Jean de Fontaney (sometimes Fonteney), French priest, mathematician-astronomer and educationist, joined the order of Jesuits in October 1658 and received his education within the order. In 1676 he was appointed professor of mathematics at the Jesuit College de Clermont (later Louis le Grand) in Paris, a position he held for eight years. In 1678, with the astronomers G.D. Cassini, J. Picard, O. Roemer and de la Hire, he observed an occultation of the planet Saturn by the moon. Their observations were published in the Journal des Savants and the Memoires de l'Academie des Sciences. In 1674 he edited a collectional of astronomical maps, Le planisphere ou globe celeste by I.G. Pardies. His observations of comets, made at the College de Clermont during 1680 and 1681, were published in Observations sur la comete de l'annee MDCLXXX et MDCLXXXI (Paris, 1681, 105p). He became a member of the Academie Royale des Sciences in 1684 and that same year published his observations of the solar eclipse of 12 July 1684 in the academy's Memoires.
In 1685 King Louis XIV, in collaboration with the Academie Royale des Sciences, sent de Fontaney to the Far East as the leader of a party of six Jesuit priests, in the company of the Chevalier de Chaumont, ambassador to the King of Siam (now Thailand). The other five priests were Fathers Guy Tachard*, Francois Gerbillon, Joachim Bouvet, Louis La Comte, and Claude de Visdelou. Described as royal mathematicians, their purpose, in addition to missionary work, was to determine the longitude and variation of the compass of places visited, correct existing maps and navigational instructions, collect scientific knowledge, and acquire interesting books for the king's library. De Fontaney in particular was to study Chinese astronomy and geography and make astronomical observations. Several of them, but particularly de Fontanay, were skilled in astronomy. They were equipped with memoranda on various subjects and the best scientific instruments of the time, provided by the Academie des Sciences. The instruments included a pendulum clock with a period of one second, a quadrant to measure the height of the sun, a telescope with a length of 12 French feet (3.9 m), thermometers, microscopes, barometers, astronomical tables and sea charts. The expedition arrived in Table Bay on 31 May 1685 and stayed to 7 June, a period which coincided with a visit to the Cape by Commissioner H.A. van Reede*. De Fontaney and Tachard were well received by van Reede and Governor Simon van der Stel* and allowed to set up a temporary observatory in a pavilion in the Dutch East India Company's garden, a site where Government House was later built.
The expedition's astronomical observations at the Cape were aimed mainly at a more accurate determination of its longitude. Observations started on the night of 2 June, when the beginning of a transit of Jupiter's brightest satelite was timed. Observations of the sun's altitude during the morning and afternoon of 3 and 4 June provided the relation between their clock time and local solar time. On the night of 4 June they timed a re-appearance of Jupiter's brightest satelite and used it to derive their longitude. The standard times (of Paris) at which Jupiter's four brightest satelittes started or ended a transit across the planet, an occultation by the planet, or an eclipse by its shadow, had been tabulated by French astronomers. Hence the difference between the standard time and local time of such an event (found to be 1h 12m) provided an estimate of the longitude of the Cape relative to Paris. The result indicated that the Cape was three degrees further west in longitude than shown on some sea charts. However, it turned out later that the method lacked precision and despite the care with which the observations were made the final result still placed the Cape almost two degrees too far east.
Some general observations were also made. Jupiter's two equatorial belts were clearly seen; the double star Alpha Crucis, which was known to be composed of two closely separated stars, was found to have a third, much fainter star associated with it; the milky way near the southern cross was seen to be made up of numerous faint stars; the clouds of Magellan could not be resolved into individual stars; and the region near the celestial south pole was found to contain more faint stars than had so far been catalogued. These constituted the first significant telescopic observations made from South Africa. The party furthermore determined the magnetic declination at the Cape to be 11 degrees west of north. On June 5 de Fontaney calculated the height of Table Mountain to be 4704 Rhineland feet (1480 m), which is, however, much too high. Though Tachard has usually been credited with the expedition's astronomical work, de Fontaney's career, publications and duties indicate that he was mainly responsible. An account by him of the observations made at the Cape to determine its longitude and the observations of the Milky Way were eventually published under the title "Observations faites au Cap de Bonne-Esperance en 1685..." in the Histoire de l'Academie Royale des Sciences in 1733.
De Fontaney observed a total solar eclipse in Siam later in 1685. In July 1687 he arrived in China, where he remained for many years. He returned to France in 1699, but two years later left for China once again with new Jesuit missionaries. Returning to Europe via London in 1704, he became vice-rector of the Paris novitiate in 1706, and rector of the Jesuit college at La Fleche in December 1707. Many of his astronomical and geographical observations were published in Observations physiques et mathematiques envoyees de Siam... (Paris, 1688, 1692), edited by T. Gouye, and in various other books and scientific journals.