French astronomer best known for his efforts to determine the astronomical unit—the distance between the Sun and Earth—by observing transits of Venus across the Sun. The astronomical unit is crucial for determining the solar system size. Edmond Halley first proposed a method for determining this value from Venus transits. Delisle simplified Halley's method and produced a mappemonde indicating favorable observation sites for the 1761 and 1769 transits. He distributed the map and instructions through worldwide correspondence.