Isosceles spherical triangle formed by the z-axis, the jet direction at t = 0 which is on the x–z plane, and the disk axis at t = −t′ which rotates an angle ϕ around the z-axis from the x–z plane. Another spherical triangle formed by the z-axis, the disk axis at t = −t′, and the crossing point of the small circle depicted by the disk wind edge around the disk axis with the x–z plane is also drawn for the calculations in appendix  1. The inclination angle of the precession axis from the z-axis is θ_p, the angular radius of the disk edge circle is θ_d and the angle of the crossing point from the z-axis is θ.