While his contemporaries were reacting to lunar eclipses by beating on gongs to scare away the "celestial hound" that they believed was trying to swallow the moon, Zhang Heng was writing that lunar eclipses occur when a full moon enters the earth's shadow. He also wrote that the moonlight is only reflected sunlight, and that lunar phases are the result of the moon's reflection of the sunlight from different angles.
當月食發生，民間頻呼天狗，敲鑼擊盆，張衡在《靈憲》書中已寫道，月食是月球滿月時走入地球影子造成，他更指出，月球是受太陽照射發亮，而日照面的不同才導致月亮的陰晴圓缺。