Joseph-Nicolas Delisle was a French astronomer, mathematician and scientist who invented the Delisle scale, and worked to find the distance of the Sun from the Earth by observing transits of Venus and Mercury across the face of the Sun.
Background
Joseph-Nicolas Delisle was born on April 4, 1688, in Paris, France. He was the ninth of eleven children of the historian-geographer Claude Delisle and Nicole-Charlotte Millet de la Croyère. Delisle became known to his contemporaries as Delisle le cadet or le jeune to distinguish him from his two older brothers.
Education
Delisle received his early education from his father, and then began to develop a taste for mathematics near the end of his formal education in rhetoric at the Collège Mazarin. A solar eclipse in 1706 stimulated this new study and led to instruction in the elements of astronomical calculation under Jacques Lieutaud. In 1714 he entered the French Academy of Sciences as pupil of Giacomo Filippo Maraldi.
Delisle was frequenting the Royal Observatory, where he was permitted to copy Jacques Cassini’s unfinished lunar and solar tables. When his first attempt to launch his own observational career in the cupola of the Luxembourg Palace was hampered by a lack of instruments, he turned temporarily to the production of various astronomical tables desired by Cassini.
Having equipped his observatory, Delisle began a regular observational program with the lunar eclipse of January 23, 1712. Forced to abandon his observatory in September 1715 - when the Duc d’Orleans became regent and installed his eldest daughter, the Duchesse de Berry, in the Luxembourg Palace - he resumed his observations at the end of 1716 in Liouville’s former apartment at the Hôtel de Taranne. After almost four years there he moved his instruments to the Royal Observatory and also had some work done in the Luxembourg Palace dome, which he regained in 1722, after the duchess’ death in 1719.
Delisle entered the Academy of Sciences 1714, and he quickly began what was to be a long series of contributions - primarily reports of observations of eclipses and occultations - to its Mémoires. Since it was also necessary for him to earn a livelihood, he gave mathematics lessons and won a small pension under the regency by drawing up astrological forecasts. An appointment to the chair of mathematics at the Collège Royal in 1718 freed him from such endeavors and also brought him students who aided him in the making and reduction of observations. His best-known students during this period were Godin, Grandjean de Fouchy, and his younger brother, Delisle de la Croyère.
Delisle’s growing reputation brought him, in 1721, an offer from Peter the Great to found an observatory and an associated school of astronomy in Russia, an invitation which was transformed into mutually acceptable contractual arrangements in 1725. Planned for four years, Delisle’s stay in Russia lasted twenty-two years. There he created an observatory which came to enjoy a good reputation while training many astronomers - with elementary treatises in the preparation of which Delisle participated. Some of these students, as well as his younger brother and the instrument maker who had accompanied him, subsequently engaged in geodetic and cartographic ventures throughout the country, the results of which they communicated to him for a projected, but unrealized, large-scale and accurate map of Russia. To provide corresponding observations for longitude determinations, Delisle published his St. Petersburg observations of eclipses of Jupiter’s satellites in each of the first six volumes of the Commentarii of Russia’s Imperial Academy of Sciences.
Various physical and meteorological data came to him as well, some of the latter inspired by a “universal thermometer” invented and widely distributed by Delisle. He described this device in a work published in 1738, which also contained his and his brother’s numerous observations of aurora borealis in Russia and a record of his own early Paris observations and experiments on light. Furthermore, Delisle returned to an interest in transits of Mercury first manifested in 1723, when he had considered, but failed to demonstrate, that the technique suggested in Halley’s famous 1716 paper on the use of transits of Venus to determine the parallax of the sun could be equally utilized in the more frequent transits of Mercury. He now treated this possibility for the 1743 transit of Mercury in a letter to Cassini, which the latter placed in the Mémoires of the Paris Academy of Sciences.
That institution took cognizance of the length of Delisle’s absence by naming him to veteran status in 1741. This did not change with his return to Paris in 1747, although he resumed attendance at the Academy’s meetings. He also returned to his Luxembourg Palace observatory to witness a solar eclipse of July 1748, about which he had prepared an avertissement to alert astronomers. In addition he regained his chair at the Collège Royal; most notable among his students in this latter period was Lalande.
One of Delisle’s long-standing activities had been the amassing of vast amounts of geographical and astronomical material through an extraordinarily extensive correspondence, through inheritance, and through laborious copying. Because of its great value the French government purchased this collection by giving Delisle the title of astronome de la marine and a life annuity of 3,000 livres. Moreover, he obtained a new observatory at the Hôtel de Cluny. It was there, in 1759, that his pupil and assistant, Charles Messier, observed the return of Halley’s comet. The place of its reappearance had been the subject of a paper by Delisle in 1757.
In other late works Delisle made some use of his meteorological and cartographic materials from Russia and devoted some attention to longitude determinations. The latter had a significance for transit studies because his perfection of Halley’s technique by a simplification of the necessary observations demanded precise longitude information. Having also corrected Halley’s planetary tables, Delisle produced an avertissement on how to observe the 1753 transit of Mercury and a mappemonde showing the favorable stations. He then determined that the Mercury phenomena were inadequate for parallax determination and concentrated his efforts on the forthcoming transit of Venus in 1761, serving as stimulator and coordinator of its worldwide observation by virtue of an other mappemonde and an accompanying memoir distributed through correspondence.
Delisle retired increasingly after this activity. Lalande began to teach in his stead at the Collège Royal in 1761. In 1763 he withdrew to the abbey of Ste.-Geneviève to devote himself to charitable and religious works, although he did publish several maps by his eldest brother, Guillaume. He died from an attack of apoplexy in 1768.