From Antiquity to the 16 th century, instruments for astronomical observation underwent minimal changes. Apart from some perfecting touches devised by individual astronomers, they consisted of revolving frames fitted with graduate scales and appropriate sights. These instruments, which equipped the major Islamic and European observatories, allowed the positions of the heavenly bodies to be precisely determined. Using quadrants, sextants, rings and rules of various kinds and sizes, the astronomer carried out his work, which consisted of updating the numerical parameters that governed the motion of the stars.

In this scientific environment, apart from the general structure of the Universe in which an astronomer could believe, the heavens were considered to have been fully explored. At the start of the 17 th century it was thought that all of the existing planets were known and all of the fixed stars had been identified and catalogued. Not even the most attentive and systematic observation of the sky would ever have revealed anything new, apart from some sporadic comet or nova. No one could ever have imagined what wondrous new things were about to be revealed by an instrument created by inserting two eyeglass lenses into the ends of a tube.