François Arago

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Dominique François Jean Arago

Catalan: Francesc Joan Domènec Aragó
Birthplace: Languedoc-Roussillon, Estagel, Pyrénées-Orientales, Languedoc-Roussillon-Midi-Pyrénées, France
Death: October 02, 1853 (67)
Paris, Paris, Île-de-France, France
Immediate Family:

Son of François Bonaventure Raymond Arago and Marie Anne Agathe Arago
Husband of Lucie Arago
Father of François Victor Emmanuel Arago; Alfred Arago and Gabriel Arago
Brother of Marie-Rose Arago; Marie-Thérèse Arago; Marie-Rose Émerenciane Arago; Marie-Victoire Françoise Arago; Général Jean Arago and 5 others

Occupation: astronome, physicien et homme politique français.
Managed by: Alex Bickle
Last Updated:

About François Arago

Dominique François Jean Arago (Catalan: Domènec Francesc Joan Aragó), known simply as François Arago (French: [f%CA%81%C9%91%CC%83swa aʁaɡo]; Catalan: Francesc Aragó, IPA: [f%C9%BE%C9%99n%CB%88s%C9%9Bsk əɾəˈɣo]) (26 February 1786 – 2 October 1853), was a French mathematician, physicist, astronomer, freemason, supporter of the carbonari and politician.

Early life and work

Arago was born at Estagel, a small village of 3,000 near Perpignan, in the département of Pyrénées-Orientales, France, where his father held the position of Treasurer of the Mint.

Arago was the eldest of four brothers. Jean (1788–1836) emigrated to North America and became a general in the Mexican army. Jacques Étienne Victor (1799–1855) took part in Louis de Freycinet's exploring voyage in the Uranie from 1817 to 1821, and on his return to France devoted himself to his journalism and the drama. The fourth brother, Étienne Vincent de (1802–1892), is said to have collaborated with Honoré de Balzac in The Heiress of Birague, and from 1822 to 1847 wrote a great number of light dramatic pieces, mostly in collaboration.

Showing decided military tastes, François Arago was sent to the municipal college of Perpignan, where he began to study mathematics in preparation for the entrance examination of the École Polytechnique. Within two years and a half he had mastered all the subjects prescribed for examination, and a great deal more, and, on going up for examination at Toulouse, he astounded his examiner by his knowledge of J. L. Lagrange.

Towards the close of 1803, Arago entered the École Polytechnique, Paris, but apparently found the professors there incapable of imparting knowledge or maintaining discipline. The artillery service was his ambition, and in 1804, through the advice and recommendation of Siméon Poisson, he received the appointment of secretary to the Paris Observatory. He now became acquainted with Pierre-Simon Laplace, and through his influence was commissioned, with Jean-Baptiste Biot, to complete the meridian arc measurements which had been begun by J. B. J. Delambre, and interrupted since the death of P. F. A. Méchain in 1804. Arago and Biot left Paris in 1806 and began operations along the mountains of Spain. Biot returned to Paris after they had determined the latitude of Formentera, the southernmost point to which they were to carry the survey. Arago continued the work until 1809, his purpose being to measure a meridian arc in order to determine the exact length of a metre.

After Biot's departure, the political ferment caused by the entrance of the French into Spain extended to the Balearic Islands, and the population suspected Arago's movements and his lighting of fires on the top of Mount Galatzó (Catalan: Mola de l'Esclop) as the activities of a spy for the invading army. Their reaction was such that he was obliged to give himself up for imprisonment in the fortress of Bellver in June 1808. On 28 July he escaped from the island in a fishing-boat, and after an adventurous voyage he reached Algiers on 3 August. From there he obtained a passage in a vessel bound for Marseille, but on 16 August, just as the vessel was nearing Marseille, it fell into the hands of a Spanish corsair. With the rest the crew, Arago was taken to Roses, and imprisoned first in a windmill, and afterwards in a fortress, until the town fell into the hands of the French, when the prisoners were transferred to Palamos.

After three months' imprisonment, Arago and the others were released on the demand of the dey of Algiers, and again set sail for Marseille on 28 November, but then within sight of their port they were driven back by a northerly wind to Bougie on the coast of Africa. Transport to Algiers by sea from this place would have occasioned a weary delay of three months; Arago, therefore, set out over land, guided by a Muslim priest, and reached it on Christmas Day. After six months in Algiers he once again, on 21 June 1809, set sail for Marseille, where he had to undergo a monotonous and inhospitable quarantine in the lazaretto, before his difficulties were over. The first letter he received, while in the lazaretto, was from Alexander von Humboldt; and this was the origin of a connection which, in Arago's words, "lasted over forty years without a single cloud ever having troubled it."

