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What is a leap year? Everything you need to know

Leap years occur every four years. But why? 12News meteorologist Lauren Rainson explains.

PHOENIX — Thursday, Feb. 29, 2024, is a leap day. So what exactly is a leap day and leap year? 

It has to do with the number of days in a year and the time it takes for the Earth to complete one orbit around the sun, which is 365 days. Well, technically, it's 365.2422 days.

So what's the deal with that extra .2422? That quarter of a day needs to be accounted for somewhere otherwise in 100 years, the calendar would be off by four weeks.

This is why we add an extra day to the calendar once every four years. Problem solved? Well, not entirely. By simply adding one extra day to the calendar every four years, we'd still be off by 18 hours in 100 years.

To correct this problem, we look at the century years. Not all century years are leap years. Only one in four century years are leap century years. For example, the years 1700, 1800, and 1900 were skipped. The year 2000 was a leap year. A century year must be divisible by 400 (evenly divided by 400) in order to be a leap century year. 

Now, with all this in place at this rate it would take over 3,000 years for us to be off by one day. Still not completely perfect, but this is why we observe leap years mostly every four years.

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