Leap Years
Leap Year
How to know if a year will be a Leap Year:

Leap Years are any year that can be evenly divide by 4 (such as 2004, 2008, etc) 


except if it can can be evenly divide by 100, then it won't be (such as 2100, 2200, etc) 



except if it can can be evenly divide by 400, then it will be (such as 2000, 2400) 
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 accurate, but will still be wrong by about 1 day every 100 years.
So every 100 years we don't have a leap year, and that results in 365.24 days per year (1 day less in 100 year = 0.01 days per year).
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 Aren't?
So 1600, 2000 and 2400 are leap years but 1700, 1800, 1900, 2100, 2200 and 2300 are not.
Apart from that, every year divisible by 4 (2004, 2008, 2012, etc.) is a leap year.
How far away each year is from the average 

As you can see, 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!)
