Hi. I'm just a hobbier of astronomy and have a question about elliptical orbit. In Before, I had calculated the elipse of a planet successfully. In this case, I wonder that can calculate velocity(red vector) using orbit time(or position on elipse). Let me assume that we know orbit's semi-major, minor axis(properties of black elipse), orbit period and green dot's location or elapsed time from passing perihelion or aphelion. I had thought that just differentiate the orbit elipse by time t but differentiate elipse is impossible operation as I know. so can't catch clue of this problem. Would anyone give me some hints?