Leap years exist because a year runs slightly longer than three hundred sixty-five days.

The longer version

Earth takes a little more than three hundred sixty-five days to orbit the sun, so a calendar with only whole days falls behind the seasons. Left alone, the gap grows by roughly a quarter of a day each year, and after a century the dates of planting and festival would drift badly.

Adding an extra day every four years absorbs most of that gap. Because the true year is slightly shorter than three hundred sixty-five and a quarter days, the rule needs an exception: century years are usually skipped unless they divide by four hundred, keeping the calendar close to the sky.

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.