Because the Sun rotates, one side is coming toward us at 2 km/s and the other is going away. The figure is a map of the speed of the Sun's surface as measured by the SOHO/MDI instrument; the colors indicate motion toward us (dark) or away (lighter). Superimposed on the dominant left-right rotational gradient, you can see smaller scale features. These features are caused by solar phenomena including supergranulation, granulation, and p modes. The small bright feature right of center was caused by an active region where the magnetic field at the surface was very strong. MDI measures the velocity and brightness at nearly a million points on the Sun's surface every minute.