Profession: astronomer

The astronomer-mathematician Fatio, originator of a mechanistic theory of gravity, is now known primarily as an associate of others. At 18 he introduced himself to Cassini with an account of Saturn’s rings, subsequently studying under him in Paris. Going to the Netherlands to warn William of Orange of a plot against him, he worked with Huygens, then settling in London became Newton’s very close friend (exciting some latter-day speculation as to its nature, Newton seemingly suffering a breakdown when the relationship ended). He was friendly with Locke and corresponded widely, including with Leibniz, whom he charged with ‘stealing’ Newton’s invention of calculus.

Notable for much more than his comet, the prodigy Halley started observing with Flamsteed even before the Royal Observatory was built. His catalogue of Southern stars made his name aged 22, though Flamsteed – never hurried, and later his professional enemy – criticised his hasty methods. Cassini, met in Paris, advised him on observing the transit of Mercury. Halley discussed the shape of an orbit with Wren and Hooke, but beaten by the maths, asked Newton for the answer. He knew Pepys and Arbuthnot well, met Leibniz en route to Vienna, and travelled to Danzig to resolve Hevelius and Hooke’s acrimonious dispute.

Lexell, who spent most of his career in Russia, is known for his work on cometary orbits and on spherical geometry. He arrived in St Petersburg when its eminence grise, Euler, was over 60 and practically blind. He helped Euler in his work as one of his disciples, became his friend and chosen successor, and was present when he died (Lexell himself however dying young, barely a year later). Linnaeus was one of his correspondents. Although Lexell wrote at least once to William Herschel, the discoverer of Uranus, confirming its status as a planet and calculating its orbit, the extent of any further contact is not clear.