The solution of this equation is u ( φ φ ) = − − α α m h 2 [ 1 + e cos ⁡ ⁡ ( φ φ − − φ φ 0 ) ] which shows that the orbit is a conic section of eccentricity e ; here, φ 0 is the initial angle, and the center of force is at the focus of the conic section. Using the half-angle formula for sine, this solution can also be written as u ( φ φ ) = u 1 + ( u 2 − − u 1 ) sin 2 ⁡ ⁡ ( φ φ − − φ φ 0 2 )