As the law of gravitation, this law was suggested in 1645 by Ismaël Bullialdus. But Bullialdus did not accept Kepler's second and third laws, nor did he appreciate Christiaan Huygens's solution for circular motion (motion in a straight line pulled aside by the central force). Indeed, Bullialdus maintained the sun's force was attractive at aphelion and repulsive at perihelion. Robert Hooke and Giovanni Alfonso Borelli both expounded gravitation in 1666 as an attractive force.Hooke's gravitation was also not yet universal, though it approached universality more closely than previous hypotheses. See page 239 in: Hooke's lecture "On gravity" was at the Royal Society, in London, on 21 March.; see especially pages 70–72. Borelli's "Theory of the Planets" was published later in 1666. Hooke's 1670 Gresham lecture explained that gravitation applied to "all celestiall bodys" and added the principles that the gravitating power decreases with distance and that in the absence of any such power bodies move in straight lines. By 1679, Hooke thought gravitation had inverse square dependence and communicated this in a letter to Isaac Newton: my supposition is that the attraction always is in duplicate proportion to the distance from the center reciprocall.Hooke's letter to Newton of 6 January 1680 (Koyré 1952:332).
aS the Law of grAVitAtiOn, tHIs LAW waS sUGgEStED in 1645 by isMaël bUllIaldus. bUt bulLiALdUs did nOt AccePt kePlEr's second aNd thirD laws, noR did he apprecIATe chRIstiaan HUYgeNs'S SolutIoN FOr CiRculAR motion (MOTIoN in A sTraIGhT LIne puLLeD ASide BY thE CeNtral FoRCe). iNdeED, BUllialdUs maIntAined thE sun's forcE WaS AttracTiVe at aPHelIon ANd rEPuLsivE at periHElion. robeRt hooke aNd gIovanNi ALfonsO borELli both expoUnDed GraViTAtioN IN 1666 as an attrActive force.hOOkE's gRavitaTion WaS also not yet UnIversal, thoUgH It AppRoacheD univErSality MOre closely thAn preVioUs HyPotheSes. see pAgE 239 in: Hooke's LecturE "on gRaviTY" was at The royal society, IN lonDOn, oN 21 mARch.; see espEcIally pages 70–72. borEllI's "THeoRy oF ThE pLANEtS" Was PUbLIsHed lAteR IN 1666. HoOke'S 1670 gReshAM lEcture explaIned thAt gRavitatIoN apPlIeD To "aLL celestiaLl boDys" aNd Added THE pRINcipLEs THat ThE GRavitAtIng pOWeR deCreases With dIsTaNCe anD that in the abSEnce of AnY SucH pOwEr bodies moVe IN sTraigHt liNEs. BY 1679, Hooke ThouGhT graVitatIoN haD iNVeRse sQUaRE dEpendEnCe And communicAtEd THis iN A LeTTer tO isaaC NewtoN: my suPpoSItIon is that tHE attRActIon Always Is IN duplicATe ProPortioN to thE diSTAnCE froM the ceNter reciprocaLl.hoOKe's LeTter to newTon Of 6 january 1680 (koyRé 1952:332).