The Vatican is the world’s smallest state both by area and population, coming in at under only 110 acres with a total population of 1000 people. However, its small stature is nothing to laugh at – the Vatican is filled to the brim with historical buildings, monuments, and other cultural works of art. From Saint Peter’s Basilica and the Sistine Chapel, to the Vatican Library or the Vatican Museums, the small nation is rich with culture. The one building most don’t know about however is the Tower of the Winds, which helped create the calendar used by nearly everyone around the globe today.

The Tower of the Winds, also known as the Gregorian Tower, is one of the highest points in the Vatican. It was originally constructed from the years 1578 to 1580 by the Bolognese architect Ottaviano Mascherino. The Torre dei Venti, Italian for Tower of the Winds, gained its moniker after the anemoscope it possesses, an instrument used to determine the direction of the wind. The tower is located above the Gallery of Maps, also constructed by the same architect, which connects the tower to the Apostolic Palace through the Villa Belvedere.

Despite the name of the tower, it was not used as a weather station. Instead, the tower was commissioned by Pope Gregory XIII for the sole purpose of determining just how inaccurate the Julian calendar was – an issue brought to light during the Council of Trent in 1563.

Beginning with the First Council of Nicaea in 325, Easter was always celebrated the first Sunday following the full moon after the Spring Equinox on March 21st. During the Council of Trent and the Counter-Reformation, one major topic was the celebration Easter on the correct date at risk of excommunication. However, it became clear the Julian calendar (the calendar preceding today’s Gregorian calendar) was inaccurate.

The tower consists of two stories both lavishly decorated with frescoes. The lower level is a windowless Meridian room containing a small pinhole on its South side that projects onto a marble meridian on the floor. On the Spring Equinox at noon, a ray of light is supposed to fall in a line on the floor, perfectly illuminating an astronomical constellation built into the marble. When it was first tested in 1582, the ray of light aligned on March 11th, instead of March 21st. This meant the Julian Calendar was 10 days off.

The Julian calendar was then found to be off by roughly two hours each year. After 1600 years of being in use, these repeated hours added up to about 10 days. Pope Gregory XIII created the Gregorian calendar, instituting leap days on any centennial year divisible by 400, e.g. 1200, 1600, 2000 which solved the problem of being two hours off. To eliminate the accumulated 10 days, October 4th of 1582 was followed by October 15th.

During the papacy of Pope Leo XIII, the roof was turned into a terrace and became the first seat of the Vatican Observatory, renamed the Specola Astronomica Vaticana. Today, the Tower of the Winds is considered part of the Vatican Secret Archives and off limits to the general public.

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