Century leap year facts for kids

A century leap year is a special kind of leap year in our modern Gregorian calendar. It's a year that can be perfectly divided by 400.

Like all leap years, a century leap year has an extra day in February, making the year 366 days long instead of the usual 365. This extra day helps our calendar stay in sync with the Earth's orbit around the Sun.

Why Do We Need Century Leap Years?

You might know that a normal leap year happens every four years. For example, 2020 and 2024 were leap years. But what about years like 1700, 1800, or 1900? These are called "century years" because they end a century.

The rule for century years is different. A century year is only a leap year if it can be divided by 400 without any remainder.

The Old Julian Calendar Problem

Before the Gregorian calendar, people used the Julian calendar. In the Julian calendar, every year that could be divided by 4 was a leap year, even century years. So, 1700, 1800, and 1900 would have been leap years in the Julian calendar.

However, this rule added too many extra days over a long time. About three extra leap days were added every 400 years. This meant the calendar slowly started to get out of sync with the Earth's actual orbit around the Sun and the seasons. For example, important dates like the start of spring would slowly shift earlier and earlier in the calendar.

The Gregorian Calendar Solution

To fix this problem, Pope Gregory XIII introduced the Gregorian calendar in 1582. This new calendar changed the rule for century years.

In the Gregorian calendar, a century year is a leap year only if it can be divided by 400. This means that out of every four century years, three of them are not leap years.

• For example, the years 1600, 2000, 2400, and 2800 are century leap years because they are perfectly divisible by 400.
• But years like 1700, 1800, 1900, 2100, 2200, and 2300 are not leap years. Even though they can be divided by 4, they cannot be divided by 400. These are called common years.

This clever change makes the average length of the calendar year much closer to the actual length of the Earth's orbit. It almost completely stops the calendar from drifting away from the seasons.

When Did Countries Start Using It?

The Gregorian calendar was introduced in 1582. But not all countries started using it at the same time. Different countries adopted it over many centuries.

This means that for dates before 1582, people generally used the Julian calendar. Also, between 1582 and when a country switched to the Gregorian calendar, things could be a bit confusing.

For example:

• The year 1700 was a leap year in places like the British and Russian empires because they were still using the Julian calendar. But in most of Europe, which had switched to the Gregorian calendar, 1700 was not a leap year.
• Similarly, 1800 and 1900 were still leap years in the Russian empire, but not in most other parts of the world.

This shows how different parts of the world followed different calendar rules for a long time!