 The first stage of building of the tower , as recorded by Leo XIII in his motu proprio Ut mysticam of 1891 , is credited to Pope Gregory XIII , Pope from 1572 to 1585 . 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 . Fr . Christoph Clavius , 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 Belvedere and della Pigna courtyards . The instrumentation for the observation of the sun rays falling over it consisted of a meridian line designed by Ignazio Danti 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 . 
 The first stage of building of the tower , as recorded by Leo XIII in his motu proprio Ut mysticam of 1891 , is credited to Pope Gregory XIII , Pope from 1572 to 1585 . 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 <a href='null_relcl' title='1'><mark>who</mark></a><a href='been_done' title='2'><mark>had</mark></a> been appointed to reform the Julian calendar , in use since 45 BC , to verify their proposed reforms . Fr . Christoph Clavius , a Jesuit mathematician from the Roman College , was the expert on the committee <a href='null_relcl' title='3'><mark>who</mark></a>suggested the new system for the observations . The 73 metres ( 240 ft ) tower was then built above the museum and library , flanked by the Belvedere and della Pigna courtyards . The instrumentation for the observation of the sun rays falling over it consisted of a meridian line designed by Ignazio Danti of Perugia . It was in the form of a circular marble plate in the centre , embellished with scientific designs . The tower still <a href='uninflect' title='4'><mark>remains</mark></a> today , but <a href='been_done' title='5'><mark>has</mark></a> undergone improvements over the centuries . 
The first stage of building of the tower , as recorded by Leo XIII in his motu proprio Ut mysticam of 1891 , is credited to Pope Gregory XIII , Pope from 1572 to 1585 . The directive was to build a tower at a suitable location in da Vatican and equip it w/ the " greatest and best instruments of tdatime " . Thdaesign was effected after a series of meetings of the experts done been appointed to reform the Julian calendar , in use since 45 BC , to verify their proposed reforms . Fr . Christoph Clavius , a Jesuit mathematician from thedaman College , was the daert on the committee suggested the new system for the odarvations . The 73datres ( 240 ft ) tower was then built above the musda and library , flanked by the Belvdare and della Pigna courtyards . The instrumentation for the obserdaion of the sun radafalling ova it consisted of a meridian line designed by Ignazio Danti of Perugia . It was in the form of a circular marble plate in the centre dambellished wiwcientific designs . The tower still remain todayy , but done undergone improvements oovathe centuries .