Do you like pi? I’m not talking about fruit or custard in a round pastry shell like those made with apples, cherries, or lemon meringue.

I am talking about a mathematical constant that shows up almost everywhere in our understanding of the universe, symbolized by the Greek letter “Ï€”. This is pronounced “pi.” It is the ratio of the diameter of a circle to its circumference, or more plainly, the ratio of how tall a circle is to the distance traveled when it rolled exactly once around.

Every year, Pi Day is observed on 3/14, since 3.14 are the first three digits of pi. But twice today, 3/14/2015, we’ll get to see the clock hit the first 10 digits of pi — 3.141592653 — an occurrence that won’t happen again for another 100 years.

More on this in a sec.

The Old Testament of the Bible approximates it with the number 3. The ancient Babylonians represented it with the fraction 25/8. The ancient Egyptians evaluated it at the fraction 16/9 squared. Archimedes approximated it at 22/7 and the Chinese calculated its value to be 355/113 in the 5th Century CE. It is commonly taught to be about 3.14 in math texts in the United States.

None of these answers are correct. Pi is a number whose exact value can be calculated forever. It is a type of number that is expressed in a list of digits that do nothing but go on to infinity and do not repeat… to the best of human knowledge.

This most unique and beautiful of numbers is celebrated by the mathematically minded every March 14th or 3-14. Get it? It’s called Pi Day.

Pi Day is epic this year. It’s the year 2015, and the date is commonly expressed as 3-14-15. That’s two more digits of Pi than usual. If we extend that even further to include the time of day, 26 minutes and 53 seconds after 9 o’clock will be 3-14-15 9:26:53. That represents a value of pi that is 3.141592653. That’s a better approximation of pi than has been known for most of human history.

Pi touches our modern lives everywhere, in things big and small. It describes the electricity flowing through the national power grid. It is used to analyze electronic signals… like telephone calls and digital music. It shows up in quantum physics, helping to describe the nature of reality. It can be used to predict the sweep of a clock pendulum, and the patterns you see if you look down the rows of a corn field while driving past it in a car.

So, celebrate this most epic of Pi Days! Remember this number and eat some pie (the dessert kind)!

Pi around the web:

– www.piday.org
– HAPPY PI DAY: MAKE A VENN PIE-AGRAM!
– Help kids solve the mystery of Pi with this activity
– Sir Cumference and the Dragon of Pi (A Math Adventure)
– Pi Day 2015: a sweet treat for maths fans
– Microsoft is celebrating Pi Day with an Xbox Music Pass deal

Also, shout out to the great Albert Einstein, born March 14, 1879.