Background

Jean Sylvain Bailly was born on September 15, 1736, in Paris, France. He was the son of Jacques Bailly, an artist. He was the grandson of Nicholas Bailly, who also was an artist and court painter. Bailly was born in the Louvre, where his father, Jacques, as keeper of the king’s paintings, had an apartment. His father also owned a house in Chaillot, a fashionable suburb of Paris.

Education

When he was about twenty, Jean Bailly received mathematics lessons from Montcarville of the Collège Royal. Shortly thereafter he met Nicolas de Lacaille, France’s greatest observational astronomer, and Alexis Clairaut, France’s greatest theoretical astronomer. The further studies that he pursued with them directed him into his lifework.

Career

Little is known of Bailly’s youth, but it appears that he received schooling that would prepare him to assume his father’s office, a position held by a member of the family since Bailly’s great-grandfather. He succeeded to the title in 1768. Bailly was relieved of his functions in 1783 but continued to receive the pension accorded to the position and to be known as an honorary keeper.

Bailly undertook his first astronomical research in 1759 - concerned, appropriately, with Halley’s comet. His paper on the comet’s theory, read to and published by the Académie des Sciences, pointed out that one could not conclude the duration of its revolution from that of its visibility. This paper was based upon the observations of Lacaille.

In 1760 Bailly established his own observatory in the Louvre. Although not ideal, the site served him well, and he felt it important for an astronomer to base his theories on his own observations. He now began to do this with Jupiter’s satellites, reading two papers on this subject in 1762 and adding a third near the end of 1763.

By the time he read the third report, Bailly had become a member of the Académie. In January 1763 Bailly was elected to the vacancy created by the death of Lacaille, probably because he had been Lacaille’s protégé and was engaged in editing an unfinished work of his. He had also shown greater promise than his competitors, Jeaurat and Messier, of significant theoretical researches.

Bailly approached the problem of inequalities in the motions of the four known satellites of Jupiter with Clairaut’s lunar theory in mind. Improvements had been made in tables of the motions since Cassini’s 1668 ephemerides, but these improvements had been made empirically. Bailly was the first to attempt to achieve better tables theoretically, by treating each satellite in turn as the third body in a three-body problem. His success was not complete - there were considerable discrepancies between his theoretical formulation of orbitary elements and their observed values - but he did demonstrate that the problem was amenable to solution by Newtonian principles.

Bailly’s memoirs sparked interest in this subject, and the Académie made it the topic of its essay contest for 1766. As an Academician, Bailly could not win the prize; nevertheless, he considered himself in competition with the astronomers who entered and therefore synthesized and extended his earlier researches to produce his major Essai. Although excellent, this work was greatly overshadowed by the prize-winning entry of Lagrange. As Bailly himself later admitted, Lagrange’s use of the new method of the variation of parameters allowed him to resolve almost completely the problem of five simultaneously perturbed bodies, as opposed to his own continuing series of three-body treatments.

In 1766 there arose the possibility of Bailly’s succeeding the ailing Grandjean de Fouchy as perpetual secretary of the Académie. To prepare for this position, Bailly wrote several éloges. He did not achieve his goal, however; in 1770 Fouchy named Condorcet as his assistant.

Bailly followed this setback with one of his best scientific papers, his 1771 memoir on the inequalities of the light of Jupiter’s satellites. Fouchy had earlier noted that a satellite disappeared before its total immersion and that, at its emersion, it was observed only when a small segment had already emerged from the planet’s shadow. He concluded that the size of the segment involved depended upon the amount of the particular satellite’s light. Utilizing a new observational technique, Bailly confirmed Fouchy’s theory while coupling his study of light intensities with determinations of the diameters of the satellites. The resultant paper added greatly to contemporary knowledge of Jupiter’s satellites and suggested a standard observing method to reduce instrument and observer errors. Because the fourth satellite had not been eclipsing during the period of his observations, the memoir dealt only with the first three satellites. Bailly never completed the study, for this paper was his last theoretical effort.

Having moved to his father’s house in Chaillot, Bailly turned his attention to literary pursuits. These were given direction by his scientific activity, as shown in the four-volume history of astronomy that he published between 1775 and 1782. These tomes represent his most lasting achievement and were responsible for additional honors. In 1783 he was elected to the Académie Française, and two years later he was named to the Académie des Inscriptions et Belles-Lettres. This made him a triple academician, the only Frenchman besides Bernard de Fontenelle to achieve this distinction.

Meanwhile, Bailly continued to work within the Académie des Sciences, although not at astronomical pursuits. In 1784 he was named to the commission appointed to investigate the extravagant claims then being made for “animal magnetism” by Mesmer and others, and he drafted the commission’s damning report on that alleged phenomenon. It was probably for this service that, near the end of 1784, Bailly was made a supernumerary pensioner within the Académie. A year later he was appointed to a commission to investigate the Hôtel Dieu, the hospital of the poor of Paris; again he prepared the findings. Three reports submitted between 1786 and 1788 deplored the miserable conditions existing at the hospital and suggested means for their correction.

It was chiefly acclaim through these reports that catapulted Bailly into public affairs at the beginning of the French Revolution; the movement culminated on 15 July 1789 in his unanimous proclamation as the first mayor of Paris. He was reelected for a second term in August 1790, but in this second year, he lost popularity, particularly after the unfortunate massacre of the Champ-de-Mars. Although he retired from office in November 1791 and left Paris in July he was not forgotten. Arrested in September Bailly was soon tried, found guilty, and condemned to the guillotine. Like those of his victims, his head fell on the Champ-de-Mars.

