Consider a particle moving on a circle of radius A with a constant angular speed ω as shown in figure.

Suppose the particle is at point P_o on the circle, at t = 0. The radius OP make an angle θ = (ωt + φ) with the X-axis at time t. Drop a perpendicular PM on X-axis and drop a perpendicular PN on Y-axis. Let us trace the path followed by the points M and N with time, as particle P moves with constant angular speed on the circular path. We find, both the points M and N makes oscillatory motion, about the mean position at the center of the circle.