The time problem
A normal calendar year has 365 days. But Earth actually takes about 365.2422 days to complete one full orbit around the sun, roughly 6 extra hours beyond what the calendar counts. Left alone, that 6-hour gap would quietly build up year after year. It doesn't sound like much, but four years of it adds up to very close to one full 24-hour day.
Keeping the seasons lined up
Adding February 29th every 4 years is how the calendar catches up. Without it, the calendar would slowly drift out of sync with the actual solar year. It wouldn't be noticeable in a single lifetime, but over centuries, the drift would be enough to shift which months line up with which seasons, eventually putting winter months where summer used to be.
Why the century rule exists
Adding a leap day every single 4 years is close, but not perfect, it slightly overcorrects. So the Gregorian calendar (the one in everyday use today) adds one more rule: century years are only leap years if they're divisible by 400. That's why 1900 and 2100 aren't leap years, but 2000 was. This extra rule trims out 3 leap days every 400 years, which brings the calendar's average year length much closer to Earth's actual orbital period.
What would happen if we skipped leap years entirely
The drift would be small at first, about 6 hours a year, but it compounds. Over a few centuries, holidays and school calendars tied to specific dates would slowly detach from the seasons they're currently associated with. It's the same reason the Gregorian calendar replaced the older Julian calendar in 1582, the Julian calendar's leap-year rule (a leap day every 4 years, no century exception) was slightly too generous, and by the 1500s the drift had become large enough to visibly throw off the date of the spring equinox.
Frequently asked questions
Why do we have leap years?
Earth takes about 365.2422 days to orbit the sun, not exactly 365. Adding an extra day every 4 years makes up for the roughly 6 extra hours that build up each year, keeping the calendar lined up with the actual seasons instead of slowly drifting away from them.
Why are century years sometimes not leap years?
The simple every-4-years rule adds slightly too many days over long stretches of time. The Gregorian calendar corrects for this by skipping the leap day in century years that aren't divisible by 400. That's why 1900 wasn't a leap year but 2000 was.
What would happen if we never had a leap year?
Without the extra day, the calendar would drift against the actual solar year by about 6 hours annually, roughly a full day every 4 years. Over centuries, that drift would add up enough that the months would no longer line up with the seasons they're associated with today, eventually putting winter months in what should be summer.
Sources
- NASA Jet Propulsion Laboratory, "Doing the Math on Why We Have Leap Day"
- Smithsonian National Air and Space Museum, "The Science of Leap Year"
- US Naval Observatory Astronomical Applications Department, "Leap Years"