▁the ▁five ▁known ▁sat ell ites ▁of ▁Sat urn ▁enabled ▁Hal ley ▁to ▁correct ▁their ▁movements ; ▁and ▁Newton ▁employed , ▁in ▁the ▁third ▁edition ▁of ▁the ▁Princi p ia , ▁his ▁mic rom et rical ▁measures ▁of ▁Jup iter ' s ▁disc , ▁of ▁Sat urn ' s ▁disc ▁and ▁ring , ▁and ▁of ▁the ▁el ong ations ▁of ▁their ▁sat ell ites ; ▁and ▁obtained ▁from ▁him ▁data ▁for ▁correct ing ▁the ▁places ▁of ▁the ▁com et ▁of ▁ 1 6 8 0 . ▁Lap lace ▁also ▁used ▁P ound ' s ▁observations ▁of ▁Jup iter ' s ▁sat ell ites ▁for ▁the ▁determ ination ▁of ▁the ▁planet ' s ▁mass ; ▁and ▁P ound ▁himself ▁compiled ▁in ▁ 1 7 1 9 ▁a ▁set ▁of ▁tables ▁for ▁the ▁first ▁satellite , ▁into ▁which ▁he ▁introduced ▁an ▁equation ▁for ▁the ▁transmission ▁of ▁light . ▁ ▁P ound ▁trained ▁his ▁sister ' s ▁son , ▁James ▁Brad ley , ▁and ▁many ▁of ▁their ▁observations ▁were ▁made ▁together , ▁including ▁the ▁opposition ▁of ▁Mars ▁in ▁ 1 7 1 9 , ▁and ▁the ▁trans it ▁of ▁Mercur y ▁on ▁ 2 9 ▁October ▁ 1 7 2 3 . ▁Their ▁measurement ▁of ▁ γ ▁Virgin is ▁in ▁ 1 7 1 8 ▁was ▁the ▁first ▁made ▁of ▁the ▁components ▁of ▁a ▁double ▁star ▁and ▁was ▁directed ▁towards ▁the ▁determ ination ▁of ▁stell ar ▁par alla x . ▁ ▁In ▁ 1 7 2 7 , ▁Brad ley ▁emb ark ed ▁upon ▁a ▁series