Pi Day is celebrated on March 14th (3/14) around the world. Pi (Greek letter “π”) is the symbol used in mathematics to represent a constant — the ratio of the circumference of a circle to its diameter — which is approximately 3.14159.

Pi has been calculated to over one trillion digits beyond its decimal point. As an irrational and transcendental number, it will continue infinitely without repetition or pattern. While only a handful of digits are needed for typical calculations, Pi’s infinite nature makes it a fun challenge to memorize, and to computationally calculate more and more digits.

##### Pi (π) is the ratio of a circle’s circumference to its diameter. Pi is a constant number, meaning that for all circles of any size, Pi will be the same.

##### The diameter of a circle is the distance from edge to edge, measuring straight through the center. The circumference of a circle is the distance around.

### History of Pi

By measuring circular objects, it has always turned out that a circle is a little more than 3 times its width around. In the Old Testament of the Bible (1 Kings 7:23), a circular pool is referred to as being 30 cubits around, and 10 cubits across. The mathematician Archimedes used polygons with many sides to approximate circles and determined that Pi was approximately 22/7. The symbol (Greek letter “π”) was first used in 1706 by William Jones. A ‘p’ was chosen for ‘perimeter’ of circles, and the use of π became popular after it was adopted by the Swiss mathematician Leonhard Euler in 1737. In recent years, Pi has been calculated to over one trillion digits past its decimal. Only 39 digits past the decimal are needed to accurately calculate the spherical volume of our entire universe, but because of Pi’s infinite & patternless nature, it’s a fun challenge to memorize, and to computationally calculate more and more digits.

# How Albert Einstein Celebrated His Birthday

Albert Einstein was not into birthdays. The legendary theoretical physicist, born in Germany on March 14, 1879, resisted being the center of attention, telling LIFE he believed that “birthdays are for children.” But on his 74th birthday, in 1953, he made an exception.

When Yeshiva University of New York requested to build a medical school in his name, he agreed—despite feeling the same way about such honors as he did about birthdays—and attended a fundraising luncheon in his honor.

“For the occasion,” LIFE reported, “Einstein shed his characteristic baggy sweater and slacks, put on a gray suit. But he found it less easy to shed a lifetime of shyness.” He failed to notice the triple-tier birthday cake presented to him by Cake Bakers Union Local 51. He eyed a slab of roast beef and objected, “This is for lions.”

When all was said and done, the luncheon had raised $3.5 million for what is now the Albert Einstein College of Medicine. “I am glad it is over,” was all the honoree had to say about it. He died two years later, on April 18, 1955.

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