It's time for an extra day on the calendar
閏年到 今年，日曆上多了一天喔！
Every four years, the calendar looks different. An extra day is added to keep our human-made calendar in sync with the astronomical year. The primary reason for this is that our calendar is 365 days in length. But Earth's orbit around the sun takes approximately 365.2422 days. To prevent our calendar from drifting out of sync, February 29 appears on the calendar every four years. Julius Caesar was behind the move back in 45 B.C.
Interestingly enough, an astronomical year is actually slightly less than 365.25 days. So adding an extra day every four years results in about three extra days every 400 years. For this reason, only one out of every four century years is appointed as a leap year. Century years are only considered leap years if they can be divided evenly by 400. Therefore, 1700, 1800 and 1900 were not leap years. And 2400 will be the next century year that is a leap year.
Keeping the calendar correct is no easy matter!
