Why Is There a Leap Day?

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This year, the last day of February won’t be February 28. Instead, it’ll be February 29. That’s because every four years, the Gregorian Calendar includes an extra day in February known as the leap day. Why is there a leap day? Read on to understand the science behind why there’s a leap day every four years.

First: understanding the leap year

To understand why there is a leap day, you first need to understand the leap year. During a leap year, February is one day longer, and the extra day – February 29 – is the leap day. Leap years occur every four years starting the first year of a century, i.e. 2000, 2004, 2008, 2012, 2016, 2020, 2024, and so on.

Why does the leap year have a leap day?

The simple explanation for why there is a leap day is that adding an extra 24 hours to the Gregorian calendar every four years ensures that the calendar actually properly tracks the location of the earth relative to the sun – after all, a year is the amount of time it takes for the earth to complete a revolution around the sun. But a year is just an approximation of the revolution time: In reality, the earth completes its orbit around the sun in a bit more time than the standard 365 days in a year.

In reality, the earth takes 365.2421 days to complete its orbit around the sun. Though 0.2421 days may appear small on paper, it translates to five hours and 48.624 minutes. This means that, at 12:00 AM on January 1 on the day after a leap year, the earth isn’t actually at the start of its orbit around the sun – instead, it’ll be there just before 5:49 AM on January 1.

With time, this discrepancy means that the earth isn’t actually at the point along its orbit that calendars dictate it is. By the time January 1 three years after a leap year arrives, the earth won’t actually be hitting the start of its orbit until just before 5:26 PM. The leap day the next year calibrates this.

Look at the amount of time by which an actual full revolution of the earth around the sun exceeds a calendar year: five hours and 48.624 minutes. If you multiply this quantity by four, you get 23.2416 hours, or 23 hours and 14.196 minutes. This quantity is only slightly less than 24 hours, so by having a leap day every four years, the Gregorian calendar becomes much more closely realigned with the actual orbit of the earth around the sun than if no leap day were instated.

Of course, this explanation about why there is a leap year overlooks one key fact: 23 hours and 14.196 minutes is not a full day. The Gregorian calendar will never perfectly track the earth’s path around the sun, but without a leap year and leap day, our summers might look more like fall or even winter. Humanity doesn’t always perfectly replicate scientific phenomena, but it’s easy to come close.

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