Pi (or ) is the ratio of the circumference of a circle to its diameter. This ratio is the same for every circle--for instance, if you double the diameter of a circle, you double its circumference as well. The observation that this ratio is constant has been known so long that historians cannot say when it was first discovered. It was certainly known by the year 2000 b.c., when the Babylonians estimated its value at 3 1/8, and the Egyptians at 3.1605. Another early estimate of the value of pi, although a less accurate one, is found in the Old Testament, which puts the value at 3 in the following passage about the great temple of Solomon: "And he made a molten sea, ten cubits from the one brim to the other: it was round all about, and his height was five cubits: and a line of thirty cubits did compass it about...