Rolling circle

[Genady]

I've made a mistake by posting this puzzle in the other thread (https://www.scienceforums.net/topic/131412-puzzle-for-the-day/?do=findComment&comment=1239451). It does not belong there. Thus, I repost it here and copy relevant comments from that thread. Hope to continue discussing this puzzle here:

@TheVat:

Seemed easy.

I'm on a tablet so the hidden button doesn't seem to appear.  It can be figured without recourse to formulae.

@Genady:

It is an easy one to make a mistake, too.

@TheVat:

Spoiler

Yep. One has to see there are two kinds of revolving, one with respect to circle B and one with respect to us, looking down on the page.  

It's a nice frame of reference puzzle. 

@Genady:

Spoiler

Exactly. Like a solar day vs. sidereal day.