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Reply from the Observatory of Cambridge
Consequently,theweightofashotwilldecrease,andwillbecomereducedtozeroattheinstantthattheattractionofthemoonexactlycounterpoisesthatoftheearth;thatistosayat4752ofitspassage.Atthatinstanttheprojectilewillhavenoweightwhatever;and,ifitpassesthatpoint,itwillfallintothemoonbythesoleeffectofthelunarattraction.Thetheoreticalpossibilityoftheexperimentisthereforeabsolutelydemonstrated;itssuccessmustdependuponthepoweroftheengineemployed.
Astothesecondquestion,"Whatistheexactdistancewhichseparatestheearthfromitssatellite?"
Answer.—Themoondoesnotdescribeacircleroundtheearth,butratheranellipse,ofwhichourearthoccupiesoneofthefoci;theconsequence,therefore,is,thatatcertaintimesitapproachesnearerto,andatothersitrecedesfartherfrom,theearth;inastronomicallanguage,itisatonetimeinapogee,atanotherinperigee.Nowthedifferencebetweenitsgreatestanditsleastdistanceistooconsiderabletobeleftoutofconsideration.Inpointoffact,initsapogeethemoonis247,552miles,andinitsperigee,218,657milesonlydistant;afactwhichmakesadifferenceof28,895miles,ormorethanone-ninthoftheentiredistance.Theperigeedistance,therefore,isthatwhichoughttoserveasthebasisofallcalculations.
Tothethirdquestion.
Answer.
