janssen also left two signifi cant legacies: • The fi rst, to the Library of the Institut de France, of all the correspondence received by the family as well as her father’s notebooks, ordered chronologically, like diaries. • The second, to the Conservatoire national des arts et métiers (CNAM), of three instruments designed by Janssen: the 1862 spectroscope, the 1870 aeronautical compass, and the 1874 photographic revolver, the recognized predecessor of the cine camera. The collection of scientifi c works saves a lot of research in scattered publications. The instruments represent a major tribute to Janssen as an inventor, who was defi - nitely more of an experimenter and engineer than a theoretician. The notebooks and the correspondence2 are an extraordinary mine of information, because, apart from the some 500 letters exchanged by the Janssen husband and wife couple, the collec- tion includes more than a thousand others sent to Janssen by more than 160 corre- spondents. The latter consist not just of people from the scientifi c world, but also the literary and artistic worlds, and even politicians. 2 An annotated edition of this correspondence, augmented by letters from other sources, is in prepa- ration, cf . Launay, Françoise, “Jules Janssen’s in and out correspondence,” in 100 years of obser- vational astronomy and astrophysics, Homage to Miklos Konkoly Thege (1842–1916) , C. Sterken and J. Hearnshaw, eds., Brussels (2001), 159–68. Preface to the French Edition Jules Janssen. This D’Artagnan of science, this bard of the Sun, and this audacious master builder, merits far more than just a statue on the public terrace at Meudon, at the opposite end to the observatory gates: a statue subject to the ravages of time. Janssen is actually the founder of astrophysics in France. After confi rmation of the triumph of Newtonian theory, and both the subtle and powerful developments of it made by Clairaut, Lagrange, and Laplace, and after Le Verrier and the discovery of Neptune, the need arose to study the nature of the celestial bodies and of that of the Fig. 8 The Janssen Medal © Author’s collection vii viii Preface to the French Edition Sun in particular. What a long delay there was, in our country, where celestial mechanics developed remarkably, but in splendid isolation, relative to the astro- physical work carried out by Anglo-Saxon and German astronomers. There was Herschel with the infrared and the “island universes”; Fraunhofer and the solar spectrum; Wolf and Schwabe and the observation of solar activity… In this book, which, let us be in no doubt, will become a source of reference, all the aspects of Janssen’s rich personality have been covered by Françoise Launay, his attentive and erudite biographer. A physicist, an inventor and instrument builder, crazy about the Sun and travel, Janssen was a sort of “self-made man,” who had acquired most of his great knowledge, practically alone, guided by his energy and curiosity. Naturally, a mathematician (he followed the courses given by Cauchy and Le Verrier), he tried (without success) to be recruited by the tetchy director of the Paris Observatory. But luck accompanied his employment as the tutor that he became, and brought him in contact with the future great traveler and geographer, Alfred Grandidier. From this began his great travels around the globe, under various pretexts. Initially, this was an effort directed toward geophysics; in Peru, Janssen measured the magnetic fi eld and determined the magnetic equator. The spirited young man had no hesitation: in preparing for his trips, he made contact with the most competent members of the Académie. He built up, if I may put it this way, a really useful address book… He threw himself into physics: his thesis discussed the propagation of heat. But in what medium? Not in optical laboratories. He became a physiologist: the medium was the eye, which was not damaged by the near infrared (then called “thermal”) radiation. It was this that formed the very personal subject of Janssen’s thesis, a thesis in which there was no intervention from the hand of any master. Janssen’s fi rst invention was, in effect, Janssen himself. Like all inventors, before arriving at the correct solution, he tried everything. The analysis of the spectrum of sunlight revealed the role of absorption in the Earth’s atmosphere. So Janssen’s research followed two paths: the Earth’s atmosphere and the Sun. And mastery of a technique, spectroscopy, that would enable him to make rapid progress toward those two ends. Studying the Earth’s atmosphere and the Sun encouraged travel, even demanded it. Nothing could have appealed more to the enthusiastic temperament of our physi- cist. And was it not on a mountain that one had the best of the Sun without being bothered by the Earth’s atmosphere? But was it not also there that the purest spectra were available? A high mountain, then. Switzerland, the Alps, Etna, the Himalaya… Janssen was restless. And not just to observe the Sun! A map of the globe devoted to his journeys would be criss-crossed by an extraordinary number of erratic round trips and covered in small fl ags “Jules Janssen was here.” Just look at Janssen’s biography, so minutely picked out by Françoise Launay in her “biographical land- marks”! It is absolutely astonishing, given that in Janssen’s day there were no TGVs or Airbus 320s… Among this almost Brownian motion, we might pick out two – no, sorry, two groups – of journeys that are particularly memorable. One was the trip to Japan in 1874 to observe the transit of Venus in front of the Sun. In 1882, recognizing that celestial mechanics was not his strong point, Janssen did not take part in the expeditions for the second transit. The transits of Venus Preface to the French Edition ix provided plenty of “grist to the mill” for the specialists in fundamental astronomy. But it was not our astrophysicist’s “cup of tea,” which he readily recognized. Admittedly, he invented the famous photographic revolver to record the precise instants of the moments of contact during the course of the transits.