phaseAngle function

double phaseAngle(
  1. double r,
  2. double delta,
  3. double rr
)

Phase angle from distances.

r planet-Sun, delta planet-Earth, rr Sun-Earth. All in AU.

Implementation

double phaseAngle(double r, double delta, double rr) {
  return math.acos((r * r + delta * delta - rr * rr) / (2 * r * delta));
}