Pi, you will recall, is the ratio of the circumference of a circle to its diameter. For most practical uses, the value of pi can be taken as 3.14, or, if added precision is needed, 3.1416. But each of these values contains a slight error. In fact, any value that is taken for pi will contain a slight error, for pi is both irrational and transcendental, which means, among other things, that its decimal expansion can be carried out forever without repeating or coming to an end.

Since Ptolemy calculated pi to four decimal places in 150 A.D., efforts have been under way to add more and more digits to it. The history of the calculation of pi is itself an interesting story. Suffice to say that the advent of electronic computers gave the enterprise a big boost. In 1976, a computer calculated a million decimal digits of pi, and by last year, about 10 million decimals were known.

Now comes word from NASA's Ames Research Center near San Francisco that a newly installed Cray-2 supercomputer there, using a program written by David H. Bailey, has calculated pi to 29,360,000 decimal places, a number that would fill more than a thousand pages of this newspaper from top to bottom. It took the machine--the world's fastest computer--some 28 hours to do the 12 trillion arithmetic operations needed to reach this result.