Bradley Schaefer in calculating the brightness of the moon, defined the scattering angle as angular separation between the Moon and the Sky Position.

He then provided diagram as shown below, and state that P is measure along the great circle on the sky passing through the moon and zenith. Therefore, the column p=60 corresponds  to the zenith

I trying to figure out what does scattering angle mean and how to calculate it.

Would appreciate if anybody could explain them to me.