Scientific studies

Arago had succeeded in preserving the records of his survey; and his first act on his return home was to deposit them in the Bureau des Longitudes at Paris. As a reward for his adventurous conduct in the cause of science, he was elected a member of the French Academy of Sciences, at the remarkably early age of twenty-three, and before the close of 1809 he was chosen by the council of the École Polytechnique to succeed Gaspard Monge in the chair of analytical geometry. At the same time he was named by the emperor one of the astronomers of the Paris Observatory, which was accordingly his residence till his death. It was in this capacity that he delivered his remarkably successful series of popular lectures in astronomy, which were continued from 1812 to 1845.

In 1818 or 1819 he proceeded along with Biot to execute geodetic operations on the coasts of France, England and Scotland. They measured the length of the seconds-pendulum at Leith, Scotland, and in the Shetland Islands, the results of the observations being published in 1821, along with those made in Spain. Arago was elected a member of the Bureau des Longitudes immediately afterwards, and contributed to each of its Annuals, for about twenty-two years, important scientific notices on astronomy and meteorology and occasionally on civil engineering, as well as interesting memoirs of members of the Academy.

Arago's earliest physical researches were on the pressure of steam at different temperatures, and the velocity of sound, 1818 to 1822. His magnetic observations mostly took place from 1823 to 1826. He discovered rotatory magnetism, what has been called Arago's rotations, and the fact that most bodies could be magnetized; these discoveries were completed and explained by Michael Faraday.

Arago warmly supported Augustin-Jean Fresnel's optical theories, helping to confirm Fresnel's wave theory of light by observing what is now known as the spot of Arago. The two philosophers conducted together those experiments on the polarization of light which led to the inference that the vibrations of the luminiferous ether were transverse to the direction of motion, and that polarization consisted of a resolution of rectilinear propagation into components at right angles to each other. The subsequent invention of the polariscope and discovery of Rotary polarization are due to Arago. He invented the first polarization filter in 1812. He was the first to perform a polarimetric observation of a comet when he discovered polarized light from the tail of the Great Comet of 1819.

The general idea of the experimental determination of the velocity of light in the manner subsequently effected by Hippolyte Fizeau and Léon Foucault was suggested by Arago in 1838, but his failing eyesight prevented his arranging the details or making the experiments.

Arago's fame as an experimenter and discoverer rests mainly on his contributions to magnetism in the co-discovery with Léon Foucault of eddy currents, and still more to optics. He showed that a magnetic needle, made to oscillate over nonferrous surfaces, such as water, glass, copper, etc., falls more rapidly in the extent of its oscillations according as it is more or less approached to the surface. This discovery, which earned him the Copley Medal of the Royal Society in 1825, was followed by another, that a rotating plate of copper tends to communicate its motion to a magnetic needle suspended over it, which he called "magnetism of rotation" but is now known as eddy current. Arago is also fairly entitled to be regarded as having proved the long-suspected connexion between the aurora borealis and the variations of the magnetic elements. In 1828, he was elected a foreign member of the Royal Swedish Academy of Sciences.

In optics, Arago not only made important optical discoveries on his own, but is credited with stimulating the genius of Jean-Augustin Fresnel, with whose history, as well as that of Étienne-Louis Malus and Thomas Young, this part of his life is closely interwoven.

Shortly after the beginning of the 19th century the labours of at least three philosophers were shaping the doctrine of the undulatory, or wave, theory of light. Fresnel's arguments in favour of that theory found little favour with Laplace, Poisson and Biot, the champions of the emission theory; but they were ardently espoused by Humboldt and by Arago, who had been appointed by the Academy to report on the paper. This was the foundation of an intimate friendship between Arago and Fresnel, and of a determination to carry on together further fundamental laws of the polarization of light known by their means. As a result of this work, Arago constructed a polariscope, which he used for some interesting observations on the polarization of the light of the sky. He also discovered the power of rotatory polarization exhibited by quartz.

Among Arago's many contributions to the support of the undulatory hypothesis, comes the experimentum crucis which he proposed to carry out for measuring directly the velocity of light in air and in water and glass. On the emission theory the velocity should be accelerated by an increase of density in the medium; on the wave theory, it should be retarded. In 1838 he communicated to the Academy the details of his apparatus, which utilized the relaying mirrors employed by Charles Wheatstone in 1835 for measuring the velocity of the electric discharge; but owing to the great care required in the carrying out of the project, and to the interruption to his labours caused by the revolution of 1848, it was the spring of 1850 before he was ready to put his idea to the test; and then his eyesight suddenly gave way. Before his death, however, the retardation of light in denser media was demonstrated by the experiments of H. L. Fizeau and B. L. Foucault, which, with improvements in detail, were based on the plan proposed by him.

