The Julian calendar was replaced by the Gregorian one to correct a slow drift.

The longer version

A year measured by the seasons is a little longer than three hundred sixty-five days but a little shorter than three hundred sixty-five and a quarter. A system that adds a whole leap day every four years therefore runs slightly fast, and the small surplus piled up over many centuries.

After enough centuries the spring equinox was arriving noticeably earlier than the date the calendar assigned to it, which mattered because the date of Easter was calculated from that moment. Fixing it meant dropping some days in one go and refining the leap year rule so that century years would usually be skipped. Different countries adopted the change at different times, so old records can clash.

Where this entry sits

This set looks at ancient civilizations, old inventions, trade routes, calendars, and the ordinary objects people once used every day. Nothing here is advice or opinion, just short notes about how daily life used to be organised long ago. The full list sits on the History page, where every entry is listed in order.