The position polar coordinates (r, θ) can now be written as a Cartesian vector p = r ⟨ cos ⁡ ⁡ θ θ , sin ⁡ ⁡ θ θ ⟩ and the Cartesian velocity vector can then be calculated as v = μ μ a r ⟨ − − sin ⁡ ⁡ E , 1 − − ε ε 2 cos ⁡ ⁡ E ⟩ , where μ μ is the standard gravitational parameter.