Jérôme Lalande measuring the distance to the Moon

Jérôme Lalande

Jérôme Lalande

On July 11, 1732, French astronomer, freemason and writer Jérôme Lalande was born. Lalande is best known for having determined the Moon’s parallax from Berlin for the French Academy in 1751. His planetary tables, into which he introduced corrections for mutual perturbations, were the best available up to the end of the 18th century.

Jérôme Lalande first studied at the Jesuit College in Lyon and later went to Paris to study law. While studying in Paris, Lalande lived at the Hotel de Cluny in Paris, where Joseph-Nicolas Delisle, the French astronomer and cartographer, had an observatory. Despite keeping up with his law studies, Lalande became also interested in astronomy and began to attend Delisle’s astronomy lectures at the Collège Royale and Pierre Lemonnier’s lectures on mathematical physics.

Around 1750 Jérôme Lalande completed his law studies and planned on returning to his home town to practise law. However, Nicolas de Lacaille was sent by the Académie des Sciences to make observations to determine the distances to the Moon and Mars. Lemonnier was made responsible for the observations to be made in Berlin, but he asked Lalande to take his place. While in Berlin, Jérôme Lalande was admitted to the Prussian Academy. He further was able to debate with Voltaire, Maupertuis and Euler. Lalande’s scientific outcomes were published in Acta eruditorum as well as through the Berlin Academy and the Paris Academy. In 1753, Jérôme Lalande was elected to the Académie des Sciences in Paris.

To determine the lunar parallax, Lalande had to calculate some assumptions first. For instance, it was important to figure out the amount of which the Earth differed from a sphere as it is flattened at the poles. Lalande and Lemonnier disagreed on the problem and the Paris Academy set up a commission, settling for Lalande’s version. Further, a more accurate orbit for Halley’s comet had to be calculated, a task in which Lalande assisted Clairaut. Next to Jérôme Lalande, also the French astronomer and mathematician Nicole Reine Lepaute contributed to the calculations significantly. By fall 1757, they figured out that Halley’s comet would probably return a year earlier than previously expected.

Between 1760 and 1776 as well as between 1794 and 1807, Jérôme Lalande functioned as editor of the astronomical almanac Connaissance des temps. He added precice tables of lunar distances from stars, especially valuable for navigation. In 1762, Lalande was appointed professor of astronomy at the Collège Royale, a post he held for 46 years. In 1791, he was elected Head of the Collège de France where one of his first acts was to admit women to all classes. Four years later, Jérôme Lalande was appointed director of the Paris Observatory.

At yovisto you can learn more about “Our nearest neighbour, the Moon” in a lecture by Professor Carolin Crawford at Gresham College.

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