enT: +39 06 9422062 | M: +39 388 7617243
Virtual Tour

Villa Mondragone and the Gregorian calendar

The measurement of time has been in human history a problem of great importance and difficulty, with the succession of seasons and years with the expiration of equinoxes and solstices but the calculations were by no means simple.

To understand episode of Villa Mondragone and the Gregorian calendar we start here.

The Sosigene Error

In the time of Julius Caesar, the Egyptian astronomer Sosigene of Alexandria calculated the time length of the year to be 365 days and 6 hours, and to compensate for the 6-hour gap he introduced the leap year by inserting an extra day every 4 years.

Thus was born in 46 B.C. the Julian Calendar, which that year, to realign the spring equinox, would have 445 days and was called ultimus annus confusionis.

But Sosigene still made a mistake, which was impossible to recognize at the time. The Earth’s revolution time is actually 365 days, 5 hours, 48 minutes and 45 seconds.

The difficult task of dating Easter

In 325 Constantine, the first Roman emperor to embrace the Catholic faith, and the ecclesiastical hierarchies promulgated the Council of Nicaea. Where the temporal role of the Catholic Church was established with a liturgy based on fixed recurrences such as “Christmas” and movable recurrences such as “Easter.”

The date of the latter was set on the first Sunday following the first full moon after the spring equinox, which in 325 fell on March 21 and officially became the date of the equinox.

As the years went by, however, it was noticed that the date of the equinox, which corresponds to a precise position of certain stars, had changed and no longer corresponded to March 21, the discrepancy increased, and thus slipped all the so-called “moving” holidays.

The ten days that never existed

By the 16th century the gap had become 10 days, and Pope Gregory XIII decided to resolve the situation therefore by promulgating from Villa Mondragone (one of the Tuscolan villas in Frascati) the papal bull Inter gravissimas (first 2 words of the bull – “Among the most serious things”) in which it was decided that October 4, 1582 would be changed to October 15, thus recovering the 10-day difference.

The very precise and accurate calculations were entrusted to the Calabrian astronomer Luigi Giglio who also established that every hundred years the leap year would be normal except, however, every four hundred years. This allowed for the accumulation of ninety-seven leap days every four hundred years instead of one hundred in total, thus making up for the actual delay.

And so the 1700s, 1800s, and 1900s were not leap years, while 2000 was and 2100 will not be.
The Gregorian Calendar had been born.

To enforce the use of the new calendar and convince Protestants who in opposition continued to use the Julian Calendar, the Church began the construction of large sundials inside churches.

The entry into modernity

In 1700, in order to definitively verify the accuracy of the Gregorian Calendar reform Pope Clement XI commissioned mathematician and astronomer Francesco Bianchini to build a monumental sundial inside the imposing church of Santa Maria degli Angeli e dei Martiri that would accurately indicate the equinox and consequently Easter.

It had entered modernity and the age of scientific verification. Even today many people are fascinated by the history of Villa Mondragone and the Gregorian calendar.

With the Blog of Rome and Latium Region Around Rome guides you to discover places and territories for the pleasure of satisfying curiosity and putting culture at the service of people and businesses.

view on Google Maps
Edited by il NETWORK
The Museum of the Walls of Rome: worth a visit to learn about ancient Rome
27 May 2024
Tolfa: the sea seen from above, the Maremma from below
22 May 2024
The French Cemetery of Rome at Monte Mario
19 May 2024

Let us help you!

Get in touch with us! We will reply as soon as possible! If you need fast reply please call us.

T: (+39) 06 94 22 062


We are using cookies to give you the best experience. You can find out more about which cookies we are using or switch them off in privacy settings.
AcceptPrivacy Settings


  • Privacy e Cookie Policy

Privacy e Cookie Policy

Prima di proseguire si prega di leggere l’informativa sul trattamento dei dati personali e le informazioni relative all’utilizzo dei Cookie.