Politics and legacy

In 1830, Arago, who always professed liberal opinions of the republican type, was elected a member of the chamber of deputies for the Pyrénées-Orientales département, and he employed his talents of eloquence and scientific knowledge in all questions connected with public education, the rewards of inventors, and the encouragement of the mechanical and practical sciences. Many of the most creditable national enterprises, dating from this period, are due to his advocacy – such as the reward to Louis-Jacques Daguerre for the invention of photography, the grant for the publication of the works of Fermat and Laplace, the acquisition of the museum of Cluny, the development of railways and electric telegraphs, the improvement of the reneile.

In 1830, Arago also was appointed director of the Observatory, and as a member of the chamber of deputies he was able to obtain grants of money for rebuilding it in part, and for the addition of magnificent instruments. In the same year, too, he was chosen perpetual secretary of the Academy of Sciences, the place of J. B. J. Fourier. Arago threw himself into its service, and by his faculty of making friends he gained at once for it and for himself a worldwide reputation. As perpetual secretary it was his duty to pronounce historical eulogies on deceased members; and for this duty his rapidity and facility of thought, and his happy piquancy of style, and his extensive knowledge peculiarly adapted him. He was elected a Foreign Honorary Member of the American Academy of Arts and Sciences in 1832.

In 1834, Arago again visited Scotland, to attend the meeting of the British Association at Edinburgh. From this time till 1848 he led a life of comparative quiet – although he continued to work within the Academy and the Observatory to produce a multitude of contributions to all departments of physical science – but on the fall of Louis-Philippe he left his laboratory to join the Provisional Government (24 February 1848). He was entrusted with two important functions, that had never before been given to one person, viz. the ministry of marine and colonies (24 February 1848 – 11 May 1848) and ministry of war (5 April 1848 – 11 May 1848); in the former capacity he improved rations in the navy and abolished flogging. He also abolished political oaths of all kinds and, against an array of moneyed interests, succeeded in procuring the abolition of slavery in the French colonies.

On 10 May 1848, Arago was elected a member of the Executive Power Commission, a governing body of the French Republic. He was made President of the Executive Power Commission (11 May 1848) and served in this capacity as provisional head of state until 24 June 1848, when collective resignation of the Commission was submitted to the National Constituent Assembly. At the beginning of May 1852, when the government of Louis Napoleon required an oath of allegiance from all its functionaries, Arago peremptorily refused, and sent in his resignation of his post as astronomer at the Bureau des Longitudes. This, however, the prince president declined to accept, and made "an exception in favour of a savant whose works had thrown lustre on France, and whose existence the government would regret to embitter."

Cape Gregory was named by Captain Cook on March 12, 1778 after Saint Gregory, the saint of that day; it was renamed Cape Arago after François Arago.

Last years

Arago remained a consistent republican to the end, and after the coup d'état of 1852, though suffering first from diabetes, then from Bright's disease, complicated by dropsy, he resigned his post as astronomer rather than take the oath of allegiance. Napoleon III gave directions that the old man should be in no way disturbed, and should be left free to say and do what he liked. In the summer of 1853 Arago was advised by his physicians to try the effect of his native air, and he accordingly set out to the eastern Pyrenees, but this was ineffective and he died in Paris. His grave is at the famous Père Lachaise Cemetery in Paris.

Craters on Mars and the Moon, and a ring of Neptune, are named after Arago, as well as the study association for Applied Physics at the University of Twente. His name is one of the 72 names inscribed on the Eiffel Tower.


Arago's works were published after his death under the direction J. A. Barral, in 17 vols., 8vo, 1854–1862; also separately his Astronomie populaire, in 4 vols.; Notices biographiques, in 3 vols.; Indices scientifiques, in 5 vols.; Voyages scientifiques, in 1 vol.; Grimoires scientifiques, in 2 vols.; Mélanges, in I vol.; and Tables analytiques et documents importants (with portrait), in 1 vol.

English translations of the following portions of Arago's works have appeared:

  • Treatise on Comets, by C. Gold, C.B. (London, 1833); also translated W. H. Smyth and Grant (London, 1861)
  • Euloge of James Watt, by Muirhead (London, 1839); also translated, with notes, by Brougham
  • Popular Lectures on Astronomy, by Walter Kelly and Rev. L. Tomlinson (London, 1854); also translated by Dr W. H. Smyth and Prof. R. Grant, 2 vols. (London, 1855)
  • Arago's Autography, translated by the Rev. Baden Powell (London, 1855, 58)
  • Arago's Meteorological Essays, with introduction by Alexander von Humboldt, translated under the supervision of Colonel Edward Sabine (London, 1855)
  • Arago's Biographies of Scientific Men, translated by Smyth, Powell and Grant, 8vo (London, 1857)

See also

About François Arago (Français)

François Arago, né le 26 février 1786 à Estagel (Roussillon) et mort le 2 octobre 1853 à Paris, est un astronome, physicien et homme politique français.

