 Different lists of climata were in use in Hellenistic and Roman time.Claudius Ptolemy was the first ancient scientist known to have devised the so-called system of seven climes (Almagest 2.12) which, due to his authority, became one of the canonical elements of late antique, medieval European and Arab geography.In Medieval Europe, the climes for 15 and 18 hours were used to calculate the changing length of daylight through the year.