Leap year

Some years have one extra day.

This day is in February. Most years have 365 days. But the Earth takes a bit longer to go around the sun.

We add this extra day to keep our calendar right. It helps our seasons stay in the same place. Without it, our calendar would drift away. This extra day is called a leap day. It happens about every four years.

This keeps our world in sync.

Have you ever wondered why February sometimes has 29 days?

Most years have 365 days. But the Earth takes a little longer to go around the Sun. To keep our calendar in sync with the seasons, we add an extra day. This is called a leap day.

We use the Gregorian calendar in most of the world. It adds a leap day almost every four years. In these years, February has 29 days instead of 28. But there is a special rule for century years. A year that ends in "00" is not a leap year unless it can be divided by 400. For example, the year 2000 was a leap year. But the years 1700, 1800, and 1900 were not.

Long ago, the Julian calendar used a simpler rule. It added a leap day every four years without fail. This rule was not perfect. It caused the calendar to drift away from the seasons. The Gregorian calendar fixed this small error. Now, our calendar stays close to the seasons we see in nature.

Have you ever noticed that February sometimes has an extra day?

This special day is called a leap day. It happens during a leap year, which has 366 days instead of the usual 365. We need these extra days to keep our calendars in sync with the seasons. The Earth does not orbit the Sun in an exact number of days. An astronomical year lasts slightly less than 365 days. Without a leap day, our calendar would slowly drift away from the seasons.

To fix this drift, we add a day to the calendar at regular times. In our modern system, we add the day to the end of February. This makes February 29 days long instead of 28. This extra day acts like a small correction for the solar system. The Earth's journey around the Sun is not a perfect match for our 365-day count. By adding a day, we pull the calendar back into place. This ensures that spring and winter always arrive at the right time.

People have used different ways to track time for a very long time. Julius Caesar helped create the Julian calendar to follow the sun. His rule was quite simple: add one leap day every four years. This system worked for about 1,600 years. However, the Julian year was about 365.25 days long. The actual tropical year is about 365.2422 days. This small difference caused the calendar to drift by three days every 400 years.

To solve this error, the Gregorian calendar was created. It is the most widely used calendar in the world today. This system uses a more careful rule for leap years. A year is a leap year if it is divisible by 4. However, years ending in "00" are only leap years if they are also divisible by 400. For example, the years 1600 and 2000 were leap years. But the years 1700, 1800, and 1900 were not leap years.

The word "leap" might come from how days move in the week. Usually, a date moves forward by one day each year. But during a leap year, the date skips over a day. This makes the day of the week seem to "leap" forward. We also sometimes use leap seconds to fix the time. This happens because the Earth's rotation can change slightly. These small changes help our clocks stay perfect with the world.

A leap year, sometimes called an intercalary or bissextile year, is a calendar year with an extra day.

In most modern systems, this extra day is added to February, making it 29 days long. This adjustment is necessary to keep our human calendars synchronized with the astronomical year. The astronomical year is the time it takes for Earth to orbit the Sun. Because this orbit does not take an exact number of whole days, calendars with a fixed number of days would eventually drift away from the seasons. By adding a leap day or a leap month, civilizations can prevent this drift.

The mechanism of the leap year relies on correcting a small mathematical mismatch. An astronomical year lasts slightly less than 365 days. Specifically, a mean tropical year is approximately 365.2422 days. If we used a standard 365-day calendar, we would lose about six hours every year. Over many years, these lost hours would add up to full days. This would cause the calendar to move out of alignment with the seasons, such as the spring equinox. To fix this, we insert a leap day into the calendar at regular intervals to pull the date back into place.

Different calendar systems use different rules to manage these corrections. The Julian calendar, created by Julius Caesar, used a simple rule. It added one leap day every four years. This resulted in a Julian year of 365.25 days. While this was close to the truth, it was still slightly too long. The Gregorian calendar, which is the most widely used civil calendar today, uses a more complex rule to improve accuracy. In the Gregorian system, a year is a leap year if it is divisible by 4. However, century years are only leap years if they are also divisible by 400.

This Gregorian rule helps correct the error found in the Julian system. Because the Julian year was 365.25 days, it drifted by about three days every 400 years. The Gregorian calendar aims for an average year of 365.2425 days. This is much closer to the actual tropical year of 365.2422 days. To achieve this, the Gregorian calendar omits three leap days every 400 years. For example, the years 1700, 1800, and 1900 were not leap years. However, the years 1600 and 2000 were leap years because they are divisible by 400.

The history of these corrections goes back to ancient Rome. Before Caesar's reform, the Roman calendar was a lunisolar system. It had 12 months totaling 355 days. To stay in sync with the sun, Romans sometimes inserted a 27-day month called Mercedonius. This month was added into February at specific times. Julius Caesar later reformed the calendar to be a consistent solar calendar. He changed the months to have 30 or 31 days. He kept February as the shortest month but added a leap day every four years.

In the original Julian system, the leap day was handled quite differently than it is now. Instead of adding February 29, the Romans doubled the day known as the sixth day before the Kalends of March. This was February 24. This doubling is why the term "bissextile" exists, which comes from the Latin for "twice sixth." For a long time, this meant there were two days both named February 24. It was not until later that the practice of simply using February 29 became the standard. In England, the use of February 29 was formally recognized in law by the Calendar Act of 1750.

The term "leap year" also relates to how days of the week move. In a common year, a specific date usually advances by one day of the week. For instance, if March 1 is a Friday in 2025, it will be a Saturday in 2026. However, during a leap year, the extra day causes the date to "leap" over a day in the weekly cycle. If March 1 is a Friday in 2024, it becomes a Sunday in 2026, skipping Saturday. Beyond days, we also use leap seconds to correct Coordinated Universal Time. These are added to account for variations in Earth's rotation period. Unlike leap days, leap seconds are not on a regular schedule because Earth's rotation changes unpredictably.