In 1678, Newton published in Principia a demonstration that the planets' orbits are ellipses (r₁+r₂=c) with the Sun at one of the foci. 7 years earlier, Cassini proposed that the orbits follow the curve with r₁r₂=c². pic.twitter.com/swl8fbdMbz

— Fermat's Library (@fermatslibrary) December 17, 2020