Note that E and M are in units of radians in this computation. This iteration is repeated until desired accuracy is obtained (e.g. when f ( E ) < desired accuracy). For most elliptical orbits an initial value of E 0 = M ( t ) is sufficient. For orbits with e > 0.8 , a initial value of E 0 = π π can be used. Numerous works developed accurate (but also more complex) start guesses. If e is identically 1, then the derivative of f , which is in the denominator of Newton's method, can get close to zero, making derivative-based methods such as Newton-Raphson, secant, or regula falsi numerically unstable. In that case, the bisection method will provide guaranteed convergence, particularly since the solution can be bounded in a small initial interval. On modern computers, it is possible to achieve 4 or 5 digits of accuracy in 17 to 18 iterations. A similar approach can be used for the hyperbolic form of Kepler's equation. In the case of a parabolic trajectory, Barker's equation is used.