March 14 is Pi Day — here’s what that means

Some circles. Wikimedia Commons
  • March 14, written out as 3/14 in the US and a few other countries, is π day, as those are the first three digits of the famous number.
  • π is one of the most important and fascinating numbers in maths.

Happy π day!

March 14, when written out in the American date style of month/day, comes out as 3/14. That coincides with the three first digits of π: 3.14.

π is one of the most important numbers in maths. As you may recall from basic geometry, π is defined as the ratio of a circle’s circumference to its diameter. So, anywhere where circles or cyclical things show up, π tends to show up as well. It would be impossible to do geometry, trigonometry, calculus, analysis of waves, or most branches of maths without the concept of π.

As a number, π is also itself fascinating. π is an irrational number – it can’t be written as a fraction of two integers, no matter what integers you choose. One consequence of that is that the infinite decimal expansion of π never adopts a repeating pattern.

π is actually a step further than an irrational number, and is what mathematicians call a transcendental number. There are plenty of irrational numbers – the square root of 2, or 3, or any prime number is irrational, as are cube roots and higher-power roots of primes. Transcendental numbers, like π, aren’t the roots of any rational number. Another way of saying that is that π raised to any power is still irrational.

Even though circles, and therefore π, have been a subject of study for millennia – the great Greek mathematician Archimedes came up with a clever way of approximating π by drawing polygons inside and outside a circle to estimate the circle’s circumference – π was only proven to be irrational in the 18th century, and transcendental in the 19th.

π day comes with a few caveats, the biggest being that most of the world doesn’t write March 14 as 3/14, but instead as 14/3. Still, for residents of the countries that do, π day offers a fun chance to reflect on a very special number.

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