Hello, I'm trying to really understand orbits. I want to be able to calculate the velocity at any given point in an orbit. Now, parametrically an ellipse can be: x = a*cos(t) y = b*sin(t) If those are position, can I take the derivative to obtain velocity? x' = -a*sin(t) y' = b*cos(t) For the overall velocity: V = sqrt( (-a*sin(t))^2 + (b*cos(t))^2) However, there is a pesky t in there, now I use: x' = -a*sin(t) Solve for t -x'/a = sin(t) asin(-x'/a) = t And subsitute t back into overall equation? Does this make sense or am I just making stuff up?