On the 12th of November of 1793, he was guillotined amid the insults of a howling mob. He met his death with patient dignity, having, indeed, disastrously shared the enthusiasms of his age, but taken no share in its crimes. Notices of his life are contained in the Eloges by Merard de Saint Just, Delisle de Salles, Lalande, and Lacretelle; in a memoir by Arago, read on the 26th of February 1844 before the Academie des Sciences, and published in Notices biographiques.

Achievements

• Bailly's major achievement was in his precise calculation of an orbit for the comet of 1759 (Hailey's) and reduction of the Lacaille's observations of 515 zodiacal stars. His chief work as an astronomer was Essai sur la thiorie des satellites de Jupiter (1766), an expansion of a memoir presented to the Academy of Sciences in 1763, which showed much original power; and it was followed up in 1771 by a noteworthy dissertation Sur les inegalites de la lumi'ere des satellites de Jupiter.
  
  Also, Bailly's career achievement was in his election Estates-General, then the president of the National Assembly, being the first to take the Tennis Court Oath and the first mayor of Paris.

Works

More photos

• book

  • Mémoires d'un Témoin de la Révolution, ou Journal des Faits Qui Se Sont Passés Sous Ses Yeux, Et Qui Ont Préparé Et Fixé la Constitution Française
  • Eloges De Charles V. De Moleère
  • Essai Sur La Théorie Des Satellites De Jupiter 1766
  • Histoire de l'astronomie moderne depuis la fondation de l'école d'Alexandrie 1775
  • Lettres sur l'Atlantide de Platon et sur l'ancienne histoire de l'Asie. l'our servir de suite aux Lettres sur l'origine des sciences 1779
  • Histoire de l'astronomie ancienne et moderne 1775
  • Traité de l'Astronomie Indienne Et Orientale, Ouvrage Qui Peut Servir de Suite À l'Histoire de l'Astronomie Ancienne 1787

Politics

Politically Bailly was moderate and, at the same time, a constitutional monarchist at heart. As mayor of Paris, he supported the National Constituent Assembly while ignoring or suppressing radical political demands. Bailly’s moderate position made him a frequent target for radical journalists like Jean-Paul Marat and Georges Danton, who ran against Bailly for the mayorship in 1790.

Views

Quotations: Bailly, as mayor, presented Louis XVI with a tricolour cockade and the keys to Paris, famously declaring that “the people have reconquered their king”.

Bailly wrote the following beautiful and eloquent statement, which reveals quite strikingly the noble character of the man: "Nature is just; she equally distributes all that is necessary to the individual put on earth to live, work, and die; she reserves to a small number of human beings, however, the right to enlighten the world, and by entrusting them with the lights that they must diffuse across their century, she says to one, you shall observe my phenomena, to the other, you shall be a geometer; she calls on this one for the purpose of legislation; she calls on this other one to paint the morals of people, of revolutions, and of empires. These geniuses pass away after they have perfected human reason, and leave behind them a great memory. But all of them have travelled on different routes: Only one man elevated himself, and dared to become universal, a man whose strong will synthesized the spirit of invention, and the spirit of method, and who seemed to have been born to tell the human race: Behold and know the dignity of your species! These are the traits by which Europe has given recognition to Leibniz."

Membership

In 1763 Bailly was elected a member of the Academy of Sciences. He was admitted to the French Academy (February 26, 1784), and to the Academie des Inscriptions in 1785.

Bailly was also a member of the Club de 1789, one of the most well-known societies at the time. Though calls on his time from his mayoral duties restricted his involvement in the group, by May 1790, Bailly had risen to presiding officer of the club. In 1791, Jean Sylvain Bailly joined the Jacobin Club, but no active role was taken by him.

Personality

Bailly’s bold leadership and gestures made him popular with the people of Paris, who routinely cheered his public appearances.

Quotes from others about the person

• A. Berville, giving recognition to a true hero of the French Revolution, Jean-Sylvain Bailly, stated: "A noble and touching picture presented for posterity is that of a man who, already famous in the domain of the sciences, commendable in all private virtues, who finds himself, almost without his knowledge, risen by public esteem to the highest functions; maintaining his modesty among the highest dignitaries, moderating in the middle of the most violent political dissensions; going through a raging revolution without being swayed by it; participating in all of its glories, yet remaining unstained by its excesses; defending liberty against power, and power against license, and whose wise life becomes crowned by the death of a hero. Such was Sylvain Bailly, first deputy of Paris to the Estates General, first president to the National Assembly, and first Mayor of Paris."

Connections

After Bailly took up residence in Chaillot, a fashionable suburb of Paris, he met Madame Gaye there, who was a widow and whom he married in 1787.

Father:

Jacques Bailly

Grandfather:

Nicholas Bailly

Wife:

Madame Gaye

teacher:

Alexis Claude Clairaut

teacher:

Nicolas-Louis de Lacaille

By 1763 Bailly was elected to the vacancy created by the death of Lacaille, probably because he had been Lacaille’s protégé and was engaged in editing an unfinished work of his.

Friend:

Pierre Simon Laplace

Late in 1793, Bailly quitted Nantes to join his friend Pierre Simon Laplace at Melun; but was there recognized, arrested and brought (November 10) before the Revolutionary Tribunal at Paris.

References

• Jean Sylvain Bailly: Astronomer, Mystic, Revolutionary, 1736-1793
  2013
• Jean sylvain bailly
  1927
• Jean Sylvain Bailly: Revolutionary Mayor of Paris
  2013
• Dictionary of scientific biography
  1970
• Encyclopaedia Britannica
  1990