Your forearm as a compass

In My Humble Opinion
1

When I was in high school, my Solid Geometry teacher (Mr. McAfee) could draw a near-perfect chalk circle on the blackboard by pivoting his forearm around his elbow . . . and he could adjust the size of the circle as well. It only took a second or two, and his elbow appeared to never leave the blackboard. I can only get about 200 degrees, before it’s no longer a circle. I have no idea what his trick was. Could he have been double-jointed in some way? Or possibly an elbow injury enable this rare talent? Or is it some arcane talent known only to mathematicians?

Can any of you do this, and if so, how?

2

I can’t even picture someone performing this in one go, let alone try it myself.

Was he an alien? He must’ve taken his elbow off the blackboard at some point? Or maybe he traced out as much of a circle as possible, and just became really good at completing the missing arc freehand.

3

My high school physics teacher could do the same thing. Weird-looking as hell.

4

Humans do it too.

5

But he was using his entire arm. That’s cheating.

6

Yes his elbow was clearly not touching the board at the end and I think it never was.

7

Wow - thought it was going to be a joke - that’s impressive.

8

Holy crap!

9

I came for a discussion on how, if you stand facing east and stick your arm straight out from your body, then your forearms point to north and south.

On the other hand, drawing a circle like that is pretty cool!