Happy (belated) Pi-Day

Over a year ago, I promised Diana of MyGurumi a post about how to use pi or π to knit a circle in any stitch pattern and any gauge. I would also show how to estimate the length of yarn you need to finish a row based on pi. I thought that pi day, March 14, would be a good day to time the post.

This pi day found me in bed with a 100F fever and simultaneous viral, fungal and bacterial infections. Immune deficiency sucks. If there is one thing I can not stress enough to young people, it is the importance of selecting your grandparents very carefully.

Well, it's a bit late and I have a burning need to write about pi in the context of circle skirts instead of knitting. Fortunately, Diana lives in Europe and they celebrate pi day on July 22. (Leave a comment if you know why!) So check back on July 22 if you are a knitter.

Fellow Cal grad Shams wrote a Pie Day post mentioning that she used to bake special pies for her kids' lunchboxes on pi day, and Iris asked me why I didn't bake her pies for pi day. Um, is it because you turned down all the pies I have baked to date with, "I am not a pie person. I am more of a brownie person...and a cake person." ???

What really set me off was Gertie's March 14 post, Are you a Right Brained or Left Brained? Normally, I enjoy her blog very much. But this post was so irritating I wanted to scream.

Let's just dispel the myth right now. Humans use our whole brains, if we are fortunate enough to have an intact brain.

Because of research with stroke and accident patients, we know that, for most people (but not all), language processing happens mainly on the left side of the brain and spatial processing happens mainly on the right side.

We also know that the dominant hand tends to be opposite the dominant side of the brain.

Math is strongly spatial. Although less than 10% of the population is left-handed, a significantly higher percentage of mathematicians are left-handed.

I recall noticing in an honors math class (Complex Analysis) at Cal that the professor and 5 of the 7 students were left-handed. I am a crypto-left-hander because teachers in Taiwan use corporal punishment to ensure that all the kids write with their right hand*. So really, there was only one right-hander in the room; the rest were left-hand dominant and, likely, right brain dominant.

So Gertie had the right-left math brain thing exactly backwards.

Furthermore, does she mean to imply that math and science do not require creativity?

The part that most made me cringe was when she flashed her "I'm worse at math than you" credentials. She couldn't master third grade arithmetic! Please tell me that is an exaggeration. Please. It brings back bad memories of Barbie math.

Ironically, she chose March 14th, pi day, to write:

I've been writing instructions for drafting a full circle, half circle, and 3/4 circle skirt to go in my book. Readers, this involves math. And not just any math: this is algebra and stuff.
...
Indeed, sewing always involves some sort of math. It can be simple, like adding 2" to a skirt to make it knee length. Or it can be pretty involved, like figuring out the radius of a 3/4 circle skirt.

Actually, the math is not very difficult if you understand the concept of pi.

Pi is simply the ratio of the circumference of a circle divided by the diameter. That's it.

It's a constant.

Use your sewing measuring tape and measure the circumferences and diameters of a bunch of round objects. Calculate the ratio for each object. They should be all just about (but not exactly) pi, because of small measurement errors and manufacturing flaws. Then average the ratios. You should get pretty close to pi.

My fifth grade teacher made it extra interesting. She had us calculate the ratio for each object individually and average them. Then we summed all the circumferences and divided by the sum of all the diameters. The two calculations agreed--a lesson in number theory. By calculating the ratios individually, we had another lesson in statistics. One object, a trash can with a dent in one side, was an outlier; it's ratio was less than the others.

Now that the kids are getting older younger, my daughter learned about pi in fourth grade. So drafting a circle skirt should be child's play, or about fourth grade level.

I drew out how to calculate the inside diameter of a full circle skirt using your waist measurement.

diameter = waist/pi = waist/3.14

It's a little more complicated for a fraction of a full circle, but not by much. Say you want a 3/4 circle skirt, your fraction, f = 3/4.

diameter = waist/(pi*f)

An easy way to check is to note that, the smaller a fraction of a full circle you use for your skirt, the larger the diameter has to be to go around your waist. In math speak, it's an inverse relationship. The diameter of a half circle skirt is double the diameter for a full circle skirt (given the same waist measurement).

It's pretty easy.

It's even easier if you have the right tools. I use a yardstick compass attachment on a metal yardstick (actually, it's a full meter).

I bought them from a sewing catalog years ago, but you can get them at Dick Blick for less. Use the links above to find them.

I'm going to show a gratuitous shot of my 2/3 circle skirt. If you are sewing with a stripe or plaid, it may look nicer if you don't make a perfect circle at the waist. The sides should be about 1/2 inch longer than the center front/back so dip out your circle at the center front/back. This will make your circumference slightly bigger (more room for dessert!), and then you will ease that back into the waistband. Easy peasy. Barely any algebra. And pink!


* I am totally not bitter about it. I love being ambidextrous and able to solve problems with a larger than average toolkit. Additionally, I can type like the wind and switch hit in volleyball. And volleyball is a very important part of growing up in California.

Read the pi-day series.