What is a leap year, and why do we need one?

A leap year is a calendar year that contains an extra day, February 29th, making the year 366 days long instead of the usual 365.

We need leap years to keep our calendar aligned with the Earth's orbit around the Sun.

It takes the Earth approximately 365.2422 days—roughly 365 days and 6 hours—to complete one full trip around the Sun. Because our standard calendar counts only 365 days, we are left with a remainder of about a quarter of a day each year.

If we didn't account for this extra time, the seasons would slowly drift out of sync with our calendar. Over a period of about 100 years, the calendar would be off by roughly 25 days, meaning our summer months would eventually arrive in the middle of winter.

Adding an extra day every four years corrects this "drift" and ensures that the seasons remain consistent with the same dates on our calendar year after year.