Joseph Jerome le Francois de Lalande


French astronomer who observed the transit of Venus and helped to calculate the size of the Solar System. He compiled a catalogue of 47,000 stars.
Lalande was born in Bourg-en-Bresse, near Lyon, and studied in Paris. He was appointed professor at the Collège de France in 1762 and during his tenure there he published Treatise of Astronomy 1764. In 1795 he was made director of the Paris Observatory.
In collaboration with French astronomer Nicolas de Lacaille at the Cape of Good Hope, South Africa, in 1751, Lalande measured the lunar parallax and thus the distance from the Earth to the Moon.
Two transits of Venus, in 1761 and 1769, offered the chance to establish accurately the size of the Solar System. Such transits occur twice within a period of eight years only every 113 years. During the transit, which takes approximately five hours, Venus can be seen silhouetted across the face of the Sun; the distance of the Earth from the Sun can be deduced by measuring the different times that the planet takes to cross the face of the Sun when seen from different latitudes on Earth. Lalande was responsible for coordinating expeditions to all corners of the world and collecting the results of observations.

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