Earth takes a little longer than 365 days to go around the Sun. So about every four years, we add one extra day: 29 February.
A year with that day is called a leap year.
The problem with 365 days
NASA’s Jet Propulsion Laboratory says Earth takes about 365.2422 days to go around the Sun. That is roughly six hours more than a 365-day calendar year.
Six hours sounds small. But it adds up. After four years, the missing time is close to one full day. Adding one day every four years fixes most of the gap.
How the leap year rule works
The Royal Museums Greenwich explains the rule. A year is a leap year if its number can be divided by four. But years that end a century, such as 1900 or 2000, must be divisible by 400. So 2000 was a leap year, and 1900 was not.
Why the exception? Adding a full day every four years is slightly too much. NASA says it overshoots by about 45 minutes in each four-year cycle. That builds up to about three days every 400 years. The century rule skips three leap days in every 400 years, which fixes this. NASA says 2100, 2200 and 2300 will not be leap years.
What would happen without leap days
Without leap days, the calendar would slowly drift away from the seasons. NASA says the dates of the equinoxes and solstices would slowly shift later in the year. After about a century, summer would be pushed to around mid-July. NASA’s Space Place adds that in a few hundred years, July would fall in the cold winter months.
Other planets have the same problem. A year on Mars is 668 sols, but Mars takes 668.6 sols to go around the Sun. So over 10 years, six Martian years have 669 sols and four have 668.
When the next leap years come
The Royal Museums Greenwich says 2028, 2032 and 2036 are all leap years. As NASA notes, 2100 will not be one.
