Definition of the right-handed coordinate system for the thin-shell bow shock model. With reference to the plane of the sky, the positive x-axis points east, the positive y-axis points north, while the positive y-axis points towards the observer. θ is the polar angle from the axis of symmetry, as seen from the star at the origin. The azimuthal angle φ (not shown) is counted from the positive z-axis towards the positive y-axis. The coordinate system may be rotated over the x-axis by an angle λ counted in the same way as φ, over the y-axis by an angle PA (the position angle) counted from the positive x-axis towards the negative z-axis (i.e. south-of-east), and over the z-axis by an angle i (the inclination) counted positive from the positive x-axis towards the negative y-axis. Shown is the Wilkin curve for a standoff distance of R₀ = 1. The star is at rest and colliding head-on with a wind moving at a velocity V_w.