1582 & the Missing Ten Days!

Tower of the Winds (Torre dei Venti), Vatican City

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October 4th 1582 was the last day of the Julian calendar. The next day the Gregorian calendar came into effect and ten days were omitted to make the adjustment from the old to the new. October 4th was therefore followed by October 15th!

The Gregorian calendar is named after Pope Gregory XIII (r. 1572-85). His funerary monument in St Peter's Basilica makes reference to his reformation of the calendar. 

The old calendar, which aligned the civil year more closely to the astronomical year, was introduced by Julius Caesar on January 1st 46 BCE. In the Julian calendar, each year had 365 days, and an extra day was added every 4 years to make up for the six-hour difference between the civil and the solar year.

The Julian calendar was a great improvement on what had gone before, but it had one fundamental flaw, it fell short 12 minutes each year. Over the years the missing minutes added up and by the beginning of the 4th century the spring equinox, which the Julian calendar had set at March 24th, was falling on March 21st. This was to have significant implications for the celebration of Easter, the most important feast in the Christian year. 

In 325 the Council of Nicaea set the date on which Easter was to be celebrated as the first Sunday after the first full moon after the spring equinox. As the centuries passed, the date of the spring equinox fell back further and further; by the start of Gregory XIII's reign, it was falling on March 11th. The pope decided to act and appointed a special commission of experts to devise a system which would correct the error of the Julian calendar. 

On February 24th, 1582, Pope Gregory XIII issued the bull Inter Gravissimas, whereby the new calendar was to be put into effect. In addition to fine tuning leap years*, the bull also decreed that the ten days from October 5th to October 14th were to be eliminated, in order to offset the shift of the spring equinox.

​The Gregorian calendar came into force in Italy, France, Spain, Portugal and Poland on October 15th, 1582. However, it was not accepted by Protestant nations until the 18th century. 

*Aloysius Lilius (c.1510-76), the brains behind the Gregorian calendar, came up with the idea of adding an extra day in years divisible by 4, unless the year is also divisible by 100. If the year is also divisible by 400, an extra day is added, regardless.

Sadly, Lilius's ingenious method of aligning the civil and solar years is not perfect and the system is still off by 26 seconds. As a result, the Gregorian calendar will be a full day ahead of the solar year in 4909. ​

​The Torre dei Venti (Tower of Winds) was expressly built to determine the extent of the inaccuracy of the Julian calendar. The eagle-eyed may spot a dragon atop the tower. (A dragon makes up the coat of arms of the Buoncompagni family, to which Pope Gregory XIII belonged.)