Why did ten days vanish in 1582?
In October 1582, several countries in Europe simply deleted ten days. Thursday the fourth was followed by Friday the fifteenth. Rents, birthdays and saints’ days all jumped. Nothing whatever had happened in the sky and nobody was any older. The man who ordered it was correcting a mistake that had been quietly growing since before the Roman Empire had emperors.
A year is not three hundred and sixty-five and a quarter days. It is very slightly less, and sixteen centuries of ignoring the difference added up to ten days.
A year is an awkward number
The length of a year is set by our journey around the Sun. Earth travels its orbit at about 30 km/s(67,000 mph), covering roughly 940 million km(584 million mi) before arriving back where it started.
Astronomers measure a year by the seasons rather than the stars, timing the interval from one spring equinox — the day when daylight and darkness are equal — to the next. That interval is the tropical year.
Its length is 365.2422 days. Not a neat number, not a fraction anybody would choose, and awkward in a way that no calendar can entirely escape.
Whole days, and an inconvenient remainder
A calendar has to use whole days, so the leftover must be handled by adding an extra day sometimes. Any year given that extra day is a leap year.
The Roman fix was very nearly right
Julius Caesar reformed a badly broken Roman calendar in 46 BCE, on advice from Egyptian astronomers, and his rule was beautifully simple: every fourth year gets an extra day.
That treats the year as exactly 365.25 days long, which is remarkably close. Caesar’s calendar kept step with the seasons far better than anything Europe had used before, and it lasted sixteen hundred years.
The Roman assumption minus the true length of a year, in days.
Eight thousandths of a day is eleven minutes. Eleven minutes a year is nothing at all, which is precisely why nobody noticed for centuries.
Nobody can feel eleven minutes. Nobody could ignore ten days, and the second is only the first, left alone for long enough.
Eleven minutes a year is one day every hundred and twenty-eight years. By the 1500s the calendar was running ten days ahead of the seasons. The spring equinox, which the Church used to fix the date of Easter, had wandered from the twenty-first of March to the eleventh.
Deleting the difference
The fix needed two separate parts, and this is what makes 1582 interesting rather than merely dramatic.
The first part was paying off the accumulated drift. Ten days had to disappear at once, so late in the sixteenth century Pope Gregory decreed that the fourth of October 1582 would be followed directly by the fifteenth.
The second part mattered far more, because deleting days once fixes nothing permanently. The leap rule itself had to change, or the same eleven minutes would start piling up again immediately.
The new rule was designed by Aloysius Lilius, an Italian doctor and astronomer, and worked through in detail by Christopher Clavius. It keeps the every-fourth-year rule, then removes three leap days every four centuries.
A year ending in double zero is a leap year only if it divides by four hundred. So 1600 and 2000 were leap years. 1700, 1800 and 1900 were not, and 2100 will not be either.
That correction leaves an error of roughly one day in three thousand years, which is a rather better piece of engineering than most things built in 1582.
The quarter that is not a quarter
The countries that refused
Here is the mundane part, and it lasted far longer than the astronomy. Half of Europe did not accept a calendar issued by a Pope.
Catholic countries changed in 1582. Protestant and Orthodox countries did not, so for a century and a half a traveller crossing a border also changed the date by ten days, and letters routinely carried two.
Britain and its colonies held out until 1752, by which time the gap had grown to eleven days. The second of September was followed by the fourteenth, roughly eight generations before you were born.
Russia waited until 1918, which is why its October Revolution took place in November. The last European country to change over did so in 1923.
There is a famous story that English crowds rioted, demanding their eleven days back. Historians have looked hard and found very little evidence for it. The slogan comes mostly from a satirical painting, and the tale spread because it is a much better story than a quiet administrative change.
Work out the drift yourself
Your third answer will land at about twelve days, which is close to the ten that were actually deleted. The difference is that the Church chose to restore the equinox to where it sat at an important meeting in 325, rather than to where it sat in Caesar’s day.
The fourth calculation is the satisfying one. You should get an average year of 365.2425 days, which misses the truth by three ten-thousandths of a day — about twenty-six seconds a year.
An error nobody could feel
The reason this went unnoticed for so long is worth keeping. Every individual mistake was far too small to detect. Eleven minutes cannot be measured by anybody looking at the sky for one year.
It only becomes visible by accumulating, quietly, for longer than any person is alive. By the time it is obvious, correcting it means doing something drastic.
So the calendar on your wall carries an old repair. Somebody noticed a difference of eleven minutes, deleted ten days to catch up, and wrote a rule about years ending in double zero that your phone is still following tonight.
