Happy Pi Day! How are you celebrating the transcendental, irrational mathematical constant central derived from circles on 3/14/15 at 9:26:53? For me, it's going to be giggling over physicists engaging in an epic chalk battle, and devouring an apple-ginger pie.

*Pi Day is the best reason ever to bake a pie for breakfast. Image credit: Mika McKinnon*

Pi Day is an annual celebration where mathematical enthusiasts take advantage of the North American custom of writing the date month-day corresponding to the first few digits of pi. (Don't follow this custom with your dates? We can work with that.) This year is particularly exciting as even the year is cooperating for the next few digits, getting us all the way out to 3.141592653. Why celebrate Pi Day? Because it's fun. (Unless you're feeling Grinchy...)

The number pi is a simple geometric ratio: divide the distance around a circle (the circumference) by the distance across it (the diameter). No matter the size of the circle, the ratio will be the same: pi.

*The Earth is circular, to first approximation. Image credit: **Allen Pope** via **NASA*

The number itself is irrational and transcendental. Consequently, it continues forever without repeating. Although it only takes the first 40 digits to provide enough accuracy for the most demanding scientific tasks, mischievous humans have calculated it to a few trillion digits for the simple reason that they can. And, of course, that means that the few and proud will undertake epic feats of memorization, like Matt Parker and Tibra Ali at the Perimeter Institute:

How can you celebrate?

- Check your local museums, observatories, and other science-oriented locations for events today.
- Take on the challenge problems crafted by scientists and engineers at Jet Propulsion Laboratory based on their own real-world experiences using pi.
- Appreciate the ways in which pi shapes our universe. (Hint: Circles are surprisingly pervasive!) Bonus: pi and the asteroid Bennu.
- Dive into these core equations that rely on pi to explain the every-day.
- Find your personal pi day, the location in pi's never-ending stream of digits that contains the number sequence of your birthday.
- Bust out edible geometry by indulging in circular foodstuffs. The pun-inspired traditional celebratory food is of course a pie, but a pizza is equally circular, maintains the right starting letters, and has its own groan-worthy joke.
- Get hands-on with these at-home activities using pi developed by founders of Pi Day, the Exploratorium.
- Approximate pi with Buffon's Needle Problem with paper and a handful of needles (No needles? Here's a simulator.), or use the Monte Carlo method during a darts game.
- Want to bring a Pi Day celebration into the classroom? Edutopia complied a list of resources specifically for teachers. (They also provided the tip on the excellent musical-pi video embedded above.)
- On the musical theme, glory in a Pi-composed playlist by adding in this piece on violin playing pi to 220 decimals (sheet music) or this piano piece in A harmonic (sheet music).
- While celebrating, take a moment to toast a pair of historic birthdays of people who certainly knew how to use the number to its fullest: Albert Einstein and Apollo astronaut Frank Borman were both born on this date.
- Learn trivia about the fascinating number. Really weird trivia.
- Read the Tau Manifesto and join the debate of which is more celebration-worthy: pi or 2pi.

As for me, my celebrations started a day early with friends and family checking in to share plans of what flavour pies they were baking and swap recipe tips.

*These circles and sines make it enchantingly clear how even complex signals can be approximated with a sufficiently-detailed Fourier Transform. Image credit: **Charles Deluga*

The celebration continued this morning with a constant stream of incoming photo-messages of naturally-occurring circles and curves in unexpected places and mesmerizing geometrical gifs.

*These fossilized stromatolites in Nunavut, Canada are beautifully circular domes. Image credit: **Marc St-Onge/Natural Resources Canada*

Do you have more ideas, or already started your celebrations? Share your adventures with us! Bonus points if you include photos of your numerically-inspired celebrations. Do you see any particularly great pi tributes today? Tell us about them, too! If you were caught flat-footed and missed your chance to celebrate at 9:26:53am this morning, bust out your math-happy self at 9:26:53pm instead.