▁of ▁Western ▁hor os cop ic ▁ast ro log y ▁was ▁the ▁ast ro lo ger ▁and ▁astronom er ▁P to le my , ▁whose ▁work ▁T et rab ib los ▁laid ▁the ▁basis ▁of ▁the ▁Western ▁ast ro log ical ▁tradition . ▁Under ▁the ▁Gree ks , ▁and ▁P to le my ▁in ▁particular , ▁the ▁plan ets , ▁H ouses , ▁and ▁signs ▁of ▁the ▁z od iac ▁were ▁rational ized ▁and ▁their ▁function ▁set ▁down ▁in ▁a ▁way ▁that ▁has ▁changed ▁little ▁to ▁the ▁present ▁day . ▁P to le my ▁lived ▁in ▁the ▁ 2 nd ▁century ▁AD , ▁three ▁centuries ▁after ▁the ▁discovery ▁of ▁the ▁pre cess ion ▁of ▁the ▁equ ino xes ▁by ▁Hi pp arch us ▁around ▁ 1 3 0 ▁BC . ▁Hi pp arch us ' s ▁lost ▁work ▁on ▁pre cess ion ▁never ▁circul ated ▁very ▁widely ▁until ▁it ▁was ▁brought ▁to ▁prom in ence ▁by ▁P to le my , ▁and ▁there ▁are ▁few ▁explan ations ▁of ▁pre cess ion ▁outside ▁the ▁work ▁of ▁P to le my ▁until ▁late ▁Anti qu ity , ▁by ▁which ▁time ▁P to le my ' s ▁influence ▁was ▁widely ▁established . ▁P to le my ▁clearly ▁explained ▁the ▁theoretical ▁basis ▁of ▁the ▁western ▁z od iac ▁as ▁being ▁a ▁tropical ▁coordinate ▁system , ▁by ▁which ▁the ▁z od iac ▁is ▁aligned ▁to ▁the ▁equ ino xes ▁and ▁sol st ices , ▁rather ▁than ▁the ▁visible ▁const ell ations ▁that ▁bear ▁the ▁same ▁names ▁as ▁the