Happy birthday, Einstein! But this is also Pi Day, and a very special Pi Day at that.

Why so special? The date is 3/14/15, the first five digits in the ratio of the circumference of a circle to its diameter.

And—get ready for this—at 26 minutes and 53 seconds after 9 a.m., we will pass a date and time represented by the first *10* digits of π. Very exciting! If you miss it, there will be one more chance at 9 p.m. After that, you’ll have to wait another 100 years for that auspicious moment to happen again, assuming that we will still be writing our Gregorian calendar dates as we do in America.* It ought to be a great moment for conspiracy theorists; they are missing a terrific opportunity to warn us about cables snapping on suspension bridges, oil rig draw-works no longer pumping in Texas, and countless highway accidents when the mechanisms of rack and pinion steering freeze.

At that moment will π continue to be representable as an infinite sum, an infinite product, and an infinitely repeated fraction? Will it still be connected to the wave formulas for light and sound? Will it still tell us which colors should appear in a rainbow, and how middle C should sound on a piano? Will Albert Einstein’s energy-mass equation continue its connection to the curvature of space time? Will it still pop up in so many seemingly unrelated places?

And most importantly, will we still be amusingly able to closely approximate it by tossing a gazillion nails on a wood planked floor? Will it be part of the normalizing constant in distribution tools of the Gallup polls soon to predict who might be our 45^{th} president? And will it still be entrenched in Heisenberg’s uncertainty principle, the equation that tells us just how precisely we can ever know the state of the universe?

Of course it will. March 14 is a marker, just a day; the next is the anniversary of the assassination of Julius Caesar. We don’t celebrate the Ides of March, at least not officially. So why celebrate π? What is so special about π that makes us devote part of a school day to it? Aren’t there other constants in the mathematician’s arsenal to parade in schools across the country? There’s the golden ratio *ϕ* = 1.61803398874 … that so often shows up in nature and could be celebrated on Jan. 6. There’s *e* = 2.7182818284 … the base for the natural logarithms, just as ubiquitous and just as wondrous as π, ready for Feb. 7. And there are plenty of other interesting constants, but none are easy ratios of simple geometric objects coming with the excuse of eating pie.

Blame it on Larry Shaw, the physicist who started celebrating Pi Day at the San Francisco Exploratorium in 1988. It was his idea to celebrate the day by eating pies and marching around circular spaces. In 2009 it became official, when House Resolution 224 of the first session of the U.S. 111^{th} Congress was passed, designating every March 14 as a day to encourage “schools and educators to observe the day with appropriate activities that teach students about Pi and engage them about the study of mathematics.” But somewhere along the way a silent E crept in to bring us to culinary merriment.

Our pi-themed movement has detoured through pie-themed activities. We now have pie-eating, pie-baking, and pie-throwing contests. The words,* pi* and *pie* as they come to us through etymological history, have nothing to do with each other. *Pi* is simply the English language spelling of the 16^{th} letter of the ancient Greek alphabet. We know it as π, the notational symbol first used by William Jones in the 18^{th} century to denote the ratio of the circumference to the diameter of a circle. *Pie*, on the other hand, comes to us from … well, it’s not really known where. Even the *Oxford English Dictionary* is stumped, except to say that it might come from the medieval Latin word for pastry, *pia*.

And with pie eating and pie recipes comes π recitations. It’s all fun, yeah, but I defy anyone to give me the defensible educational advantage of having kids memorize as many of π’s infinite string of digits as possible. Why have any more than the first six digits encoded in the head when you can get them off your smartphone, which nowadays is alarmingly smart?

Some people spend a lifetime calculating π. Ludolph van Ceulen did, and got it to a 35 decimals, 3.14159265358979323846264338327950288. But that was in the 17^{th} century when calculations were always cleverly, yet excruciatingly, done by hand.

Today—at this most momentous Pi Day—π has been calculated to 10 trillion decimal places, far less than the spaceship *Enterprise*’s madly ambitious attempt to get to the nonexistent last decimal. You don’t need to get there to whiff a trace of Pi, Givenchy’s eau de toilette. And you certainly don’t need to get there in order to know π’s aura.

You might ask, what’s so special about π that urged Congress to agree on something? Yeah, it’s a simple ratio of a simple geometric object. Maybe that’s impressive. But as the mathematical video artist Vi Hart told us last Pi Day when her satirical YouTube doodling rant went viral, “Pi is not special. Yeah, pi can be fun, and I would never deny your dessert, but why not try some real food once in a while?”

I’m with Vi, but I’m not the Grinch. I’m all for means of engaging students in learning math. But ask a school kid how he or she was engaged in math on Pi Day and invariably you will hear, “We ate pie and memorized digits.” Ask what π is, and you are likely to get, “Something to do with a circle.”

Want kids to really know what π is? Have them try to bake a square cake of the same height and area of a circular pie. They will find that they cannot measure the cake and pie with the same ruler, exactly. They would learn that there is absolutely no relationship between π and pie! Yes, most pies are circular, but that’s where the analogy ends. You bake pie. You esteem π, or maybe you don’t.

**Correction, March 16, 2015: **This article originally misstated the type of calendar we use in the United States. It is the Gregorian calendar, not Julian. (Return.)