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
(Slightly delayed) #FridayPhysicsFun – Today my husband's car had trouble with the battery. Which made me thankful for relativity theory: the reason lead acid car batteries work in the first place is relativity. pic.twitter.com/vVXwiZ3wlQ