# Leap Day: A Unit Study

It happens once every four years: February 29th! The leap day of leap years — those years ending in a number divisible by four…well, sort of.

The earth takes 365.24219 days to travel around the sun (equinox to equinox). It is this trip that gives us our year: 365 days each year. And then there is leap year, when we pick up an extra day in our calendar. Why? It’s that extra 0.24219 of a day. So every four years we add one day, which brings us close to 0.24219 x 4 or 0.96876 or roughly 1 day.

It’s that “roughly 1 day” part that prompted Pope Gregory XIII to change the calendar. The Julian calendar created in AD 325 was ten days off! The Gregorian calendar, the one we currently use, follows these rules:

• Leap year occurs every four years in years that are evenly divisible by four (2016/4=504).
• There is no leap year in years ending with 00 unless the first two digits of the year are evenly divisible by 4 (1800 was not a leap year).

So happy Leap Day! Celebrate with a few of the fun finds below.

##### Further Investigation

Happy Leap Day!
Article explaining the day at Time for Kids.

Why Do We Have Leap Year?
A brief explanation from the History Channel.

##### Activities

Leap Year Calculations
An interactive calculator to determine if a year is a leap year.

Leap Day
Calculate the probability that a baby will be born on Leap Day.

##### Unit Studies & Lesson Plans

Celebrate Leap Day!
Lesson plan from Scholastic that looks at Leap Day math and … frogs.

Gregorian Calendar: A Unit Study
More about leap year along with lots of calendar activities, printables, and more!

##### Printables & Notebooking Pages

Leap Year 2016 Printables