Figure showing the coordinate transformation from the origin-equator of the ICRF (xyz axis in the figure) into the equinox-ecliptic (x′y′′z′′ in the figure) of the epoch. First, we proceed with the rotation R_z(ϕ) (ϕ being the bias between the x-axis of the ICRF, and the equinox of the epoch, located at the vernal point γ). To obtain the ecliptic coordinates, we proceed with a second rotation R_x′(ϵ) (ϵ being the obliquity).