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Every four years, the world that uses the Gregorian calendar gets an extra day added to the calendar at the end of February. Other than making it hard for Leap Day babies to celebrate their birthdays, what is the purpose of a Leap Year? Here’s what you need to know.
Leap Years Help Synchronize The Calendar With the Seasonal Year
A regular year on the Gregorian calendar is called a “common year” and it has 365 days. But astronomical or seasonal years do not keep to a strict 365-day schedule. One full trip around the sun actually takes 365 days, 5 hours, 48 minutes and 45 seconds. Because of that, we have to periodically adjust the calendar or over time things would get a little wonky.
If we never had a leap day, the calendar would eventually shift so much that the seasons would start to change — winter would be in the spring months, spring would be in summer, summer would be in fall, and fall would be in winter. And over enough time, the seasons would be completely opposite, until they eventually came all the way back around to be in line again.
The math works out so that if there was no leap day, in 100 years, the calendar would be 24 days off. In 200 years, it would be nearly 50 days off and so forth.
To make up for the extra six hours it actually takes to go around the sun one time, the Gregorian calendar started adding an extra day (almost) every four years. This began back with Roman Emperor Julius Caesar.
Leap Year started when Caesar noticed that the lunar calendar had become out of sync with the seasons. To restore things to their natural order, he decreed a single 445-day-long Year of Confusion, which was the year 46 B.C. Then after that, he mandated a 365.25 day year that added a leap day every fourth year.
But Caesar’s Solution Added a Bit Too Much to the Calendar
Caesar’s solution seemed fine for a while, but it didn’t entirely solve the problem because if there is an extra day every year, eventually that adds up to three extra days in the leap cycle, which is 400 years. That meant eventually, the seasons were still getting thrown off.
So in 1582, Pope Gregory XIII unveiled the Gregorian calendar, which that year dropped 10 days from the month of October to get things back on track, and then moving forward, he instituted a new leap year rule wherein every 100 years, we drop a leap day except for the century years that can be exactly divided by 400. So 1600, 2000, and 2400 are leap years, but 1700, 1800, 1900, 2100, 2200, and 2300 are not leap years.
And that’s why we have leap year (almost) every four years.
What Do Other Calendars Do?
Not every culture or country uses the Gregorian calendar. So what do those calendars do? Well, it varies.
The Bengali Calendar of the Indian subcontinent and the Indian National Calendar both put their leap day as close to February 29 on the Gregorian calendar as possible. The Buddhist calendar of southeast Asia became synced with the Gregorian calendar in 1941, so that calendar has the same leap day.
The Chinese and Hebrew calendars are lunisolar, which means their dates indicate both a moon phase and a time of the solar year. These calendars add a thirteenth month every few years to make up for the seasonal shifts, but it can be a whole month because their overall yearly calendar is shorter than 365 days. Similarly, the Islamic calendar adds a leap day every two to three years because its calendar is shorter as well.
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