The Gregorian Tower, also called the Tower of the Winds, is a square early modern observatory in Vatican City. It sits above the Gallery of Maps and was built between 1578 and 1580. Ottaviano Mascherino was the architect. Pope Gregory XIII ordered it to support astronomy for the Gregorian calendar reform, promulgated in 1582. The tower is 73 meters tall.
The papal cosmographer Ignazio Danti designed a north–south marble meridian line across the floor of the Meridian Room. It measured the height of the Sun at noon through the seasons. The Jesuit mathematician Christoph Clavius helped suggest the observation system. Sundial observations provided key confirmation that the Julian calendar needed reform.
Simon Lagi and the Bril brothers painted the hall between 1580 and 1582. In 1891 Pope Leo XIII designated a second tower as the seat of the Vatican Observatory.
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