This paper is interested in circularity definitions and algorithms which most precisely model human sentiment regarding what it means to be circular. Existing research surprisingly does not define the concept of circularity, it assumes it is apparent like the linearity concept. This paper limits its discussion to simple open shapes that can be drawn in a single stroke (edges). Can a line be regarded as part of a circle with a very large radius? Although what humans or computers may perceive to be circularly shaped seems trivial, existing computer measures disagree about the answer, giving opposite measures. Similar disagreement was observed among humans. Circularity was tested on a set of 20 open curves (edges), using both human participants and existing computer algorithms. Human measurements correlate with computer measurements based on center of gravity but not with those based on the true center.