Why Do We Have Leap Years? The Science Behind February 29
A year is not exactly 365 days. Here is why calendars add a leap day every four years, why some century years do not count and when the next leap year arrives.
Most years have 365 days, but every four years we add an extra day: 29 February. The reason is astronomical, and the rule for deciding which years count is a little more subtle than “every four years”.
The Earth does not take exactly 365 days
The Earth takes about 365.2422 days to orbit the Sun once, a period called the tropical year. A calendar with exactly 365 days is therefore short by almost a quarter of a day each year.
That might not sound like much, but it adds up. After four years the calendar would be about a day behind the seasons. After a few centuries, the calendar dates for the equinoxes and solstices would have drifted by weeks, and festivals tied to the seasons would slide into the wrong time of year.
The leap day fix
Adding one extra day every four years makes the average calendar year 365.25 days, which is very close to the real figure. That is the idea behind the leap year, introduced in the Roman Julian calendar.
Why the Gregorian calendar refined it
A calendar year of 365.25 days is slightly longer than the real tropical year of 365.2422 days, so the Julian calendar slowly drifted the other way, by about three days every four centuries. By the 1500s, the date of the spring equinox had moved noticeably.
In 1582 Pope Gregory XIII introduced a reformed calendar with a more precise rule. Leap years are:
- years divisible by 4, except
- century years (such as 1900), unless they are also divisible by 400.
So 2000 was a leap year, but 1900 and 2100 are not. This tweak brings the average year to 365.2425 days, close enough that the calendar stays accurate for thousands of years.
Adoption took time
Catholic countries adopted the Gregorian calendar quickly, but others did so much later. Britain and its colonies, including the American colonies, switched in 1752. To catch up with the drift, they skipped 11 days that September.
Leap years in practice
| Year | Leap year? | Why |
|---|---|---|
| 2024 | Yes | Divisible by 4 |
| 2026 | No | Not divisible by 4 |
| 2028 | Yes | Divisible by 4 |
| 1900 | No | Century year not divisible by 400 |
| 2000 | Yes | Century year divisible by 400 |
| 2100 | No | Century year not divisible by 400 |
A note on leap seconds
Separately from leap years, international timekeepers have occasionally added a “leap second” to keep clocks aligned with the Earth’s slightly irregular rotation. Leap years fix the calendar’s length, while leap seconds fix the clock.
Frequently asked questions
Is 2100 a leap year?
No. Century years are leap years only if they are divisible by 400. So 2000 was a leap year, but 1900 and 2100 are not.
When is the next leap year?
The next leap year after 2026 is 2028, which will include a 29 February.
What happens to people born on 29 February?
They are sometimes called leaplings. In non-leap years they usually celebrate on 28 February or 1 March, depending on custom or local law.
