Geometry of the conversion of the given observed coordinates of a body, A, relative to a comparison body, B. The north poles of the original and target reference systems (ORS and ICRS) are P and P′, respectively. and are the Right Ascensions of B and P referenced to the ICRS, and δ_B and are the Declinations of B referenced to the ORS and ICRS, respectively. , , p = ∢PBA, and p′=∢P′BA.