They seem to have been deliberately selected and placed at specific points in the circle that mark certain calendrical events (sunsets and solstices related to the four seasons, for example).
The negative shows that the acceleration is pointed towards the center of the circle (opposite the radius), hence it is called centripetal (i.e. center-seeking).
He stressed that the rhumb line distance is an acceptable approximation for true great circle distance for courses of short or moderate distance, particularly at lower latitudes.
It becomes sensible from an initial quiescence or emptiness (wuji, sometimes symbolized by an empty circle), and continues moving until quiescence is reached again.
