I wrote a spreadsheet that simulates the trajectory of 3 planets in 2D space. The method is simple: for each moment in space, calculate the force felt and velocity of each planet, then for the next $\delta$t period, assume that the planet continues to move with the same acceleration. Then recalculate the position, velocity, and acceleration of each object. Proceeds until desired time frame is simulated.
The computation works roughly correct, but there is a problem with it, that is: I can not produce closed curves typical seen in a planet's orbit. Regardless of what velocity and mass I chose, the orbit's radius always gradually increases.
This is probably caused by the approximation method I chose. I am wondering, what are some improvements I can make to the numerical method?