Uniform Circular Motion . Kepler's First Law: All planets move about the sun in an elliptical orbit with the sun at one foci. ... In the above P is the orbital period measured in years and A is the semi-major axis measured in units of AU (the distance from the earth to the Sun). ... The discovery of retrograde motion in the orbit of mars was a serious challenge to the standard geocentric cosmology which demanded that all the planets orbit the earth in the same direction. ...