The Julian calendar added a leap day every four years, which slowly drifted against the seasons.

The longer version

The Julian reform set the year at 365 days with an extra day every fourth one, giving an average of 365.25. The true figure is a little shorter, so the calendar gained roughly one day against the sky every 128 years. The rule was simple and easy for officials to apply.

Over many centuries that small excess added up to about ten days, enough to move the equinox and confuse seasonal planning. The scheme worked well enough for ordinary life, which is why it lasted so long, but astronomical users noticed the error and pressed for change. The correction adopted in 1582 trimmed three leap days every four centuries.

Where this entry sits

These facts wander through clocks, calendars, time zones, and the long human effort to measure passing hours. You will find leap years, odd month lengths, ancient sundials, and the reasons modern timekeeping works the way it does. The full list sits on the Time and Calendars page, where every entry is listed in order.