# Pi Day: What Is Pi?

Pi Day is celebrated each year on March 14, or 3/14. This year, we’re celebrating this famous number with a week-long series all about pi.

Here is some background on pi:

- Pi is a constant that describes the ratio of a circle’s circumference to its diameter. For all circles, that ratio is a constant.
- Pi is not just irrational (when written as a decimal, its digits never end and never hit a never-ending repeating pattern), but it’s also transcendental (never mind what that means, but it’s pretty special and has nothing to do with Ralph Waldo Emerson or Henry David Thoreau).
- Pi comes up in a ton of mathematical formulas. Honestly, it’s pretty startling how often it comes up.

For more detail, check out Scientific American’s “What is pi, and how did it originate?”

The video below has some good animations and explanations.

*This post was updated from the original version.*

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### Paul Thomas

Paul Thomas is director of mathematics for K12. Previously, he taught mathematics and computer science at the Thomas Jefferson High School for Science and Technology in Fairfax County, Virginia. While at Jefferson, he was a contributing author for the mathematics textbook series Mathematics: Modeling Our World. Paul also has managed corporate training departments, and developed classroom, computer-based, and web-based training products that have been used by companies such as Oracle, IBM, Microsoft, and Novell. He holds a B.S. in Mathematics and an M.S. in Operations Research from George Mason University.