Guy Tachard, Jesuit scientist and missionary, entered the order in 1668. He taught grammar and rhetoric, and studied mathematics in the fraternity of St Ignace in the province of Guyenne, in south-western France. During 1680-1684 he accompanied Marshall d'Estrées to South America, for a stay of nearly four years. In 1685 he was included in an expedition sent to the East by King Louis XIV of France. The expedition, led by Chevalier de Chaumont, comprised an embassy to the King of Siam (now Thailand) and a mission of six Jesuit priests who would try to reach China from Siam to study its arts and sciences. In addition to Tachard they were Fathers Jean de Fontanay* (the superior), Francois Gerbillon, Joachim Bouvet, Louis Le Comte, and Claude de Visdelou, all scholars of repute. 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. Several of them, but particularly de Fontanay, were skilled in astronomy. They were equipped with memoranda on many subjects and the best scientific instruments of the time, provided by the Académie 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 mainly aimed 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 satellite 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 satelites 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. Though Tachard has usually been credited with the astronomical work, de Fontaney's career, publications and duties during the expedition indicate that he was mainly responsible.
As part of an exchange of presents at their departure the priests presented the governor with a (single lens) microscope and a small burning glass. After spending some time in Siam, Tachard and two other members of the French embassy were sent back to France with a Siamese embassy to request that soldiers be sent to Siam, as well as more Jesuit astronomers who were to set up an observatory there. On their way to France they anchored in Table Bay from 13 to 26 March 1686 to take in supplies. In answer to the request of the king of Siam, Louis XIV sent a party of 14 Jesuit priests (including Father de Bèze*) fitted out by the Académie des Sciences, two diplomats and over 600 soldiers to Siam in 1687. Tachard was a member of this expedition, which travelled in a fleet of six ships and stayed at the Cape from 11 to 27 June to enable more than 300 seriously ill men to recuperate. Tachard visited governor van der Stel and obtained his permission to use the same building as before to make astronomical observations. On 19 and 21 June the Jesuits observed two emersions of Jupiter's brightest satellite, even though the weather was generally cloudy and unfavourable. The observers included Tachard, Father Du Chatz, who fell ill and was left behind, and others. Tachard and Father de Bèze visited van der Stel, who told them about some of the plants found on his travels.
After a stay of three months in Siam Tachard again returned to France with a Siamese embassy, visiting the Cape from 21 April to 1 May 1688. After his first voyage he wrote Voyage de Siam des pères Jesuites, envoyés par le Roy aux Indes & à la Chine... (Paris, 1686), which included an account of their astronomical observations and remarks on the places visited. An English translation, A relation of the voyage to Siam performed by six Jesuits... appeared in 1688. His second voyage was described in Second voyage du Père Tachard et des Jesuites envoyez par le Roy au royaume de Siam... (Paris, 1689). Both works were translated into German, Dutch, Italian and Spanish and various later editions and reprints were issued during the next sixty years. The first book in particular contains much information about the Cape, most of it supplied by Van der Stel, Van Reede and others, for example, an account of the history, flora, fauna and Hottentot cultures of the Cape by Heinrich Claudius*. In his second book Tachard included an account of the shipwreck of the Portuguese ship Nossa Denhora dos Milagros west of Cape Agulhas in April 1686.
In addition to his travel books, Tachard, with various collaborators, was the author of a Latin-French dictionary (1687) and a French-Latin dictionary (1689). After his return to Paris he accompanied some Siamese nobles to Italy to meet Pope Innocent XI. He returned to the East in 1690 as one of the first Catholic evangelists in Bengal, India. Except for a few more adventurous journeys he remained there for the rest of his life. He eventually died in Bengal in 1712 of an infectious disease.
