In order to get an equation for the orbit from equation (1), we need to eliminate time. (See also Binet equation.) In polar coordinates, this would express the distance r of the orbiting object from the center as a function of its angle θ θ . However, it is easier to introduce the auxiliary variable u = 1 / r and to express u as a function of θ θ . Derivatives of r with respect to time may be rewritten as derivatives of u with respect to angle.