Little bits of a cycle

Engineers break the ac cycle down into small parts for analysis and reference. One complete cycle can be likened to a single revolution around a circle.

Degrees

One method of specifying the phase of an ac cycle is to divide it into 360 equal parts, called degrees or degrees of phase. The value 0 degrees is assigned to the point in the cycle where the magnitude is 0 and positive-going. The same point on the next cycle is given the value 360 degrees. Then halfway through the cycle is 180 degrees; a quarter cycle is 90 degrees, and so on. This is illustrated in Fig. 1.

Fig. 1: A cycle is divided into 360 degrees.

Radians

The other method of specifying phase is to divide the cycle into 6.28 equal parts. This is approximately the number of radii of a circle that can be laid end-to-end around the circumference.
A radian of phase is equal to about 57.3 degrees. This unit of phase is something you won’t often be needing to use, because it’s more common among physicists than among engineers.
Sometimes, the frequency of an ac wave is measured in radians per second, rather than in hertz (cycles per second). Because there are about 6.28 radians in a complete cycle of 360 degrees, the angular frequency of a wave, in radians per second, is equal to about 6.28 times the frequency in hertz.