Sine and cosine waves. Animation showing the relationship between a circle (red) and the phases of sine (purple) and cosine (green) waves. The sine wave describes a repetitive oscillation of constant amplitude. As the wheel rotates, the attached rods trace the waves. The maximum amplitude (height) of the sine wave is equal to the radius of the wheel. The amplitude at any given time is dependent on the sine of the angle that the wheel's radius makes with the horizontal plane. The cosine wave lags behind the sine wave at a rate of half pi.