He argued against Ptolemy 's refraction theory for why people perceive the Sun and Moon larger at the horizon than when they are higher in the sky .
He redefined the problem in terms of perceived , rather than real , enlargement .
Arguing against Ptolemy 's refraction theory for why people perceive the Sun and Moon larger at the horizon than when they are higher in the sky , he redefined the problem in terms of perceived , rather than real , enlargement .