The vector relationships are shown in Figure 1. The axis of rotation is shown as a vector ω perpendicular to the plane of the orbit and with a magnitude ω = dθ / dt. The direction of ω is chosen using the right-hand rule. With this convention for depicting rotation, the velocity is given by a vector cross product as v = ω ω × × r , which is a vector perpendicular to both ω and r (t), tangential to the orbit, and of magnitude ω r. Likewise, the acceleration is given by a = ω ω × × v = ω ω × × ( ω ω × × r ) , which is a vector perpendicular to both ω and v (t) of magnitude ω | v | = ω r and directed exactly opposite to r (t).