Hi Is "orbital velocity" supposed to be the same as "rotational velocity"? it seems that a "rotation curve" is supposed to plot the rotational velocity of a star, but then some articles e.g. http://en.wikipedia.org/wiki/Galaxy_rotation_problem claim "orbital speed" is plotted. The equation for rotational velocity seems to be in the shape of V sin(i) but the equation for orbital velocity doesn't depend on sin. Any help will be much appreciated. Thanks!