Geometry of a perfectly aligned spinning prolate particle with long-side symmetry axis a. The sky is in the xy plane. The angle Φ (red) is between the x axis and the projection of a onto the plane of the sky. The sightline is the z axis, with electromagnetic wavevector (k) approaching us. The magnetic field (B) lies in the yz plane with the magnetic field angle (Ω) measured from z towards B. The grain spinning plane is perpendicular to B (dashed blue circle) and includes a. The grain spinning angle (φ; blue) is measured on the x axis. The angle of incidence (α) is given between a and k. Note the relation between Ω, α, and φ as given in Eq. (3).