 The Gregorian Tower ( Italian : Torre <unk> ) or Tower of the Winds ( Italian : Torre <unk> <unk> ) is a round tower located above the Gallery of Maps , which connects the Villa <unk> with the Apostolic Palace in Vatican City . The tower was built between <unk> and 1580 to a design by the <unk> architect <unk> <unk> ( who was credited with building the Apostolic Palace ) mainly to promote the study of astronomy for the Gregorian Calendar Reform which was commissioned by Pope Gregory XIII and promulgated in <unk> . It was then also known as the Tower of Winds . The tower is now called the " <unk> <unk> <unk> " , the Vatican Observatory . Four stages of progressive development have occurred since it was first established . The tower was an edifice of great value for astronomical observations made using a sundial as they provided essential confirmation of the need to reform the Julian calendar . 
