March 14: Pi day

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The Greek symbol π, pronounced "pie", is used to refer the ratio of the diameter d of a circle to its circumference c.

c = π * d = π d

The radius r of a circle is half the diameter d, so the following holds.

c = π * d = π*2*r = 2 π * r = 2 π r

Numbers such as π are transcendental and, like irrational numbers, have no exact representation and can only be approximated.

More: Mathematical constants and Hebrew pi

Transcendental numbers can be approximated by a rational number as the ratio of two integers.

Consider the transcendental number π and the approximation of 1 over π which provides a ratio between 0.0 and 1.0. The smallest set of integers that gets close to π, from a brute-force analysis, is 22/7.

Pi approximations

More: Mathematical constants and Hebrew pi

In the United States, March 14, or 3/14, is PI day, from the Greek π symbol for the ratio of the circumference to diameter of a circle. In Europe, Pi day is July 22 or 22/7, from the Archimedes approximation of pi, from the 3rd century BC, of 22/7.

Month	day	Europe	United States
March	14	14/3	3/14
July	22	22/7	7/22

Europe puts the day first, then the month.
The United States puts the month first, then the day (not the military).

Albert Einstein (1879-1955) was born on pi day on March 14, 1879.

Albert Einstein was born on March 14., 1879. This day of the year, 3/14, is Pi day in the United States.

More: Albert Einstein