Today is 25th Pi Day. March 14 (3.14) is Pi Day, a celebration of the ratio of a circle’s circumference to its diameter — one of the most beautiful and confounding numbers in mathematics. It’s technically written as 3.14159, or 3.14 for short.

Of course this only works in the American date format, but let’s all look past that to celebrate this crazy mathematical constant together. Bonus: it’s also Einstein’s birthday!

Even if you hate math, you’ll love Pi Day. Why? Because there’s pie.

**Why does “pi” deserve its own day?**

It’s a special number. It shows up everywhere. In chemistry, physics, math, whether you’re talking circles or cycles or anything to do with a curve, you’re going to find pi in there somewhere. Pi is irrational, which means that you can’t calculate pi by dividing an integer by any other integer. Pi never ends and it never repeats. People have to wrap their brain around that. Twenty-two over seven gives you a pretty good approximation of pi, but it’s not pi.

**History of pi (π)**

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 (circa 1900–1680 BCE) indicates a value of 3.125 for pi, which is a closer approximation.

The Rhind Papyrus (circa1650 BCE) 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 first calculation of pi was done by Archimedes of Syracuse (287– 212 BCE), one of the greatest mathematicians of the ancient world. Archimedes approximated the area of a circle by using the Pythagorean Theorem to find the areas of two regular polygons: the polygon inscribed within the circle and the polygon within which the circle was circumscribed. In this way, Archimedes showed that pi is between 3 1/7 and 3 10/71.

A similar approach was used by Zu Chongzhi (429—501), a brilliant Chinese mathematician and astronomer. He calculated the value of the ratio of the circumference of a circle to its diameter to be 355/113.

The famous Indian mathematician-astronomer Aryabhatta (476—550 CE) worked on the approximation for pi (π), and may have come to the conclusion that π is irrational. In the second part of the Aryabhatiyam (gaṇitapāda 10), he writes in Sanskrit:

caturadhikaṃ śatamaṣṭaguṇaṃ dvāṣaṣṭistathā sahasrāṇām

ayutadvayaviṣkambhasyāsanno vṛttapariṇāhaḥ.

English translation: add four to 100, multiply by eight, and then add 62,000. By this rule the circumference of a circle with a diameter of 20,000 can be approached.

This implies that the ratio of the circumference to the diameter is ((4 + 100) × 8 + 62000)/20000 = 62832/20000 = 3.1416, which is accurate to five significant figures. It is speculated that Aryabhata used the word *āsanna* (approaching), to mean that not only is this an approximation but that the value is incommensurable (or irrational). It is quite a sophisticated insight, because the irrationality of pi was proved in Europe only in 1761 by Lambert.

After Aryabhatiya was translated into Arabic (circa 820 CE), this approximation was mentioned in Al-Khwarizmi’s book on algebra

Mathematicians began using the Greek letter π in the 1700s. Introduced by William Jones in 1706, use of the symbol was popularised by Leonhard Euler, who adopted it in 1737.

**How far have we calculated pi now?**

The current record for this noble endeavor is 10,000,000,000,050 decimal places, held by Alexander Yee and Shigeru Kondo. It took them 371 days to make the calculation with a dual-core Xeon PC running 96GB of RAM and 58TB in total storage.

**It’s hard to imagine Pi Day didn’t exist until 1988**

The first party in honor of the amazing mathematical entity known as “pi” began in 1988 when Larry Shaw, a physicist at the Exploratorium, the San Francisco science museum, looked at the calendar and said, “March 14 — it’s the number pi. It’s Pi Day. Let’s celebrate!” Now Pi Day is marking its 25th anniversary. I read this in Nat Geo.

We celebrated the 25th Pi Day by dividing the round pizza diagonally before eating it in our dinner. Happy Pi Day!