月球没有自转

安徽省滁州市沙河中学 陈 军

月球自转与不自转是一个简单的圆周运动问题, 历史上可能多次争论过,对月球自转与不自转产生分歧的根本原因，第一个原因，可能是对 “假平动”的认识不清楚造成的，另一个原因，就是不清楚宇宙中物体不自转的标准到底是什么？

平动是物体在一条直线上的运动，物体上各点的路线都是平行的，平动的物体是不自转的。从动力学角度看，平动是物体不受力或者受到的合力方向，与物体的运动方向在一条直线上。

什么是假平动呢？有一个很好的实际例子，可以解开了假平动的真相。

在观光的摩天轮上，人乘座的观光廂的运动，就是假平动，而刚体力学却认为是真平动，认为平动可以在曲线上运动，依据刚体力学逻辑，手端着一碗水，在做上下运动的同时又沿着水平方向的运动（波动），也叫做平动，这种假平动物体上各点的路线，即不是直线，又相互交叉在一起，这种假平动与平动是矛盾的，是错误的。从动力学角度看，假平动是物体受到的合力方向，与物体的运动方向不在一条直线上的运动，是由两种简单运动的组合而产生的运动。

观光廂一边绕着摩天轮中心公转，一边在重力的作用下，绕着上边的轴，发生了同步逆向自转，换句话说：如果将观光廂与它的上边的轴焊接死了，观光廂只能公转而无法自转。就像楼房、地面静止的篮球等物体，只绕着地轴公转，而没有发生自转。同理，月球只绕着地球公转，而没有自转。地球如果不自转，就是有一面始终面向太阳，另一面始终背向太阳，就没有昼夜交替和一天的概念。

在宇宙中，没有绝对的平动，只有绝对的公转。公转和自转都是动力学问题，它们的参照物是不可以任意选择的，自转只能相对自转轴而言，自转不是绝对的，并不是所有天体或者物体都在自转。 公转是绝对的，也是相对的，是相对公转中心的，这就是“日心说”和“地心说”的区别。

公转和自转之间，没有任何内在的必然联系，在宇宙中，虽然天体无数，但是要做到自转和公转同步，几乎是不可能的，只有人为机械控制，才可以演示出来。

在宇宙中，物体不自转的标准：其几何特征是物体有一面始终面向公转中心，另一面始终背向公转中心；其物理特征是自转力矩和自转角动量（角速度）均为零，而公转角动量（角速度）不为零。“天”上的和地上的不自转物体都是如此。平动只是此标准中，在公转半径为无穷大条件下的特例或者叫做一种理想状态，我们不能用此标准，来判断自然界中的物体，是否自转。

月球在公转的过程中，只发生了来回摆动（物理天平动，彗星最明显）现象，而没有发生连续自转现象，连续自转的物体只有自转的快慢之分，是不可能出现物理天平动的，也并不是所有的天体都在自转。在太阳系中，如果我们能发现一个同步逆向自转的天体，那真是奇迹！

The Moon No Rotation

The moon with rotation or without rotation is a simple question of circular motion. It is probably debated for many times in the history. There are two root causes for this debate. The first one may be that the understanding of "false translation" is not clear. The other one is that the standard of objects with no rotation in the universe is unknown.

Translational motion is the movement of object in a straight line. The route of each point in the object is parallel and object will not have a rotation.From the kinetic point of view,translation motion is that there is no force act on the object or the force direction and the move direction are in a straight line.

What is the false translation motion? There is a good practical example, which finally solved the false translation the truth of the false translation.

The motion of observation cab in ferris wheel is a false translation motion. However,in mechanics of rigid bodies,this motion is considered as a true translation motion and translation motion can happen in curve line. According to the logic of mechanics of rigid bodies,the motion of a bowl of water,which moves up and down at the same time along the horizontal direction (fluctuation), is also called translation motion.The route of each point in the object with a false translation motion is not a straight line and intersects with each other.This false translation motion is contradictory with translation motion and it is incorrect.From the kinetic point of view,the force direction and the move direction of the false translation motion are not in a straight line. It is consisted of two simple motions.

The observation cab orbits around the center of ferris wheel, at the same time, it has a synchronous reverse rotation around the above axle. If the observation cab and the above axle are fixed, it can only have a revolution but not rotation.Like the building and basketball which is stationary on the ground, they can only have a revolution around the Earth without rotation. Similarly, the moon orbits around the Earth without rotation. If the Earth did not rotate, one side of the Earth will face the sun all the time and the other side will opposite the sun, i.e., there is no concept of day and night in the Earth.

T h e r e i s n o a b s o l u t e translation motion in the universe but there exists absolute revolution. Revolution and rotation are dynamic problems; their references cannot be chosen discretionarily. The rotation is only relative to the spin axis and is not absolute.Not all celestial bodies or objects have rotation. However,revolution is both absolute and relative and is relative to orbiting center. This is the difference between heliocentric theory and geocentric theory.

There is no necessary inherent connection between revolution and rotation. In the universe, even though there are countless celestial bodies, the synchronization of rotation and revolution is almost impossible.It can only be presented by manmade mechanical control.

In the universe, there are two criteria of object which does not rotate. One is described in geometrical features: objects have one side always face the center of revolution and the other side always opposite the center of revolution. The other is described in physical characteristics: the rotation torque and the angular momentum(angular velocity) of object are zero; the angular momentum of revolution (angular velocity) is not zero. The objects without rotation on the ground and in the heaven are all like the foregoing statement. Translation motion is just a special case with an infinite large orbit radius in the criteria, or to say,an ideal state,we can not use this standard to determine the nature of the object, whether the rotation.

In the process of moon revolution, there is only a phenomenon of swinging back and forth (The phenomena of physical libration and comet are most obvious).There is no phenomenon of continuous rotation. The continuous rotation only has a distinction of fast and slow rotation; it is impossible for physical libration to happen.Also not all the celestial bodies rotate.

图1 月球绕地球公转，图2 小球圆周运动，小球无法自转。

图3 同步顺向自转，图4 只公转不自转，图5 同步逆向自转（假平动）。