Angle of incidence Ѳ the angle between solar beam and surface normal.
For the solar beam
(Solar) zenith angle Ѳz. The angle between the solar beam and the vertical
Note that Ѳz and Ѳ are not usually in the same plane.
Solar altitude αs (= 90⁰ − Ѳz). The complement to the (solar) zenith angle; angle of solar beam to the horizontal.
Sun (solar) azimuth angle γs. Projected on the horizontal plane, the angle between the solar beam and the longitude meridian. Sign convention is as for γ. Therefore, on the horizontal plane, the angle between the beam and the surface is (γs – γ).
(Solar) hour angle ω (as in (4.4)). The angle Earth has rotated since solar noon (when γs = 0 in the northern hemisphere).