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It was Galileo who first accurately described projectile motion.
He showed that it could be understood by analyzing the horizontal
and vertical components separately. No one had done this prior to Galileo.

This illustration reflects the general opinion before Galileo which followed largely Aristotelian lines but incorporating a later theory of "impetus" -- which maintained that an object shot from a cannon, for example, followed a straight line until it "lost its impetus," at which point it fell abruptly to the ground.