 However, precise long-term observations show that the constellations change shape, albeit very slowly, and that each star has an independent motion.This motion is caused by the movement of the stars relative to the Sun and Solar System.The Sun travels in a nearly circular orbit (the "solar circle") about the center of the Milky Way at a speed of about 220 km/s at a radius of from the center, which can be taken as the rate of rotation of the Milky Way itself at this radius.