As someone who only studied first year physics and maths and have taken no interest in it since than I was rather surprised to wake up one morning and realise along with 99.5% of the population that I really had no idea how the planets orbits worked beyond the vague word ellipse which I didn't really understand. http://en.wikipedia.org/wiki/Keplerian_harmonic_law#Kepler.27s_first_law Is Kepler's Law correct? Or rather is it roughly that simple with very minor perterbutions due to other planets' gravity? From my memory the 17th century observations revolved around comets that had heavily ellipitical orbits with the Sun foci very much as one end. Equally, the Earth appears to go around the Sun at at least an approximately circular orbit with no easily observable perigee or apogee. Are the other planets like this? Or do they have large recognisable elipse shapes? I thought that celestial mechanics had been worked out in the 18th century with some relatively simple and robust equations that allowed us (or them) to work out the position of the planets at every point in history. I have briefly browsed this forum and seems that there are some highly knowledgable people here and I am sure this is not the first time this question has been asked. Hopefully an answer or directions to a thread that explain this simply for a non-expert might be available?