kottke.org: In 1424, Persian astronomer and mathematician Jamshid al-Kashi calculated pi to 17 digits. via JPL
'Mathematicians have calculated
pi out to more than 13 trillion decimal places, a calculation that took
208 days. NASA's Marc Rayman explains that in order to send out probes
and slingshot them accurately throughout the solar system, NASA needs to use only 15 decimal places, or 3.141592653589793.'