In this chapter we briefly discuss most of the remaining perturbing forces. None of them have an explicit
time dependence, which simplifies the resulting differential equations for the orbital elements (making
them autonomous). Furthermore, the results are developed only to the first-order-in-ε accuracy. With the
exception of the distant-source radiation, all perturbing forces discussed here act within the orbital plane;
we can thus treat all quantities as complex (no quaternions needed) — this also eliminates θ and φ from
our orbital-element list. Finally (with the same single exception), the perturbing forces involve no special,
fixed direction, which eliminates ψ from the right hand side of the resulting differential equations, making
them functions of a and β only.
