Schematic diagram. In left panel, the primary (P, here Haumea) and the secondary (S, here Hi’iaka) rotate mutually around their center of mass C_M , but only their photocenter C_L is visible and measurable. Thus our observed trajectory of the photocenter will be its rotation motion around the center of mass C_M along an approximate ellipse when the center of mass is chosen as a reference origin. The angular separation of the secondary relative to its primary is supposed to be d (it is usually less than the atmospheric seeing for a ground-based telescope, thus only an unresolved image is observed) and its projections to right ascension and declination are (x_c, y_c), respectively. Correspondingly the separation between the photocenter and the center of mass is Δ. In right panel, the center of mass in the ephemeris for the primary and its secondary component is also shown as the reference origin. Here, the true center of mass C_M has an offset (A, D) from . In addition, an ellipse is drawn for the trajectory of the photocenter C_L going around the center of mass C_M. The projections of the angle separation Δ in right ascension and declination are Δ_x and Δ_y, respectively.