This quantity corresponds to the first integral c0 (“c-sub-zero”) in the double-averaged restricted three-body problem. Approximate solutions to the secular Kozai Hamiltonian are most commonly found by perturbatively expanding it as a function of a ratio of two forms of this quantity. In the Kepler problem, this quantity equals negative standard gravitational parameter over two times specific orbital energy. Averaging orbital distance over the (*) eccentric anomaly yields this quantity. One over this quantity is subtracted from two over distance on the right hand side of the vis-viva (10[1])equation. (10[1])This quantity (10[1])equals (10[2])the (10[1])average (10[1])of apoapsis (10[1])and periapsis. (10[4]-5[1])The (10[1]-5[1])cube (10[1]-5[1])of this quantity is proportional (10[1])to the square of (10[1])orbital period, according to (-5[1])Kepler’s (-5[1])Third (10[1])Law. For 10 points, (-5[1])name this quantity equal to half an elliptical orbit’s longest diameter. (10[2])■END■ (10[2]0[2])