The Gregorian Tower () or Tower of the Winds () is a square tower and early modern observatory located above the Gallery of Maps, which connects the Villa Belvedere with the Apostolic Palace in Vatican City. The tower was built between 1578 and 1580 to a design by the Bolognese architect Ottaviano Mascherino (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 1582. It was then also known as the Tower of Winds. The tower was also called "Specola Astronomica Vaticana", a reference to 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.
Early history
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 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.
Second stage
The second stage of construction in the 17th and 18th centuries, when the tower was under the charge of the Vatican librarian, involved Mgr. Filippo Luigi Gilii, a clergyman of St. Peter's Basilica. Earlier in 1797, Pius VI gave approval to placing a Latin inscription Specula Vaticana at the entrance to the upper part of the tower, which was implemented by Cardinal Zelada with plans to enhance the instrumentation system in the tower's observatory. The original observatory was then set up above the second level of the tower with the agreement of Pope Pius VI. Its instrumentation, apart from many normal devices (such as meteorological and magnetic equipment, with a seismograph and a small transit and pendulum clock,) was noted for the Dolland Telescope. The instrumentation facilitated recording of occurrences of eclipse, appearance of comets, Satellites of Jupiter and Mercury’s transit. As an addition, under the patronage of Pope Pius X, four rotary observatory domes were also added at strategic locations on the long fortification walls, more than a thousand years old. Mgr. Gilii, highly respected as a polyglot with a knowledge of physics, biology, archeology and the Hebrew language, was in charge of the observatory from 1800 to 1821. He carried out continuous meteorological observations (twice a day at 6 AM and 2 PM) conforming to the programme of the Mannheim Meteorological Society. While the observation records for seven years were published, the balance data in a manuscript form was preserved in the Vatican Library. Subsequent to Gilii's death in 1821, the observatory on the tower was discontinued and the instruments were moved to the observatory at the Roman College. Established in 1787, it was considered more suitable for making observations than the Vatican.