Il est l'ainé et le plus célèbre des six frères Arago, les autres étant Jean (1788-1836), général au service du Mexique, Jacques (1790-1855), écrivain et explorateur, Victor (1792-1867) et Joseph (1796-1860), militaires, et Étienne (1802-1892), écrivain et homme politique.


Il est le fils aîné de François Bonaventure Arago, propriétaire terrien, maire d'Estagel et juge de paix du canton en 1790 puis directeur de l’Hôtel de la Monnaie à Perpignan en 1795, et de Marie Anne Agathe Roig, fille d'un paysan aisé de la région.

Carrière scientifique

Il fait ses études secondaires au collège communal de Perpignan (actuel lycée François-Arago), puis ses études supérieures à l'École polytechnique1 qu'il intègre en 1803, âgé de dix-sept ans. Remarqué par Monge et Laplace, il est nommé en 1805 secrétaire-bibliothécaire de l'Observatoire de Paris. En 1806, il est envoyé en Espagne, à Majorque avec Jean-Baptiste Biot pour poursuivre le relevé du méridien de Paris. Pris dans la guerre d'Espagne, alors qu'il pratique seul une opération de triangulation6, il est fait prisonnier. Interné au Château de Bellver7, il s'évade plusieurs fois, et parvient à rejoindre Paris où il entre en héros en 1809. Cela lui permet d'être élu membre de l'Académie des sciences le 18 septembre 1809, à seulement vingt-trois ans8.

La même année, il est choisi par Monge pour le suppléer comme professeur de géométrie analytique à l'École polytechnique ; il prend le titre de professeur adjoint (de Monge) en 1812 : il reste près de vingt ans professeur à Polytechnique. En 1816, il crée un cours original d'« arithmétique sociale », donnant aux élèves des notions de calcul des probabilités, d'économie mathématique et de démographie.

Parallèlement, il poursuit sa carrière à l'Observatoire de Paris, qui dépend du Bureau des longitudes. Après avoir été secrétaire-bibliothécaire, il est nommé membre adjoint du Bureau en 1807 ; il en devient membre titulaire en 1822, à la mort de Delambre. En 1834, il prend le titre, dont il avait proposé la création au Bureau, de « directeur des observations à l'Observatoire de Paris », que dirige l'astronome Alexis Bouvard. À la mort de Bouvard, en 1843, il devient directeur de l'Observatoire et le reste jusqu'à sa mort.

Carrière politique

La mort de son épouse, en août 1829, est parfois avancée comme l'une des raisons qui l'ont poussé à se tourner vers la vie publique, tant sur le plan scientifique que politique. Après avoir été élu secrétaire perpétuel de l'Académie des sciences, il remporte ses premiers succès électoraux : conseiller général de la Seine en septembre 1830, député des Pyrénées-Orientales en juillet 1831.

Il est pendant les Trois Glorieuses colonel de la Garde nationale, puis une des figures du parti républicain pendant la monarchie de Juillet. Candidat aux élections législatives dans les Pyrénées-Orientales, le journal L'Indépendant des Pyrénées-Orientales est fondé en 1846 afin de le soutenir. Il est alors élu avec succès cette même année remportant 98,9 % des suffrages exprimés. Il choisit cependant de représenter la Seine, où il est élu simultanément et avec un score aussi enthousiaste.

Après la révolution de 1848, il devient ministre de la Guerre, de la Marine et des Colonies dans le gouvernement provisoire de la Seconde République, mis en place par Lamartine puis président de la Commission exécutive, assumant de fait durant un mois et demi une charge proche de celle de chef de l'État12. Il contribue à ce titre à l'abolition de l'esclavage dans les colonies françaises. Il refuse par la suite de prêter le serment de fidélité à Louis-Napoléon Bonaparte qui est exigé des fonctionnaires et préfère démissionner de son poste au Bureau des longitudes. Le prince-président refuse sa démission, le dispensant implicitement du serment d’allégeance. Après le coup d’État du 2 décembre 1851 qui aboutit à la création du Second Empire, il démissionne de ses fonctions. Napoléon III demande à ce qu'il ne soit pas inquiété. Malade, souffrant de diabète et de diverses affections, Arago meurt dans l’année qui suit. Il est inhumé au cimetière du Père-Lachaise (division 4).

