What does "the arc of the moral universe is long but it bends toward justice" (said by Martin Luther King) mean in terms of the actual meaning of the word "arc?" I mean, I understand that Dr. King meant that justice was a long time coming more or less but would arrive eventually.
What I don't get is why this hypothetical path of evolving justice is an arc, that is, a portion of the circumference of a circle.
If you started at the beginning of any arbitrary segment of a circumference the curve would begin at a low point, rise to a high point, then descend to the identical original level, if you consider the arc to have been cut out of the circumference and examined on a line with which it intersects only at its beginning and ending point (like a rainbow on the horizon). This configuration is typically the conception of an arc, for example, in cathedrals, in bridges (if you will allow that an arch is the architectural implementation of an arc).
This would be a metaphor for a world beginning with no justice, rising/evolving to widespread justice, then descending to chaos and brutality again.
Am I being obtuse or is there some problem with the metaphor and it is simply not mentioned?