Inancienttimes,manypeoplebelievedtheearthwasaflatdisc.Wellover2,000yearsago;theancientGreekphilosopherswereabletoputforwardtwogoodargumentsprovingthatitwasnot.Directobservationsofheavenlybodieswerethebasisofboththesearguments.First,theGreeksknewthatduringeclipsesofthemoontheearthwasbetweenthesunandthemoon,andtheysawthatduringtheseeclipses,theearth'sshadowonthemoonwasalwaysround,theyrealizedthatthiscouldbetrueonlyiftheearthwasspherical,Iftheearthwasaflatdisc,thenitsshadowduringeclipseswouldnotbeaperfectcircle;itwouldbestretchedoutintoalongellipse.ThesecondargumentwasbasedonwhattheGreekssawduringtheirtravels.TheynoticedthattheNorthStar,orPolaris,appearedlowerintheskywhentheytraveledsouth,inthemorenortherlyregions,theNorthStarappearedtothemtobemuchhigherinthesky.Bytheway,itwasalsofromthisdifferenceintheapparentpositionoftheNorthStarthattheGreeksfirstcalculatedtheapproximatedistancearoundthecircumferenceoftheearth,afigurerecordedinancientdocumentssays400.000stadium,that'sthepluraloftheworldstadium.Today,it'snotknownexactlywhatlengthonestadiumrepresents,butlet'ssayitwasabout200meters,thelengthofmanyathleticstadiums.ThiswouldmaketheGreeksestimateabouttwicethefigureacceptedtoday,averygoodestimateforthosewritingsolongbeforeeventhefirsttelescopewasinvented.