March 14, 2015, is Pi Day. But this year’s math celebration is more important because 3/14/15 are the first five numbers of pi. And more happens at 9:26:53 a.m. and p.m. This only occurs once in a lifetime — or once every one hundred years.

Here’s what you need to know:

## 1. The Holiday Originated in San Francisco in 1988

The first modern version of Pi Day came to fruition in 1988, when physicist Larry Shaw decided to hold a large-scale celebration of pi on March 14 at the San Francisco Exploratorium. The San Francisco Exploratorium still holds an annual celebration of the event. It will be the 27th annual Pi Day.

According to the San Francisco Exploratorium website, events include both math related activities and pie-eating activites, with both pizza pies and standard fruit pies available. Similar activities can be found at Pi Day celebrations the world over, particularly in the United States. According to the official Pi Day website, a House Resolution passed in 2009 by Congress which officially recognized every March 14 as “supporting the designation of Pi Day.”

The long-winded resolution ended with these bullet points “That the House of Representatives”:

(1) supports the designation of a ‘Pi Day’ and its celebration around the world;

(2) recognizes the continuing importance of National Science Foundation’s math and science education programs; and

(3) encourages schools and educators to observe the day with appropriate activities that teach students about Pi and engage them about the study of mathematics.

## 2. Pi Is Constant

Pi is not just the sixteenth letter of the Greek alphabet “π”. It also the symbol used for the numerical value of pi, which is an infinite number that is usually rounded to about 3.14159. The number comes from the ratio of a circle’s circumference to its diameter.

Pi useful because it is always the ratio of the circumference of a circle to its diameter. It doesn’t matter how big or small the circle is made, the ratio stays constant.

## 3. Pi Has Been Known for Nearly 4,000 Years

Most people credit Archimedes as cementing the art of calculating pi, but pi goes back much further.

According to the San Francisco Exploratorium website, pi has been known for nearly 4,000 years. They include some of the earliest examples:

The ancient Babylonians calculated the area of a circle by taking 3 times the square of its radius, which gave a value of pi = 3. One Babylonian tablet (ca. 1900–1680 BC) indicates a value of 3.125 for pi, which is a closer approximation.

The Rhind Papyrus (ca.1650 BC) gives us insight into the mathematics of ancient Egypt. The Egyptians calculated the area of a circle by a formula that gave the approximate value of 3.1605 for pi.

The method of deriving the digits in pi was improved upon until sometime between 1340 and 1425, Indian mathematician-astronomer Madhava of Sangamagrama calculated pi to the 11th digit. But it wasn’t known as pi then. According to TechHive.com:

Later, in 1707, Welsh mathematician William Jones was the first to use the Greek letter pi (π) to denote the constant ratio, though it wasn’t until 1737 that Swiss mathematician and physicist Leonhard Euler popularized the use of the symbol. The π symbol was taken from the Greek word for “perimeter.”

The most accurate calculation of pi to date took place on October 19, 2011 when Shigeru Kondo calculated pi to 10 trillion digits, which is a world record calculation.

## 4. Pi Day 2015 Is Special

Pi Day 2015 is special because of the numbers sequence for this year’s date.

On March 14, 2015, at 9 o’clock and 26 minutes and 53 seconds, the first 10 numbers of pi will be in sequential order.

This will happen twice in countries that observe the 12-hour clock like the United States and Canada. In other parts of the world that use the 24-hour clock, this magic moment will only occur once in the morning.

The next time this will happen will be 2114, when most of us will be long gone.

## 5. There Is Another Math Holiday

/>Pi Day is by far the most popular “math holiday”, but there is another. However, the other isn’t officially recognized by the United States. Probably because of the difficulty it would entail for most people to know when Square Root Day is.

Square Root Day… is celebrated on days when both the day of the month and the month are the square root of the last two digits of the year.[1] For example, the last Square Root Day was March 3, 2009 (3/3/09), and the next Square Root Day will be April 4, 2016 (4/4/16).

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