This means that the acceleration vector r ¨ ¨ of any planet obeying Kepler's first and second law satisfies the inverse square law r ¨ ¨ = − − α α r 2 r ^ ^ where α α = n 2 a 3 is a constant, and r ^ ^ is the unit vector pointing from the Sun towards the planet, and r is the distance between the planet and the Sun.