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MEETING VENUS C. Sterken, P. P. Aspaas (Eds.) The Journal of Astronomical Data 19, 1, 2013 The Important Role of the Two French Astronomers J.-N. Delisle and J.-J. Lalande in the Choice of Observing Places during the Transits of Venus in 1761 and 1769 Simone Dumont1 and Monique Gros2 1Observatoire de Paris, 61 Avenue de l’Observatoire, 75014, Paris, France 2Observatoire de Paris, Institut d’Astrophysique de Paris – Universit´e Pierre et Marie Curie, 98 bis Boulevard Arago, 75014, Paris, France Abstract. Joseph-Nicolas Delisle, as a member of the Acad´emie Royale des Sci- ences of Paris and professor at the Coll`ege Royal de France, went to England in 1724 to visit Newton and Halley. The latter suggested observations of the transits of Mercury and of Venus in order to obtain the solar parallax. Delisle was also in- terested in the Mercury transits. After a stay of 22 years in Saint Petersburg, on his return to Paris, he distributed avertissements (information bulletins) encouraging all astronomers to observe the same phenomena, like the solar eclipse of 1748. Later, in 1760, Delisle presented an Adresse to the King and to the Acad´emie in which he detailed his method to observe the 1761 transit of Venus. This was accompanied by a mappemonde showing the best places for observations. Copies of the text, together with 200 maps, were sent to his numerous correspondents in France and abroad. Following the advanced age and finally death of Delisle, his assistant and successor Joseph-J´erˆome Lalande presented a m´emoire related to the 1769 transit of Venus and an improved map of the best observing places. We detail the role of Delisle and Lalande in the preparation of the international collaboration related to these two transits. 1. Joseph-Nicolas Delisle and Joseph-J´erˆome Lalande, two activist- astronomers? Joseph Nicolas Delisle (or de L’Isle) (1688–1768) was born in Paris; being very gifted in Astronomy, he was admitted in 1714 as the student of Giacomo Filippo Maraldi (1669–1729) at the Acad´emie Royale des Sciences. Two years later he was appointed a member of the Academy, first as an adjunct (1716), and then as an associated member (1719, see Fig. 1). He was known as Delisle le cadet since his older brother, Guillaume (1675–1726) was also a member of the Academy. In 1719 he succeeded the late Philippe de La Hire (1640–1718) as professor of mathematics at the Coll`ege Royal. In 1717 Tsar Peter the Great (1672–1725) came to France and visited the Royal Observatory where he met Delisle. In 1721 the Tsar invited Delisle to come and teach astronomy and geography in Russia. After his death, his wife and successor Tsarina Catherine I (1684–1727) maintained his invitation. From 1726 to 1747 Delisle developed the teaching and practice of both astronomy and geography at the Russian Academy of Sciences in Saint Petersburg. He equipped an observatory and began the measurement of a meridian. His younger brother Louis (1687–1741), known as Louis de la Croy`ere, also a member of the Paris Academy, came with him to stay in Russia. Louis de la Croy`ere took part in the exploration of the Northern part of the Pacific Ocean by the Russian navy. 131 132 Dumont & Gros Figure 1. Old Joseph Delisle. Engraving by Westermeyer (1765–1834). Biblioth`eque de l’Observatoire de Paris. On his return to Paris in 1747, Delisle was appointed to the chair of mathematics at the Coll`ege Royal again; he was appointed astronomer of the French royal navy – astronome de la Marine – and set up an observatory in the Hˆotel de Cluny (nowadays the Museum of the Middle Ages in Paris). During his stay in Russia Delisle had created a large network of correspondents, over Europe and among the Jesuit missionaries in China and India. He had supplied numerous astronomers with Avertissements (information bulletins) encouraging them to observe the same phenomena: the first one concerned the solar eclipse of July 25, 1748, the second was an exhortation from Nicolas-Louis de La Caille (1713–1762), asking astronomers to observe the Moon, Mars and Venus in conjunction with him being stationed at the Cape of Good Hope (1751–1752). Joseph J´erˆome Lefran¸cois (1732–1807), born in Bourg-en-Bresse on July 11, after studying in Jesuit schools first in Bourg and then in Lyon, arrived in Paris in the autumn of 1748 to study law. His name was then changed to Lefran¸cois de Lalande, which soon became Delalande or Lalande when he reached fame (Fig. 2). By chance he got accommodation with a procureur (royal prosecutor) at the Hˆotel de Cluny where he was happy to have a look at the observatory and to meet Delisle. In Bourg, while he was a young boy, he had admired the comet of 1744; later, when he was a pupil at the college of Lyon, father B´eraud (1702–1774) had shown him the solar eclipse of July 25, 1748. So, while studying law to satisfy his parents, Lalande had the opportunity to observe and learn about astronomical facts with Delisle; the following year he studied with Pierre Charles Lemonnier (1715–1799), also professor at the Coll`ege Royal; in 1751, Lemonnier provided him with an astronomical mission to Berlin to observe the Moon in conjunction with La Caille, who was then, as already mentioned, at the Cape of Good Hope. On his return to Paris with some good observations, Lalande was admitted to the Academy (1753). From his own observations of the Moon and those sent to the Academy by La Caille, Lalande deduced the parallax of the Moon. Later, in 1758, The Role of Delisle and Lalande in Choosing Observing Places 133 Figure 2. Lalande, 1783, work by Gatteaux (1751–1832). Biblioth`eque de l’Observatoire de Paris. following a request of Alexis Clairaut (1713–1765), with the help of Mme Lepaute (1723–1788), he calculated the perturbations induced by Saturn and Jupiter on the orbit of the comet now known as Halley’s comet, and, as a consequence, he obtained a more precise time of perihelion passage; thanks to his successful results, Lalande gained the reputation as an able astronomer, both as an observer and as a calculator. In 1760, 72-year old Delisle appointed his young friend Lalande his “survivancier” in his teaching at the Coll`ege Royal. Lalande thus lectured here, as Delisle did before him, and, when Delisle passed away in 1768, he became the successor to his chair of mathematics that became, in 1774, a chair in astronomy. Lalande stayed at the Coll`ege de France until his death. Like Delisle, Lalande developed a large network of correspondents and exerted a marked influence on the international community of astronomers. 2. Delisle and the transits of planets over the solar disc Assiduously observing, Delisle was much interested in important astronomical events, like the transit of Mercury over the solar disc of 9 November 1723 and the total solar eclipse of 22 May 1724, both of which he observed at the Royal Observatory in Paris. This same year, to prepare for his mission to Russia, Delisle went to London with a member of the Acad´emie Fran¸caise. There, he visited Isaac Newton (1643–1727), who gave him his portrait, and the Astronomer Royal Edmond Halley (1656–1742). In 1677, Halley had proposed a method to evaluate the solar parallax. First he proposed to use the observations of the Mercury transits over the Sun’s disc, then the Venus transits predicted to occur later, in 1761 and 1769. Halley developed his method while, on the Island of Saint-Helena, he attended and observed a transit of Mercury; his method consisted in recording the local time of both the second and third contacts of the transit from two observing places with different latitudes. This information was supposed to infer the distance Earth–Mercury, and, by using the 134 Dumont & Gros third law of Kepler (1571–1630), the distance Earth–Sun. However, in the case of Mercury, the precision proved to be insufficient; for this reason, Halley recommended to observe the 1761 transit of Venus over the solar disc from two specific places: the Hudson Bay and some site in India. Delisle, who had already observed a transit of Mercury, is likely to have discussed this subject with Halley during his visit. Be that as it may, when Delisle returned to Paris, he was a Newtonian natural philosophy enthusiast and still a strong supporter of observations of the Mercury transits, the next transits of Venus being in a so remote future, i.e., more than forty years ahead! Delisle later, while in Russia, embarked upon an expedition to Siberia to observe the Mercury transit of May 2, 1740, from Beresov on the banks of the River Ob near Tobolsk, – after asking some of his correspondents to observe the Moon in con- junction with him. Delisle proceeded through Russia as quickly as possible to reach his destination on the Ob. Unfortunately, the transit day was cloudy, and his cor- respondent Johann Wilhelm Wagner (1681–1745), wrote to him that it had rained in Berlin during the whole transit. By the end of August/beginning of September Delisle returned to Saint Petersburg; he soon sent advice to astronomers in vari- ous countries including a letter to Jacques Cassini (1677–1756) for improving the observation of the next transit of Mercury, predicted to occur on November 4, 1743. Back in Paris in 1747, Delisle suggested a method to obtain the solar parallax that differed from Halley’s; one needed to record the exact time of just one single inner contact of the planet with the Sun from two different observing places, the latitude and longitude of which must be precisely known.

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