How did Giotto draw a perfect circle freehand?

Like this

Who said it was freehand? There are many ways he could have done this—like the old pencil tied to a string tied to a nail trick.

@JeanPaulSartre: Well-researched.

I don’t know if it was Giotto, but my dad said somebody put his elbow in one spot on the chalkboard and used the length between the chalk and the elbow as the radius. He would have to turn backwards halfway. I can’t remember who it was, but obviously Giotto could have done it that way.

@JeanPaulSartre Wow!

It’s on google.

I saw a Japanese Zen roshi do it with a large brush on a piece of paper that was probably 8 or 10 feet wide. It was part of his practice to focus well enough to do that. It was not only a full circle but a full beautiful circle like this.

My physics professor used to draw perfect circles all the time in the same way that @JeanPaulSartre linked to, he just used his elbow as the pivot point and whipped his arm around. It was pretty cool.

@ChocolateReigns Ahh beat me to it :).