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In this animation the ellipse and eccentricity parameters of the orbit of the smaller mass, `m_1`, are adjustable by the learner . In the link below I compute the rate of change, `d/dttheta`, of the angle `theta` that describes the position of `m_1` with respect to the focus of its ellipse. We integrate `d/dttheta` to obtain `theta`, and using the value, `r`, of the distance of `m_1` from its focus, the position in the (x,y) plane of `m_1` is obtained.
The position versus time of `m_1` is then shown by a moving circle. Since the center of mass of the large mass, `m_2`, and `m_1` must not move, the position of the larger mass is just the negative of the product of mass ratio `m_1/m_2` times the vector position of `m_1`. It will probably be noted that `m_1` strays from its (black) supposed elliptical path when the eccentricity is large. That is due to the very simple way that the program integrates `d/dttheta` to obtain `theta`. With a longer period this straying is greatly reduced.
Click Here for Orbit Equations