The number you get when you divide the circumference of a circle by its diameter is roughly 3.14, or pi -- so today, March 14 (or 3/14) is Pi Day, David Blatner, the author of "The Joy of Pi," writes in BBC News online.

Pi Day, coincidentally or not, is also the birthday of Albert Einstein, who probably knew a thing or two about pi, Blatner writes.

As math buffs know, 3.14 is just a start, and a surprising number of students have memorized 50 or even 100 digits beyond the decimal point, according to Blatner.

But pi, the 16th letter of the Greek alphabet (π) and the world's most widely known mathematical constant, also represents a deep universal mystery, Blatner writes.

How, he asks, is it that something this basic, this fundamental to mathematics and science, can be so incredibly difficult to pin down?

"In fact, it's literally impossible to know what pi is, because its digits rattle off into infinity," Blatner said.

Despite 3,500 years of solving the puzzle of pi or "squaring the circle" -- calculating the exact ratio of a circle's circumference to its diameter -- no one has been able to find anything but a new, perhaps more precisely expressed, approximation.

So, according to Blatner, Pi Day is a time to honor "the essential truth that there are some things we simply cannot know. We can only get close to knowing."

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