Mean Elements considering just the short period terms from the Brouwer-Lyddane theory. Included perturbations are the J2, J3, J4 and J5 zonal harmonics including terms involving J2^2. These elements are not time averages of the osculating values, but are the results of a specific averaging procedure defined by the theory. In the Brouwer theory, a variation of Delaunay elements are averaged and are therefore the actual mean elements. Other "mean" elements are derived using of osculating orbit element conversions on the computed mean elements. Mean elements are computed in the selected reference frames by first transforming the osculating state to the selected frame then applying the mean element conversion. The available coordinate systems depend on the object's central body. Nominally, the systems Fixed, Inertial, J2000, TrueOfDate, and MeanOfDate are supported, although some central bodies (notably the Earth and Sun) have more. The following lists the systems available for Earth.