Let us set up a conventional Cartesian coordinate system
which is such that the (apparent) orbit of the
Sun about the Earth–Moon barycenter lies in the - plane. This implies that the
- plane corresponds to the so-called ecliptic plane. Approximating the solar orbit as a low-eccentricity Keplerian orbit,
we can write
where
, and
is the orbital eccentricity (Yoder 1995). (See Section 4.13.) Here, we have
neglected terms that are
, and smaller.