▁coordinate ▁system . ▁According ▁to ▁calculations ▁by ▁modern ▁ast ro physics , ▁the ▁z od iac ▁was ▁introduced ▁between ▁ 4 0 9 - 3 9 8 ▁BC ▁and ▁probably ▁within ▁a ▁very ▁few ▁years ▁of ▁ 4 0 1 ▁BC . ▁Un like ▁modern ▁astronom ers , ▁who ▁place ▁the ▁beginning ▁of ▁the ▁sign ▁of ▁A ries ▁at ▁the ▁place ▁of ▁the ▁Sun ▁at ▁the ▁v ernal ▁equ ino x , ▁Bab yl onian ▁astronom ers ▁fixed ▁the ▁z od iac ▁in ▁relation ▁to ▁stars , ▁placing ▁the ▁beginning ▁of ▁Can cer ▁at ▁the ▁" R ear ▁T win ▁Star " ▁( β ▁Gem in orum ) ▁and ▁the ▁beginning ▁of ▁Aqu arius ▁at ▁the ▁" R ear ▁Star ▁of ▁the ▁Go at - F ish " ▁( δ ▁Cap ric or ni ). ▁ ▁The ▁divisions ▁don ' t ▁correspond ▁exactly ▁to ▁where ▁the ▁const ell ations ▁started ▁and ▁ended ▁in ▁the ▁sky ; ▁this ▁would ▁have ▁resulted ▁in ▁an ▁irregular ▁division . ▁The ▁Sun ▁in ▁fact ▁passed ▁through ▁at ▁least ▁ 1 3 , ▁not ▁ 1 2 ▁Bab yl onian ▁const ell ations . ▁In ▁order ▁to ▁align ▁with ▁the ▁number ▁of ▁months ▁in ▁a ▁year , ▁design ers ▁of ▁the ▁system ▁omitted ▁the ▁major ▁const ell ation ▁O phi uch us . ▁In cluding ▁smaller ▁figures , ▁astronom ers ▁have ▁counted ▁up ▁to ▁ 2 1 ▁el ig ible ▁z od iac ▁const ell ations . ▁Ch anges ▁in ▁the ▁orientation ▁of ▁the ▁Earth ' s ▁axis ▁of