axis complex of a quadric

例句：

1. 1.The axis of the planet revolves around its center.
   行星的轴围绕其中心旋转。
2. 2.The spindle of the needle is made of steel.
   针的轴由钢制成。
3. 3.The axis of the Earth's rotation is in the North Pole.
   地球自转的轴位于北极。