Transit of Venus of 1761

Background. A transit of Venus occurs when the planet passes directly between the Sun and the Earth, appearing as a small black circle moving across the face of the Sun. Edmond Halley had proposed in papers of 1691 and 1716 that transit measurements could yield accurate calculations, and Kepler had predicted a transit for 1761, which fell on 6 June N.S. (26 May O.S.). Venus was generally thought to possess an atmosphere before 1761, but detecting it during a transit had apparently not been considered.

Lomonosov's Observation. Mikhail Lomonosov, a Russian polymath and scientist, observed the 1761 transit from a small observatory near his house in St Petersburg, associated with the Imperial Academy of Sciences. He inferred the atmosphere of Venus from an arc of light around the planet at its ingress and egress, correctly attributing the phenomenon to refraction of sunlight in Venus's atmosphere. The effect became known in the literature as "Lomonosov's arc" and has since been observed by many astronomers watching later transits.

Aftermath and Legacy. The atmospheric arc phenomenon bearing Lomonosov's name was further clarified by later transits, notably those of 1874, 1882, 2004, and 2012, together with 18th-century observations by others, including David Rittenhouse in June 1769. Separately, combined 1761 and 1769 transit data allowed Jérôme Lalande in 1771 to calculate the astronomical unit at 153 ± 1 million kilometres.