As you’re nearing the end of February, dates can get confusing. The irregular month, already unique in its 28-day length, sometimes has a 29th day, also known as Leap Day.
This is even worse for the 5 million people with Leap Day birthdays, according to a History estimate, whose real birthdays come only once every four years.
This phenomenon may cause you a small inconvenience, but leap years are not just a cruel trick calendar companies play to keep you guessing. There’s actually a very good astronomical reason for the added day — but we’ll get into that later.
Generally speaking, leap years come every four years: 2024, 2028 and 2032, for example, will all be leap years.
Usually, if a year is divisible by 4, that makes it a leap year, but this rule does not hold for years which are multiples of one hundred, according to History.com. Although the Earth’s orbit around the sun was previously thought to be 365.25 days, it actually takes closer to 365.24 days says History.com.
To fix this small discrepancy, century years which are not divisible by 400 are not leap years. The year 2000 was a leap year, but the year 2100 will not be.
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Because the amount of time the Earth takes to spin on its axis is not dependent on the amount of time the Earth takes to complete a full orbit around the sun, the number of 24-hour rotations that fit into a calendar year is not a whole number.
According to History, a leap year is the solution to this. Just one extra day every four years keeps our calendar lined up correctly with the orbit of the Earth, meaning winter stays winter, and summer stays summer.
There are 366 days in a leap year — one more than the typical 365-day year.
Until 2024, 2020 would have been the most recent leap year.