Dear Johsan,

One of my favorite holidays is Pi Day. On March 14, people who love the math constant called pi celebrate by eating the other kind of pie. Like apple pie, pumpkin pie and even pizza pie.

I talked about the number pi with my friend Kristin Lesseig. She studies how kids learn math.

She told me pi is the ratio between the distance around a circle and the distance across a circle. A ratio is the relationship between two numbers. We usually think of pi as about 3.14—but there’s more to it than that.

“Pi is a fabulous ratio that’s constant across every single circle,” Lesseig said. “It doesn’t matter how big or how small the circle is. That ratio is always a wee bit more than three.”

Lesseig teaches this idea using licorice laces. Her students draw circles then stretch licorice laces across the middle of their circles. They cut several lengths of licorice equal to the distance. Then, they wrap those pieces around the circles. For every circle, it takes three pieces plus a little more to go around the circle.

One way to look at that relationship is with division. Let’s say the distance around a circle—its circumference—is 6.28 inches. The distance across the circle—its diameter—is 2 inches. If we divide 6.28 by 2, the answer is 3.14. That’s pi rounded to two decimal places.

Using that short version of pi is easy and works fine for simple problems. If you’ve ever used the pi button on a calculator, that probably uses pi rounded to ten decimal places. That’s 3.1415926535.

As we build more powerful computers, we’re able to calculate pi with more precision. Right now, scientists know pi to 63 trillion places. That’s 63 trillion numbers after the decimal. But pi goes on forever.

I remember lying in my nest as a kitten, trying to wrap my brain around the idea of forever. I would think as far as I could then double it. Then double it again and again—until I felt dizzy with the bigness of it.

“The idea of infinity is so cool,” Lesseig said. “And it’s not something that you have to wait for college to study. You can make sense of it now.”

But pi is rad because it’s useful, too. People use pi to figure out the distance around a circle or how much space is inside it. Let’s say you had a circular garden. You could use pi to determine how much fence you need. Or how much soil will fill the garden.

Space scientists use pi to work out how big planets are, what asteroids are made of and the size of fuel tanks they need to power a spacecraft. Back on Earth, people use pi to aim satellites, figure out what size water heater they need and understand how motors work.

To me, the most amazing thing about pi is that it’s a pattern in nature and that ancient people noticed that pattern and used it to explore the world around them. Scientists are still doing that today—and they’ll keep doing it as long as there are people thinking about the universe.

That’s something you can count on.

Sincerely,

Dr. Universe