The Gregorian Reform of 1582: The Year Ten Days Vanished

On the night of Thursday 4 October 1582, people in Italy, Spain, Portugal and Poland went to sleep as usual. When they woke, it was Friday 15 October. Ten days had disappeared from the calendar by order of the Pope. It sounds like a strange story, but it solved a problem that had been growing quietly for more than a thousand years.

Eleven minutes a year

The Julian calendar, introduced by Julius Caesar in 45 BC, assumed the year was exactly 365.25 days long. That is why it added a leap day every four years. The true solar year, measured from one spring equinox to the next, is a little shorter, about 365.2422 days.

The difference is only about eleven minutes per year. But those minutes add up. Every 128 years or so, the Julian calendar slipped one full day ahead of the Sun. By the sixteenth century, the spring equinox was arriving around 11 March instead of 21 March.

Why Easter mattered

For most people a few days of drift went unnoticed. For the Church it was a serious problem. The date of Easter depends on the spring equinox and the full Moon. In AD 325 the Council of Nicaea had linked the equinox to 21 March. As the calendar drifted, Easter was being celebrated further and further from the moment the early Church intended.

Popes and scholars had discussed reform for centuries. Finally, under Pope Gregory XIII, a commission went to work using a proposal by the Italian physician and astronomer Aloysius Lilius, with the Jesuit mathematician Christopher Clavius playing a leading role.

Inter gravissimas

On 24 February 1582 the Pope issued a papal bull, a formal decree, called Inter gravissimas, after its opening words "among the most serious". The title page shown on the cover of this post comes from the printed perpetual calendar issued to go with it. The reform had two parts.

First, to bring the equinox back to 21 March, ten days were removed. In the Catholic countries that followed immediately, 4 October was followed by 15 October.

Second, to stop the drift happening again, the leap year rule was changed:

• A year divisible by 4 is a leap year, as before.

• But a century year, such as 1700, 1800 or 1900, is not a leap year.

• Unless it is divisible by 400, like 1600 and 2000, which are leap years.

This removes three leap days every 400 years and brings the average calendar year to 365.2425 days, very close to the real solar year. The Gregorian calendar is still slightly off, but only by roughly one day in three thousand years.

A slow and uneven adoption

The reform came from the Pope at a time when Europe was divided by the Reformation. Catholic countries adopted it quickly. Protestant and Orthodox countries were suspicious of anything that came from Rome, and many waited a very long time.

• Most Protestant German states switched in 1700.

• Great Britain and its colonies, including those in America, changed in 1752. By then the gap was eleven days, so Wednesday 2 September was followed by Thursday 14 September. Britain also moved the start of the legal year from 25 March to 1 January at the same time.

• Japan adopted the Gregorian calendar in 1873.

• Russia changed only in 1918, after the revolution. That is why the October Revolution of 1917 is remembered in November on the modern calendar.

• Greece switched for civil purposes in 1923, one of the last European countries to do so.

Echoes of the change

The switch left traces that are still visible. In Britain the tax year begins on 6 April, a date that can be traced back to the old New Year of 25 March plus the eleven days lost in 1752. George Washington was born on 11 February 1731 by the old calendar, but we celebrate 22 February because of the change. Many Orthodox churches still use the Julian calendar for religious feasts, which is why Orthodox Christmas falls on 7 January.

The calendar we share

Today the Gregorian calendar is the civil calendar of almost the entire world, used for business, travel, science and daily life, even in countries that also keep traditional calendars. Every printed year planner, every wall calendar and every phone calendar follows the rules worked out by a small group of astronomers in the 1570s.

So the next time a leap year comes round, or you notice that 2100 will not have a 29 February, remember the ten days that vanished in 1582. They are the reason your calendar still matches the seasons, and the reason a calendar printed for any year, past or future, can be calculated with complete confidence.

Cover image: Inter gravissimas, Public domain, via Wikimedia Commons.