It can be calculated using recursive formulae, but you need a time difference between the position now and the next step, the larger the time interval the less acurate the results, and as it's a chaotic system and sensitive to minor errors no matter how small you make the time interval there will always be a small error which will lead to wrong results later on, this wasn't the point I'm making however, I was trying to explain why the bodies in his simulation had unpredictable orbits.