Phase angle
Phase angle is defines as the fraction of the time interval that has lapsed since the particle has crossed the mean position of rest in the positive direction. It is also equal to the angle swept by the radius vector since the vibrating particle last crossed its mean position of rest.
For a particle vibrating simple harmonically, the displacement is
Here, 
or () is the phase angle.
or (
) is the phase angle.Epoch
The initial phase angle when t=0 , is called the epoch. Thus α is called the epoch in the given equation of displacement.
