⚫ Since Hipparchus knew that the basis of his astronomy work were observation records made with reliable observation instruments , he had made numerous observation instruments . Hipparkhos is also said to have established an observatory in Rhodes , although it is not very certain . He used an observation tool called dioptra by Ptolemaios in calculating the time of the stars ' birth and sinking . Hipparchus also invented the plane spherical astrolabe . Hipparchus , who managed to catalog as many as 850 stars during his life and determine their latitude and longitude , is the scholar who also developed the method of classifying stars according to their brightness .