We’re counting down to our favorite math holiday—**Pi (π) Day**, which takes place every year on **March 14** (3/14 = the first three digits of pi)! Math enthusiasts the world over celebrate Pi Day every year, and with all this hullabaloo, you may be wondering… *what exactly is pi*?

**Pi is the ratio of a circle’s circumference (the distance around the circle) to its diameter (the distance across the circle)**, expressed as C/d (where C is the circumference and d is the diameter). When you take the distance around the circle and divide it by the distance across the circle, you always get the same number—pi! It doesn’t matter how big the circle is—

*it could be as small as a dime or as big as the sun*—the distance around divided by the distance across always comes out the same number. Which brings us to our next pi fact…

**Pi is a mathematical constant. **As the term implies, “constants” are fixed quantities—they don’t change in value within their given context.

**Pi is also an irrational number. **This means that it cannot be expressed exactly as a common fraction. Furthermore, while you may find patterns of some sort in irrational numbers, these patterns don’t repeat. Every irrational number—including pi—can be written as a **non-repeating, non-terminating decimal**. In late 2013, a computer successfully calculated pi to 12.1 trillion digits, stopping only because it ran out of disk space! Per the Guinness Book of World Records, the current (human) record holder (as of March 21, 2015) for the most digits of pi memorized is Rajveer Meena of Vellore, India, who memorized a whopping **70,000 digits of pi**. PiDay.org has pi written out to the millionth digit—how many can* you* memorize?

Many mathematicians and scientists throughout ancient and modern history have conceptualized and worked with pi! References to the ratio have been found in the Bible, and the Ancient Babylonians, Egyptians, Greeks, and Chinese have also approximated values for pi. Most notably, the Ancient Greek mathematician Archimedes used polygons and circles to arrive at a geometric interpretation of how to calculate the value of pi. Throughout history, pi’s been used in calculations by astronomers, architects, engineers, computer programmers, sociologists, statisticians, physicists… the list goes on. **Indeed, the world as we know it would be very, very different without pi!** Read about how pi is used to find new planets, and more.

Considering that pi is written as a non-terminating decimal, just how many digits of pi do we really need for all these calculations? While 3.14159… will do for most schoolwork, this awesome video shows how 39 digits of pi will suffice when calculating very large or very small things, from the width of a hydrogen atom all the way to the circumference of the observable universe!

How will you celebrate Pi Day? Pizza, pie, fun, and games are on our agenda—check out our ultimate Pi Day Party Guide!

P.S. We're running a social media Pi Day Trivia Contest! **Follow us on Facebook, Twitter, Google+, Pinterest, & Instagram; answer any of the Pi Day trivia questions posted between Monday, March 7 and Friday, March 11; and email your answers to ilovemath@mathnasium.com before Sunday, March 13 for a chance to win an awesome Mathnasium prize pack! We'll announce all winners on Pi Day, March 14!**