Jacques-André Mallet

Jacques-André Mallet (); also Mallet-Favre;
23 September 1740 – 31 January 1790)
was a Genevan mathematician

and astronomer.

In 1772, Mallet established and co-financed the first Geneva Observatory, and served as its director until his death in 1790.
His research primarily concerned occultations,

especially lunar and solar eclipses,

sunspots,
planetary orbits, and the orbits of the moons of Jupiter.

Biography

Early life and education

Jacques-André was born in Geneva to Jean-Robert, a captain in the French Army, and his wife, Dorothée Favre, both of noble lineage.

Jean-Robert was from a branch of the Mallet family of Huguenot merchants and bankers
who had fled from Rouen to Geneva in 1557 to escape growing religious persecution.

Through his mother, Dorothée, Jacques-André was also a direct descendant of the Eidguenot patriot François Favre, who famously clashed (with later support from his son-in-law, Ami Perrin) with the Calvinist rule of Geneva.

Jean-Robert wanted his son to become a career soldier, but a severe burn to the thigh in childhood rendered Jacques-André permanently disabled and unfit for future service. Jacques-André instead pursued an education in science and research, joining the Academy of Geneva in 1755. There, he was a pupil of the mathematician Louis Necker, brother of Jacques. Mallet also studied privately with Le Sage before leaving Geneva in 1760 to study at the University of Basel, where he was a student of Daniel Bernoulli.

He completed his studies in 1762, and journeyed to France and England in 1765, where he became friends with astronomers Jérôme Lalande, John Bevis, who discovered the Crab Nebula, and Nevil Maskelyne, among others. Mallet's first introduction to the science of astronomy came from Lalande and Maskelyne during this trip.

Expedition to Lapland

In April 1768, upon the recommendation of Lalande and Bernoulli, Mallet was invited to Russia by Catherine the Great and the Saint Petersburg Academy of Sciences to prepare to observe the 1769 transit of Venus from , Lapland.
He was accompanied by another Genevan astromer, Jean-Louis Pictet, whose own assignment was along the Umba.
Others who were engaged to observe the transit include Stepan Rumovsky, Christian Mayer, and Christoph Euler, son of the mathematician Leonhard Euler. While in Saint Petersburg, readying themselves for their expedition north, Mallet and Pictet met with many members of high society, including Jean-Baptiste Charpentier and the Baron Stroganov, who, like Mallet, had been a pupil of Necker at the Geneva Academy.

The transit of Venus occurred on 3 June 1769. From 76 points globally, astronomers and navigators, like James Cook in Tahiti, were charged with observing the rare phenomenon.
Unfortunately for Mallet, rain obscured the transit, and he was only able to view its beginning. Despite the weather, he still managed to publish useful data regarding solar parallax, allowing for better estimation of the Earth-Sun distance by him and his colleagues. In December 1776, perhaps owing to his contributions in Lapland, Mallet was awarded honorary membership in the Saint Petersburg Academy of Sciences.

Geneva Observatory

After his return from Russia, Mallet was elected by the bourgeoisie to the Council of Two Hundred of Geneva, the canton's legislative authority. In 1771, he accepted an honorary professorship at the Academy of Geneva, acting as lifetime chairperson of the astronomy department. The same year, Mallet convinced his fellow members of the council to approve construction of a permanent observatory on the casemate of the ''Bastion St-Antoine'',
provided he settle a portion of the financing himself and supply accurate time calculations to the city watchmakers. Although the observatory, Geneva's first, was founded by Mallet in 1772, some sources suggest the project was completed in 1773.

For the structure's design, Mallet had departed from a traditional, utilitarian plan, instead building a single-story octagon, capped with a drum and small dome. To furnish the observatory, Mallet purchased and installed a 10-foot achromatic telescope, likely manufactured by the English optician John Dollond, a high-precision clock from the French ''maître'' Jean-André Lepaute, and a meridian bezel, crafted by Jeremiah Sisson, to better calculate the duration of Earth's rotation for timekeeping.
With his pupils and assistants, including Marc-Auguste Pictet and Jean Trembley