It’s not hard to figure out why March 14 is celebrated annually as “Pi Day” by math fans: The date resembles the ratio of a circle’s circumference to its diameter — the number that begins with 3.14, perhaps better known Pi ( π ), which the holiday’s official website describes as an “irrational and transcendental number” whose decimals “continue infinitely without repetition or pattern.”

What *was* hard to figure out was the number itself.

Ancient Greek mathematician Archimedes of Syracuse, who lived in the third century B.C. and is considered the greatest mathematician of the ancient world, is credited with doing the first calculation of pi.

However, not too many generations after his lifetime, the world experienced a “real decline in math,” according to John Conway, mathematics professor emeritus at Princeton University who once won the school’s Pi Day pie-eating contest. “Math and science in general went into a great decline from roughly the year zero to the year 1,000, and then the Arabs developed lots of math after that, like trigonometry.”

As the Mathematical Association of America (MAA) explains, modern arithmetic techniques, which were probably discovered in India before the fifth century, took centuries to spread throughout Europe: “Even though the Indo-Arabic system, as it is now known, was introduced to Europeans first by Gerbert of Aurillac (c. 946–1003, who became Pope Sylvester II in 999) in the 10th century, and again, in greater detail and more successfully, by Fibonacci in the early 13th century, Europe was slow to adopt it, hampering progress in both science and commerce.”

Ancient research on real numbers likely “didn’t get improved upon until the age of Newton,” says Conway. Sir Isaac Newton recorded 16 digits of pi in 1665, later admitting that he was “ashamed” of how long he had worked on the computations, as it meant that he had “no other business at the time,” per the MAA.

It was not until the 18th century — about two millennia after the significance of the number 3.14 was first calculated by Archimedes — that the name “pi” was first used to denote the number. In other words, the Greek letter used to represent the idea was not actually picked by the Ancient Greeks who discovered it.

British mathematician William Jones came up with the Greek letter and symbol for the figure in 1706, and it was popularized by Swiss mathematician Leonhard Euler, Catherine the Great’s mathematician, a few decades later.

“Euler was a much better mathematician than the people who used [pi] before, and he wrote very good textbooks,” says Conway. “He used it because the Greek letter Pi corresponds with the letter ‘P’… and pi is about the perimeter of the circle.”

Pi Day as holiday for math whizzes to eat pie and dress up in pi-themed hats and costumes originated much later, about 30 years ago, at the Exploratorium, a science museum in San Francisco, where physicist Larry Shaw organized such a celebration. (The day also happens to be Albert Einstein’s birthday.) In 2009, the U.S. House of Representatives passed a resolution designating March 14 as “National Pi Day” to encourage “schools and educators to observe the day with appropriate activities that teach students about Pi and engage them about the study of mathematics.”