= 3.14159 26535 89793 23846 26433 83279 50288 41971 ...
is the ratio of the circumference of any convex curve of constant diameter to that diameter. Usually a circle is stated, but this more precise definition includes e.g. Wankel rotors, and others of that ilk with more than the Wankel's three lobes :-)
In the bible's old testament, I Kings 7:23 implies that = 3.
Ahmes, an egyptian scribe, wrote that the area of a circle is like a square on 8/9 of its diameter, thus giving = 3.16049...
Archimedes used a 96-sided polygon to limit to between 3 + 10/71 (3.14085...) and 3 + 10/70 (3.142857...) . Archimedes also gave the schoolchild's approximation 22/7 (3.142857...)
Ptolomy worked with = 377/120 (3.1416...)
About 480 AD in China, Tsu Ch'ung-Chi gave = 355/113 (3.1415929...) which is 7 digits of accuracy.
Not until the 15th century did Al-Kashi reach 16 digits. Ludolph van Ceulen (1540-1610) reached 20, then 32, then 35 digits. They are carved on his gravestone in the church in Leyden.
In 1706 John Machin calculated 100 digits.
Finally, Johann Lambertz (1728-1777) proved that is irrational (i.e. is not expressible as a fraction). His personal best approximation was 1,019,514,486,099,146 / 324,521,540,032,945. Here is a shorter proof that is irrational.
In 1853 William Shanks calculated to 707 places, but made a mistake in the 528th position. All these calculations so far were done manually. The error was not found until 1945 when Ferguson used a desk calculator.
Starting in 1949 computers were used, ENIAC managing 2037 places in 70 hours runtime :-)
In 1897 in the State of Indiana the House of Representatives (in House Bill 246) proposed to rule that =3. I guess US politicians were just as stupid then as they are now, and provably so ;-)
Now go visit my blog please, or look at other interesting maths stuff :-)