The Year 1582 and the Missing Ten Days!

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Funerary monument to Pope Gregory XIII by Camillo Rusconi

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In the year 1582, October 4th was followed not by October 5th, but by October 15th! What happened to the missing ten days? The answer is carved into the funerary monument to Pope Gregory XIII (r. 1572-85), which stands in St Peter's Basilica. 

The monument, the work of Camillo Rusconi (1658-1728), a sculptor from Milan, was completed in 1723. The pope is flanked by personifications of Religion and Wisdom, and sits enthroned above an urn, under which crouches a dragon, the heraldic device of his family, the Boncompagni. 

Wisdom raises a drape to draw our attention to the relief on the urn, which depicts the most significant achievement of the pope's reign, namely the promulgation of a new calendar. 

The old calendar, which aligned the civil year more closely to the astronomical year, had been 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. ​

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Relief by Carlo Melloni depicting Pope Gregory XIII and some of the mathematicians and astronomers behind the creation of the new calendar, including Ignazio Danti and Aloysius Lilius