Why do we have leap years?
Every four years an extra day, called a leap day, is added to the end of February. That day, Feb. 29, doesn’t “exist” except for during a leap year, but why does this happen?
We use the sun to determine the seasons. One Earth rotation around the sun actually takes 365 days, five hours, 48 minutes and 45 seconds, not a flat 365 days as you commonly hear, according to The Atlantic. Philosophers have rounded this to a quarter of a day.
Every four years an extra day is added, making leap years have 366 days. This is because four years of an extra quarter day adds up to one full day.
A calendar
But if we ignored that quarter of a day, the months would eventually shift with the sun. For example, December would be in what we know as the summer months in the Northern Hemisphere. Every year we would also lose almost six hours. Over the course of 100 years, the calendar would be off by 25 days, according to the Farmer's Almanac. To adjust for this, an extra day is added to the shortest month of the year—February.
However, it's not perfect. According to the Farmer's Almanac, adding a leap day every four years adds a few extra seconds each leap year. Over time, this adds up to about three days every 10,000 years.
A good rule of thumb to determine if a year is a leap year is to divide it by four. An exception to the rule is that century years aren't leap years unless they are divisible by 400, according to the Farmer's Almanac.
A leapling is someone born on a leap day. The chances of being born on a leap day is one in 1,461.
Leap day brings a few traditions, including a day where women can propose to their significant other. This is an old Irish legend that dates back to St. Patrick and St. Bridget. According to the Farmer's Almanac, St. Bridget came to St. Patrick frustrated with waiting for a man to propose to her. They came up with a deal that women could propose to a man only on a leap day.
The next leap days will be Saturday, Feb. 29, 2020 and Thursday, Feb. 29, 2024.
