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This article was originally published on March 14, 2021.

March 14th is Pi Day – what does it mean? A holiday to celebrate a number (and eat a lot of pie)? Well, yes, but pi is not an arbitrary number. There is much more to the meaning of pi day than that.

Why is Pi Day celebrated on March 14?

Pi is simply (well, not very simple, but bear with me) the ratio of the circumference to the diameter of a circle. If you take a circle – any kind circle — and divide its circumference by its diameter, you’ll get the same answer every time: a number that’s just over three. Stop at two decimal places and you have 3.14, which is why we celebrate Pi Day on March 14th.

But the ratio of diameter to circumference is not exactly 3.14. Pi is an irrational number, meaning it cannot be expressed as a simple fraction. When you express it as a decimal, you get an infinite, non-repeating sequence of numbers that have no pattern at all. So the value of pi is actually 3.141592654 and so on… forever.

Read more: Here are the exact numbers for Pi to 100,000 digits

Meaning of Pi: A very old number

Pi was discovered at least 4,000 years ago. It is not known exactly who first scored this special number, but historians say both Babylonian and Egyptian cultures used the concept (although they didn’t nail down the first few decimal places correctly).

Then, around 250 BC, the Greek mathematician Archimedes of Syracuse developed a method to obtain a much closer approximation of pi. He inscribed a polygon in a circle, then placed the circle inside another polygon. He used to Pythagorean theorem to find the areas of the two polygons. This gave him upper and lower bounds for the area of ​​the circle. He started with a hexagon, then repeatedly increased the number of sides of the polygons. With each iteration, the polygons got a little closer to the size of the circle they enclosed, a technique called the depletion method, or as mathematician Stephen Strogatz calls it, the “squeezing technique.” Using this method, Archimedes was able to show that pi is between 3 1/7 and 3 10/71, or in decimal, 3.1429 and 3.1408.

Pi symbol

Over the centuries, Chinese, Indian, and Arab mathematicians continued to calculate pi with more and more decimal places. In 1706, the Welsh mathematician William Jones came up with the idea of ​​using the Greek letter π or pi to represent this special constant. Isaac Newton worked out how to calculate pi to 16 digits. In the early 20th century, the Indian mathematician Srinivasa Ramanujan came up with even more efficient ways to calculate pi. And now, in the 21st century, computers are doing the work (using in part algorithms developed by Ramanujan). In January 2020, cybersecurity analyst Timothy Mullican calculated pi to 50 trillion places, breaking the 2019 record of $31 trillion seats held by computer scientist Emma Haruka Iwao.

Pi is useful for more than finding the face and circumference of a circle. It appears in all sorts of places, in almost any calculation that involves curves. Pi is used in fields from construction to quantum physics, from music theory to medicine. Humans would never have reached the moon without pi. These days we probably wouldn’t have made it across town; pi is essential to GPS systems because, of course, the Earth is round.

What is Pi Day and why do we celebrate it?

It was Pi Day itself spawn of the Exploratorium in San Francisco in 1988 and is now an international holiday. People celebrate in all sorts of ways: trying to calculate pi po throwing toothpicks on the floor (annoying but doable), competing to see how many decimal places of pi they can remember, and of course, baking and eating pies.

But you have to wonder: Why exactly does this number get people—even those who aren’t particularly math-savvy—excited enough to have a holiday in its honor? Really, why do we celebrate pi day? It might be because besides being useful, pi is a bit spooky and very mysterious which makes pi day more special.

in Infinite Powers: How Calculus Unlocks the Secrets of the UniverseStrogatz puts it this way:

“There is something so paradoxical about pi. On the one hand, it represents order embodied by the shape of a circle, long considered a symbol of perfection and eternity. Pi, on the other hand, is unruly, disheveled in appearance, its digits obeying no obvious rule, or at least none that we can perceive. Pi is elusive and mysterious, forever out of reach. Its combination of order and disorder is what makes it so enchanting.

As computers get faster and faster, we’ll get closer and closer to fully uncovering pi — but we’ll never get there. Nobody ever will. And that’s one of the coolest things about the pi. But if that’s not reason enough to celebrate, March 14 is also Albert Einstein’s birthday.

Read more: 5 interesting things about Albert Einstein

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