A body that is at two different locations on a circular path with radius $$r$$. At one time, the body is at point $$S_1$$ and has orbital velocity $$\boldsymbol{v}_1$$ there. After a short period of time, the body has moved further along the circular path and is now at the point $$S_2$$. At this point it has a different orbital velocity $$\boldsymbol{v}_2$$, because the direction of the velocity has changed. The body has traveled the angle $$\delta \varphi$$.