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How do leap years work?

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Quick answer

A leap year has 366 days because Earth's orbit takes about 365.24 days. The extra 0.24 days accumulate, so every four years an extra day is added to February to keep the calendar aligned with the seasons. The Gregorian calendar refines this with century rules.

A leap year has 366 days instead of 365 because Earth's orbit around the Sun takes about 365.24 days. Adding an extra day every four years keeps the calendar aligned with the seasons.

Step by step

  1. Earth completes one orbit around the Sun in about 365.24 days, which is the length of a tropical year.
  2. A standard calendar year has 365 days, so it falls short of the tropical year by about 0.24 days each year.
  3. After four years, the shortfall adds up to nearly one full day, causing the calendar to drift ahead of the seasons.
  4. To correct the drift, an extra day is inserted into February every four years, creating a leap year of 366 days.
  5. The Gregorian calendar adds a further rule: century years are not leap years unless divisible by 400, refining the correction.

Key things to understand

  1. Earth takes about 365.24 days to complete one orbit around the Sun, which is a little longer than a 365-day calendar year.
  2. If the calendar ignored that extra 0.24 days, it would drift by roughly one day every four years.
  3. To correct the drift, an extra day is added to February every four years, making a leap year of 366 days.
  4. The extra day is called a leap day, and it keeps the calendar's dates matched to seasonal events like equinoxes.
  5. The rule is refined further: years divisible by 100 are not leap years unless they are also divisible by 400.

Labelled diagram

  1. Earth's orbit

    Earth takes about 365.24 days to orbit the Sun once.

  2. Calendar year

    A common year has 365 days, slightly shorter than the orbit.

  3. Drift

    The 0.24-day gap accumulates, so the seasons slip about six hours later in the calendar each year.

  4. Leap day

    An extra day in February every four years corrects the drift.

  5. Gregorian rule

    Century years skip the leap day unless divisible by 400.

How a leap year keeps the calendar aligned

Real-world examples

  • The year 2024 is a leap year, so February 2024 had 29 days instead of 28.
  • The year 1900 was not a leap year because it is divisible by 100 but not by 400.
  • The year 2000 was a leap year because it is divisible by 400, so it had 366 days.
  • In the Gregorian calendar, the average year length is 365.2425 days, very close to the tropical year.

Common mistake

✗ Myth: A leap year happens every four years without exception.

✓ Fact: The Gregorian calendar skips leap years in century years not divisible by 400, so 1900 was not a leap year but 2000 was.

People also ask

What is a leap year?
A leap year is a calendar year with 366 days instead of 365. The extra day, called a leap day, is added to February. Leap years occur roughly every four years to keep the calendar synchronized with Earth's orbit and the seasons.
Why do we have leap years?
We have leap years because Earth's orbit around the Sun takes about 365.24 days, not exactly 365. Without the extra day every four years, the calendar would drift, and eventually summer would start in December in the Northern Hemisphere.
When is the next leap year?
The next leap year is 2028. Leap years occur every four years, so after 2028 the next ones are 2032, 2036, and 2040. However, 2100 will not be a leap year because it is a century year not divisible by 400.
Why is February the month that gets the extra day?
February was the last month of the original Roman calendar, so it was the natural place to add or remove days when officials adjusted the year. That tradition carried into the modern Gregorian calendar, where February is the only month shorter than 30 days.
What is a leap second?
A leap second is a one-second adjustment occasionally added to Coordinated Universal Time to keep atomic clocks aligned with Earth's slightly irregular rotation. Unlike leap years, leap seconds are not on a fixed schedule and are decided by international timekeeping bodies.
Do other planets have leap years?
Any planet with a calendar can have a leap-year-like correction if its orbital period is not a whole number of its days. For example, Mars's year is about 687 Earth days, and a Mars calendar would need its own adjustment rule to stay aligned with the seasons.

Part of the guide: Space & Astronomy →