A slice of pi

Posted by Daniel Pendick
on Thursday, March 15, 2007

Yesterday was pi day. I knew because it was March 14, or 3/14, or 3.14 — the crudest approximation of (pi), the number that describes the ratio of a circle's circumference to its diameter. As most of us learned in grade school, pi is a weird number. It just goes on and on, like a Senate filibuster, as in 3.14159265358979323846 (etc., etc.). Click here to see pi calculated out to a million digits.

The other reason I knew it was pi day was that Dave Eicher, editor-in-chief of Astronomy, came back from lunch with a mischievous grin on his face and several boxed cream pies under his arm. Fearing that my ever-expanding waistline would go from saturnian to positively jovian, I passed on the pie. This morning, alas, two of the pies were staring me in the face when I arrived for work. Ah, what the heck — just one slice! It was banana cream, with real banana chunks. Yum.

But I digress. Each year, people around the world celebrate pi on March 14. Science museums often join in the fun, as did the Exploratorium in San Francisco. The festivities started at 1 P.M. with "an introduction to Pi and a special rap performance by none other than Albert Einstein (in puppet form)."

But this blog is about astronomy. What does the cosmos have to do with pi?

Not much, oddly enough. Pi reflects the dimensions of a perfect circle; yet the universe contains no perfect physical objects — no planets, moons, or stars — in the sense of pi-perfect (except maybe that banana cream pie).

However, perfect circles played a central role in the history of astronomy. To account for the apparent motions of the Sun, Moon, and planets, the Greek mathematician Apollonius of Perga (c. 262 B.C.–190 B.C.) created what came to be known as the Ptolemaic system of astronomy.  Planets moved on small circles, called epicycles, which, in turn (turn, turn) rode on larger circles, called deferents.

It made sense to the Greeks that the perfect, immutable cosmos ran as a complex clockwork of perfect circles. The cosmos, in a sense, was pi-powered. Much later, Copernicus posited a new, Sun-centered system for calculating the motions of the planets, but he still relied on epicycles and deferents. Kepler finally broke with the circle and cast his lot with elliptical orbits.

Because I have no particularly clever way to wrap up this blog, I'll just leave you with a few examples of pi humor, courtesy of the John Handley High School (Winchester, Virginia) math humor page:

Q: What do you get if you divide the circumference of a jack-o-lantern by its diameter?
A: Pumpkin pi.

Q: What do you get when you take a bovine and divide its circumference by its diameter?
A: Cow pi.

Q: What do you get when you take green cheese and divide its circumference by its diameter?
A: Moon pi.

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