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To study orbital motion, you need to use Kepler's laws, which will help you calculate how different factors affect the trajectory of an orbiting body (the object that is orbiting another). Kepler's first law determines that the orbiting body's orbit is not a circle, but an ellipse. Because of this, when the orbiting body is closer to its primary body (the object that it is orbiting), its angular velocity will increase, and when it's further away, it will slow: this is Kepler's second law. Finally, Kepler's third law states that there is a ratio between how far an orbiting body is from its primary body, and how fast it is going. This ratio depends on the size and mass of the central body of the orbit. So, the further the orbiting body is from the primary body, the slower it is going to orbit.