# Leap Years

* ..., 2008, 2012, 2016, 2020, 2024, 2028, ... etc*

## Leap Year

- A normal year has 365 days.

- A
**Leap Year**has 366 days (the extra day is the 29th of February).

## How to know if it is a Leap Year:

Leap Years are any year that can be exactly divided by 4 (such as 2016, 2020, 2024, etc) |
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except if it can be exactly divided by 100, then it isn't (such as 2100, 2200, etc) |
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it can be except ifexactly divided by 400, then it is (such as 2000, 2400) |

Try it here:

## Why?

Because the Earth rotates about **365.242375** times a year ...

... but a normal year is **365** days, ...

... so something has to be done to "catch up" the extra **0.242375** days a year.

So every 4th year we add an extra day (the 29th of February), which makes **365.25** days a year. This is fairly close, but is wrong by about 1 day every 100 years.

So every 100 years we **don't** have a leap year, and that gets us **365.24** days per year (1 day less in 100 year = -0.01 days per year). Closer, but still not accurate enough!

So another rule says that every 400 years **is** a leap year again. This gets us **365.2425** days per year (1 day regained every 400 years = 0.0025 days per year), which is close enough to 365.242375 not to matter much.

## So, Which Are and Which Aren't?

So 2000 and 2400 **are ** leap years but 1800, 1900, 2100, 2200 and 2300 **are not**.

Apart from that, **every year divisible by 4** (2012, 2016, 2020, 2024, etc.) is a leap year.

How far away each year is from the average

*Example: look just before 2100, the worst year is 1.2 days ahead, but because 2100 is not a leap year they all get adjusted back by 1 day.*

So this keeps us pretty close, and any other adjustments can be done way in the future (when the Earth may be rotating a little slower, anyway!)

## Only since 1582 (Gregorian Calendar)

These leap year rules were introduced in **1582** by the **Gregorian Calendar**, named after Pope Gregory XIII.

(It replaced the old **Julian Calendar** by Julius Caesar that only has one rule of a leap year every 4th year, and is now about 13 days behind our current date.)