It’s Pi Day (March 14th) so I figured I’d briefly talk about circles and pi. First, let’s talk define a circle.

Okay, so a long time ago people wondered how to find the area of a circle, it’s tricky because square units don’t fit within nicely unless they are teeny tiny. People found that if you square the radius (distance from center of circle to any point on the circle) and multiply that number by about 3 you get a pretty good estimate for the area of the circle. I made a quick video to show one way you might show this to students, though there are other ways to do this. This means that the area of a circle divided by the radius squared is about 3.

Later folks came to determine a more precise value of this ratio (disclaimer: I’m not a historian so I encourage you to look up more on the history of pi). They found that the circumference (distance around the circle) to the diameter was the same value and that value is what we now know as pi (approximated commonly by 3.14). So if you have a tiny circle or a huge circle, whenever you take the circumference and divide it by the diameter you get pi (and this value is the same as the area divided by the square of the radius). A good question to pose to young students is to take a piece of string across a circle and ask them how long they think the string would have to be stretched to go around the outside of the circle (circumference). Then do this with all kinds of different sizes of circles and let them experiment. They should notice that it is always just a bit more than three times the diameter string.

So basically Pi Day is a fun excuse to talk about circles and pi and eat pie. If you want to visit other resources to learn more about Pi see the links below.

- What is Pi, and How Did it Originate? (Steven Bogart, Scientific American)
- A Brief History of Pi (The Exploratorium)
- Celebrate Pi Day with NCTM (more activities for Pi Day)

Remember, pizza also comes in pie form and is my favorite type of pie!