A particle moves from point 0 to point 2. Both positions are known. The center of the circle and it's radius is also known. Im trying to work out the position of 1 where the particle strikes the circle. This is for a 2D astronomy simulator to work out where a particle will strike a 2D representation of Earth. I've exhausted my brain and Im hoping you guys can lend a hand. I've tried using SecTheta but its an imperfect solution, just like Tan Theta. Purple lines and points are known magnitudes and positions and any other colors are unknowns. The way the simulator works is it uses the Euler method to increment the position of a moving object, so the particle will move from say point 0 to point 2 between a computer cycle. The time passed typically is 0.1 seconds. Please help. Kind Regards, David And may the force be with u.