Complex sentence: The theory of impetus was an auxiliary or secondary theory of Aristotelian dynamics, introduced initially in order just to explain projectile motion against gravity, first by Hipparchus in antiquity and subsequently by Philoponus in the 6th century AD.
Split into simpler sentences: The theory of impetus was an auxiliary or secondary theory of Aristotelian physics put forth initially to explain projectile motion against gravity. It was first introduced by Hipparchus in antiquity, and subsequently by Philoponus in the 6th century AD.