We start with a simpler case first, the Earth-Sun system in two dimensions only. The gravitational force $F_G$ is $$F=\frac{GM_{\odot}M_E}{r^2},$$ where $G$ is the gravitational constant, $$M_E=6\times 10^{24}\mathrm{Kg},$$ the mass of Earth, $$M_{\odot}=2\times 10^{30}\mathrm{Kg},$$ the mass of the Sun and $$r=1.5\times 10^{11}\mathrm{m},$$ is the distance between Earth and the Sun. The latter defines what we call an astronomical unit AU. From Newton's second law we have then for the $x$ direction $$\frac{d^2x}{dt^2}=\frac{F_{x}}{M_E},$$ and $$\frac{d^2y}{dt^2}=\frac{F_{y}}{M_E},$$ for the $y$ direction.