Leap Year

If a clock runs a tiny bit slow every day and you never reset it, it will be minutes off in a month and flip day and night within a few years. The exact same thing happens between Earth and our calendar. We count a calendar year as 365 days flat. But Earth takes about 365 days and 6 hours (more precisely, 365 days, 5 hours, and 48 minutes) to complete one cycle of seasons, known as a solar or tropical year. This is slightly different from a sidereal year, which measures an orbit against distant stars.

That leaves about six leftover hours unaccounted for on the calendar each year. Six hours in a single year may sound small. But after two years, it becomes 12 hours. After four years, it adds up to 24 hours—a full extra day.

If we ignored this gap, our calendar would race ahead of the real seasons by one day every four years. To stop the calendar from outrunning the Sun, humanity decided to slip in an extra day, February 29, at the end of February every four years.

If the leftover time were exactly six hours, adding one day every four years would be perfect. But a solar year is actually about 11 minutes and 14 seconds shorter than 365 days and 6 hours. By blindly adding 24 hours every four years, we are actually overcounting by about 11 minutes each year.

Eleven minutes might not seem like much, but it adds up to a full day over 128 years, and about three extra days every 400 years. To shave off this extra drift, our modern calendar (the Gregorian calendar) uses three clever rules.

First, any year divisible by 4 is generally a leap year. However, if the year is also divisible by 100, it is skipped and remains a common year. Finally, years divisible by 400 are leap years after all. That is why the year 2000 was a leap year (divisible by 400), but the year 2100 will be a normal 365-day year ending on February 28, because it is divisible by 100 but not 400.

What if we scrapped leap years entirely and lived with 365 days every single year? For the first few years, you would hardly notice. But after 100 years, the calendar would run ahead of the seasons by about 24 days.

Fast forward about 700 years, and someone in the Northern Hemisphere would experience blizzards in July and head to the beach for summer vacation in January. Farmers would lose track of when to plant crops, and calendar holidays would completely drift away from seasonal weather.

In the end, leap years are not just abstract astronomy. They are how we keep our calendar in step with Earth's seasons. Thanks to that quiet February 29 every four years, summer stays right where it belongs under the warm sun.