Ses travaux scientifiques

L'expérience d'Arago (1810)

Les premiers travaux de physique d'Arago concernent l'optique. Il fait en 1810 une expérience importante, qu'il présente oralement à l'Académie des sciences le 10 décembre 1810 ; il ne la consigne par écrit que juste avant sa mort, plus de quarante ans après : il s'agit de mesurer la vitesse de la lumière venant des étoiles, en comparant la valeur le matin à 6 h et le soir à 18 h. Étant donné qu'on n'observe pas d'étoiles pendant la journée, Arago fait son expérience à ces heures-là. À 6 h, quand on observe une étoile au zénith, la Terre s'en approche, on devrait mesurer c + V, où V est la vitesse de rotation de la Terre autour du Soleil ; à 18 h, pour une autre étoile au zénith, la Terre s'en éloigne, on devrait mesurer c - V. Or l'expérience est négative, on mesure c dans les deux cas : cette première expérience négative ouvre plus tard la voie à la théorie de la relativité.

Travaux en optique

Arago, d'abord adepte de la théorie corpusculaire de la lumière, est convaincu par la théorie ondulatoire de son collègue Fresnel, qu'il aide pour faire ses expériences à l'Observatoire ou présenter ses résultats à l'Académie des sciences. Avec Biot, il détermine l'indice de réfraction de l'air et d'autres gaz.


En 1825, il est chargé avec Dulong de déterminer la tension de la vapeur d'eau à des pressions dépassant 3 MPa, soit 30 atm. Ses autres études sont consacrées à l'astronomie, au magnétisme et à la polarisation de la lumière. Il détermine, par exemple, le diamètre des planètes et explique entre autres la scintillation des étoiles à l'aide du phénomène des interférences.

Touche-à-tout, il se mêle aux expériences de mesure de la vitesse du son et étudie les cuves sous pression. Il fait creuser le premier puits artésien de Paris par Louis-Georges Mulot, dans la cour de l'abattoir de Grenelle, dans l'actuel 15e arrondissement. Il inspire à Foucault son expérience des miroirs tournants, qui permet ensuite de mesurer la vitesse de la lumière avec précision.

Conscient de l'importance potentielle du procédé en astronomie, il promeut la photographie alors naissante en soutenant le daguerréotype mis au point par Louis Daguerre : en janvier 1839, il présente devant l'Académie des sciences et l'Académie des beaux-arts réunies les premiers clichés.

Travaux de vulgarisation

Arago est un orateur redoutable, capable de défaire les plus brillants contradicteurs. Il est aussi pédagogue et grand vulgarisateur scientifique. Afin de faire connaître les travaux de l'Académie des sciences, il crée en 1835 les Comptes rendus de l'Académie des sciences, qui existent toujours. Avant lui, il n'y avait pas de transcription écrite des séances de l'Académie.

Il donne aussi, de 1813 à 1846, un cours public d'astronomie populaire13, qui remporte un immense succès. Ce sont ces cours qui donnent naissance à son Astronomie populaire en quatre tomes, parue à titre posthume en 1854. Dans l'« Avertissement » qui ouvre le premier tome, Arago explique ainsi son projet : « Je maintiens qu’il est possible d’exposer utilement l’astronomie, sans l’amoindrir, j’ai presque dit sans la dégrader, de manière à rendre ses plus hautes conceptions accessibles aux personnes presque étrangères aux mathématiques ».

Famille et descendance

Le 11 septembre 1811, Arago épouse Lucie Carrier-Besombes ; le couple a ensuite trois fils :

   Emmanuel Arago (1812-1896), avocat et homme politique républicain, brièvement ministre pendant le siège de Paris en 1870 ;
   Alfred Arago (1815-1892), peintre et inspecteur général des beaux-arts ;
   Gabriel Arago (1817-1832).

Il est le beau-frère du physicien Alexis Petit, leurs épouses respectives étaient sœurs, et de l'astronome Claude-Louis Mathieu.

Un de ses petit-fils, Pierre Jean François Arago, fils d'Emmanuel Arago, est député des Alpes-Maritimes sous la Troisième République.

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François Arago's Timeline

February 26, 1786
Estagel, Pyrénées-Orientales, Languedoc-Roussillon-Midi-Pyrénées, France
August 6, 1812
Paris, Paris, Île-de-France, France
June 20, 1816
Paris, Paris, Île-de-France, France
October 2, 1853
Age 67
Paris, Paris, Île-de-France, France