 Prior to Galileo 's construction of his version of a telescope , Thomas Harriot , an English mathematician and explorer , had already used what he dubbed a " perspective tube " to observe the moon . Reporting his observations , Harriot noted only " strange <unk> " in the waning of the crescent , but was ignorant to the cause . Galileo , due in part to his artistic training and the knowledge of chiaroscuro , had understood the patterns of light and shadow were in fact topographical markers . While not being the only one to observe the moon through a telescope , Galileo was the first to deduce the cause of the uneven waning as light occlusion from lunar mountains and craters . In his study he also made topographical charts , estimating the heights of the mountains . The moon was not what was long thought to have been a translucent and perfect sphere , as Aristotle claimed , and hardly the first " planet " , an " eternal pearl to magnificently ascend into the heavenly <unk> " , as put forth by Dante . 
