The main axis of the planet's orbital ellipse shifts each time when it goes around the sun.
当行星绕太阳运行时，其轨道椭圆的主轴每次都会改变。
The axis perpendicular to the main axis is called the cross axis. It extends in the cross dimension.
和主轴垂直的轴叫做交叉轴，它在交叉轴方向上延伸。
The axis perpendicular to the main axis is called the cross axis. It extends in the cross dimension.
和主轴垂直的轴叫做侧轴，它在侧轴方向上延伸。
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