 The first stage of building of the tower , as recorded by Leo XIII in his <unk> <unk> <unk> <unk> of 1891 , is credited to Pope Gregory XIII , Pope from <unk> to <unk> . The directive was to build a tower at a suitable location in the Vatican and equip it with the " greatest and best instruments of the time " . The design was effected after a series of meetings of the experts who had been appointed to reform the Julian calendar , in use since 45 BC , to verify their proposed reforms . <unk> . Christoph <unk> , a Jesuit mathematician from the Roman College , was the expert on the committee who suggested the new system for the observations . The 73 metres ( 240 ft ) tower was then built above the museum and library , flanked by the <unk> and della <unk> courtyards . The instrumentation for the observation of the sun rays falling over it consisted of a meridian line designed by <unk> <unk> of Perugia . It was in the form of a circular marble plate in the centre , embellished with scientific designs . The tower still remains today , but has undergone improvements over the centuries . 
