## Happy Pi Day from Alan Boss and Alycia Weinberger!

Pi might be one of the most popular, interesting, and used mathematical constants ("special" numbers that just never change). It is defined as the ratio of any circle's circumference to its diameter. Pi is irrational, a number that is impossible to represent as a fraction and whose digits never end. No matter what, pi goes on and on. Forever.

Despite the endless digits of pi, most simple calculations only need a few of its digits, like 3.14. For more robust calculations, more digits of pi are needed. DTM scientists, for example, use it to study planets in our Solar System and beyond.

Scientists around the world celebrate Pi Day every year on March 14 (3/14), and today two DTM astrophysicists shared their thoughts on pi to remind us of the importance of this math symbol:

DTM's Alycia Weinberger uses ground and space telescopes to study disks around young stars such as the ones shown here from her research. Disk images courtesy of Alycia Weinberger."Pi is so integral to everything I do, it's like air—happily it's there for me, and I don't have to think about it. Anything involving a circle or sphere or ellipse or any kind of curvature winds up bringing in pi. That includes orbits of planets around stars or stars around each other (ellipses), the intrinsic and observed shapes of circumstellar disks (ellipses), the volume or mass of dust grains we see around stars (estimated as the sum of spheres of a certain density), the densities of planets (spheres), and how we actually do astronomy, since the sky appears to us as a sphere above the Earth."

A snapshot of some of Alan Boss' old code from 1979 showing the line with fourteen digits of pi. Courtesy of Alan Boss.

"I was attending a seminar as a Physics Department graduate student at U.C. Santa Barbara in 1974 when Professor Hal Lewis stopped the speaker and noted that the value of pi on the screen was incorrect in the eighth decimal place. It blew me away that someone would know the value of pi to that many decimal places. Most think pi = 3.14 or so and leave it at that. Professor Lewis later advised me that my PhD thesis project was premature, because computers were not yet fast enough to be able to calculate the three dimensional hydrodynamical models of binary star formation that I was developing. I ignored his advice about my thesis project but never forgot his ability to remember the value of pi. Because my computer code used "double precision" arithmetic, my code used the fourteen-digit value of pi = 3.1415926535898, and I memorized those fourteen digits, hoping to get a chance one day to pull the Professor Lewis stunt on a speaker. No luck, so far!"