一天文学家要确定一棵小行星的运动轨道，他在其轨道内建立了以太阳为原点的直角坐标系，由开普勒第一定律知小行星的运动轨道为一椭圆，试确定行星的运动轨道。-One day in literature to determine the orbit of an asteroid, he set up in its orbit to the sun for the origin of the coordinate system, by Kepler s first law of motion track known asteroid is an ellipse, the orbits of planets.