▁Mun ja ala . ▁ ▁The ▁discovery ▁of ▁pre cess ion ▁is ▁attributed ▁to ▁Hi pp arch us ▁around ▁ 1 3 0 ▁BC . ▁P to le my ▁quotes ▁from ▁Hi pp arch us ' ▁now ▁lost ▁work ▁entitled ▁" On ▁the ▁Dis p lacement ▁of ▁the ▁Sol st it ial ▁and ▁Equ ino ct ial ▁Point s " ▁in ▁the ▁sevent h ▁book ▁of ▁his ▁ 2 nd ▁century ▁astronom ical ▁text , ▁Al mag est , ▁where ▁he ▁describes ▁the ▁phenomen on ▁of ▁pre cess ion ▁and ▁estimates ▁its ▁value . ▁P to le my ▁clar ified ▁that ▁the ▁convention ▁of ▁Greek ▁mathematical ▁astronom y ▁was ▁to ▁commence ▁the ▁z od iac ▁from ▁the ▁point ▁of ▁the ▁v ernal ▁equ ino x ▁and ▁to ▁always ▁refer ▁to ▁this ▁point ▁as ▁" the ▁first ▁degree " ▁of ▁A ries . ▁This ▁is ▁known ▁as ▁the ▁" t rop ical ▁z od iac " ▁( from ▁the ▁Greek ▁word ▁tr ó pos , ▁turn ) ▁because ▁its ▁starting ▁point ▁revol ves ▁through ▁the ▁circle ▁of ▁background ▁const ell ations ▁over ▁time . ▁ ▁The ▁principle ▁of ▁the ▁v ernal ▁point ▁acting ▁as ▁the ▁first ▁degree ▁of ▁the ▁z od iac ▁for ▁Greek ▁astronom ers ▁is ▁also ▁described ▁in ▁the ▁ 1 st ▁century ▁BC ▁astronom ical ▁text ▁of ▁Gem inus ▁of ▁Rh odes . ▁Gem inus ▁explains ▁that ▁Greek ▁astronom ers ▁of ▁his ▁era ▁associate ▁the ▁first ▁degrees ▁of ▁the ▁z od iac ▁signs ▁with ▁the ▁two ▁sol st ices ▁and ▁the