On a recent vacation trip to Rome I was lucky enough to also visit the Vatican. Our group toured the Vatican Museums, Sistine Chapel, St. Peter’s Basilica, Papal Crypts and St. Peter’s Square. It was an amazing experience to be sure but one tower seen from the Pigna (Pinecone) Courtyard caught my eye. I had read about the “Tower of the Winds” and it’s historical importance in astronomy prior to our trip.

The Tower of the Winds, also known as the Gregorian Tower, is a prominent structure in the Vatican Gardens in Rome. The tower was designed by the famous Jesuit German mathematician and astronomer Father Christopher Clavius and built in the late 16th century. Its primary purpose was to serve as an astronomical observatory for the Vatican, as well as a platform for viewing Rome.

The Tower of the Winds was equipped with several astronomical instruments, including a meridian line, a solar quadrant, and a celestial globe. These instruments allowed astronomers to accurately measure the position of celestial bodies and to determine the time of day with great precision.

The tower’s most significant contribution to astronomy was its use in the reform of the Julian calendar. Over centuries of use the Julian calendar’s issues became evident when astronomical events such as the vernal equinox and winter equinox started to drift away from the actual events. It was clear that the Julian calendar did not accurately account for the time it takes for the Earth to complete an orbit around the Sun.  Even when the Julian calendar was in use leap days were added every four years to try and keep it in sync but it was still inadequate.

In the late 16th century, Pope Gregory XIII commissioned a committee of astronomers to revise the calendar, which had become out of sync with the astronomical year due to errors in the original calculations. Father Clavius was a member of this committee and played a key role in the reform of the calendar. The committee used the Tower of the Winds to make observations and measurements necessary for the new calendar, which became known as the Gregorian calendar.

The Gregorian calendar is still used today and is the standard calendar in most countries. It replaced the Julian calendar, which had been in use since the time of Julius Caesar, some 1600 years. The reform of the calendar was a significant achievement in the history of astronomy and has had a lasting impact on the world.

The Gregorian calendar while more accurate than the Julian is still not without its flaws. Leap days are still necessary but the calculation for adding them is more precise, and with the invention of atomic clocks even leap seconds are added when warranted. Interestingly enough when the big switch was made from the Julian to the Gregorian calendar in 1582 some days just sort of disappeared from the calendar that year. With the Julian calendar’s error known Pope Gregory XIII and the committee of astronomers agreed that to get the new calendar off on the right track it would be advantageous to drop 10 whole days from the calendar. So October 4, 1582 was followed by October 15, 1582. This change was successful in moving the vernal equinox from March 11 back to line up with the actual event on March 21st. It should be noted that not all countries quickly fell into line and switched to the new calendar. In fact, only a handful made the change that year. However, slowly over the next few centuries more countries made the switch to the Gregorian calendar. Turkey was the last in 1927 and with that the majority of the countries in the world  are now using the “new” calendar.

Admittedly, the Gregorian Tower is a modest looking tower that most visitors don’t even bother to notice or see while visiting the Vatican. I wish it was open to the public to tour but it’s not. The tower played a crucial role in the history of astronomy. It served as an astronomical observatory, and its instruments were used in the creation of the Gregorian calendar that we use today. The tower remains an important historical landmark and a testament to the advances made in astronomy during the Renaissance.

Clear Skies,

Scott