Why October 1582 lost ten days: a calendar year 11 minutes too long

In 1582, Thursday 4 October was followed by Friday 15 October. Ten dates were simply deleted across Catholic Europe.

The reason was an error of about 11 minutes a year. Julius Caesar's calendar made every year 365.25 days long, but the seasons repeat every 365.2422 days. Over 1,257 years that tiny overshoot had pushed the spring equinox ten days away from 21 March. Pope Gregory XIII's fix cut the ten days and tweaked the leap-year rule so it wouldn't happen again. Drag through the centuries to watch the calendar drift.