What is a leap year?
Quick answer
A leap year is a year with 366 days instead of 365, with an extra day, February 29, called leap day. Leap years happen because Earth takes about 365.2422 days to orbit the Sun. Adding a day roughly every four years keeps the calendar in step with the seasons. Recent and upcoming leap years include 2024 and 2028.
The rules
The Gregorian calendar, used in most of the world, has three rules:
| Rule | Example |
|---|---|
| A year divisible by 4 is a leap year | 2024, 2028 |
| Except years divisible by 100 are not | 1900, 2100 |
| Unless they’re also divisible by 400 | 2000 was a leap year |
So 1900 wasn’t a leap year, 2000 was, and 2100 won’t be.
Why we need it
| Calendar year | Actual solar year |
|---|---|
| 365 days | About 365 days, 5 hours, 49 minutes |
Without leap years, the calendar would drift by about a quarter of a day each year, almost a full month every 120 years. Eventually, summer would fall in what the calendar calls winter.
A short history
| Year | Event |
|---|---|
| 45 BC | Julius Caesar’s Julian calendar adds a leap day every four years |
| Over centuries | Adding a day every 4 years was slightly too much; the calendar drifted about 10 days |
| 1582 | Pope Gregory XIII introduces the Gregorian calendar, skipping 10 days and adding the century rule |
Britain and its colonies, including America, switched in 1752, skipping 11 days.
Leap day facts
- People born on February 29, called leaplings, often celebrate on February 28 or March 1 in common years.
- The chance of being born on leap day is about 1 in 1,461.
- In an Irish tradition, women could propose marriage to men on leap day.
Leap seconds
Clocks have occasionally added leap seconds to match Earth’s slightly irregular rotation. In 2022, international timekeepers agreed to stop adding leap seconds by 2035.
Months and days are explained further in what day it is today. Earth’s orbit and the planets are explored in who discovered the laws of planetary motion.
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