月球沒有自轉(zhuǎn)

安徽省滁州市沙河中學(xué) 陳 軍

月球自轉(zhuǎn)與不自轉(zhuǎn)是一個(gè)簡(jiǎn)單的圓周運(yùn)動(dòng)問題, 歷史上可能多次爭(zhēng)論過,對(duì)月球自轉(zhuǎn)與不自轉(zhuǎn)產(chǎn)生分歧的根本原因，第一個(gè)原因，可能是對(duì) “假平動(dòng)”的認(rèn)識(shí)不清楚造成的，另一個(gè)原因，就是不清楚宇宙中物體不自轉(zhuǎn)的標(biāo)準(zhǔn)到底是什么？

平動(dòng)是物體在一條直線上的運(yùn)動(dòng)，物體上各點(diǎn)的路線都是平行的，平動(dòng)的物體是不自轉(zhuǎn)的。從動(dòng)力學(xué)角度看，平動(dòng)是物體不受力或者受到的合力方向，與物體的運(yùn)動(dòng)方向在一條直線上。

什么是假平動(dòng)呢？有一個(gè)很好的實(shí)際例子，可以解開了假平動(dòng)的真相。

在觀光的摩天輪上，人乘座的觀光廂的運(yùn)動(dòng)，就是假平動(dòng)，而剛體力學(xué)卻認(rèn)為是真平動(dòng)，認(rèn)為平動(dòng)可以在曲線上運(yùn)動(dòng)，依據(jù)剛體力學(xué)邏輯，手端著一碗水，在做上下運(yùn)動(dòng)的同時(shí)又沿著水平方向的運(yùn)動(dòng)（波動(dòng)），也叫做平動(dòng)，這種假平動(dòng)物體上各點(diǎn)的路線，即不是直線，又相互交叉在一起，這種假平動(dòng)與平動(dòng)是矛盾的，是錯(cuò)誤的。從動(dòng)力學(xué)角度看，假平動(dòng)是物體受到的合力方向，與物體的運(yùn)動(dòng)方向不在一條直線上的運(yùn)動(dòng)，是由兩種簡(jiǎn)單運(yùn)動(dòng)的組合而產(chǎn)生的運(yùn)動(dòng)。

觀光廂一邊繞著摩天輪中心公轉(zhuǎn)，一邊在重力的作用下，繞著上邊的軸，發(fā)生了同步逆向自轉(zhuǎn)，換句話說：如果將觀光廂與它的上邊的軸焊接死了，觀光廂只能公轉(zhuǎn)而無法自轉(zhuǎn)。就像樓房、地面靜止的籃球等物體，只繞著地軸公轉(zhuǎn)，而沒有發(fā)生自轉(zhuǎn)。同理，月球只繞著地球公轉(zhuǎn)，而沒有自轉(zhuǎn)。地球如果不自轉(zhuǎn)，就是有一面始終面向太陽(yáng)，另一面始終背向太陽(yáng)，就沒有晝夜交替和一天的概念。

在宇宙中，沒有絕對(duì)的平動(dòng)，只有絕對(duì)的公轉(zhuǎn)。公轉(zhuǎn)和自轉(zhuǎn)都是動(dòng)力學(xué)問題，它們的參照物是不可以任意選擇的，自轉(zhuǎn)只能相對(duì)自轉(zhuǎn)軸而言，自轉(zhuǎn)不是絕對(duì)的，并不是所有天體或者物體都在自轉(zhuǎn)。 公轉(zhuǎn)是絕對(duì)的，也是相對(duì)的，是相對(duì)公轉(zhuǎn)中心的，這就是“日心說”和“地心說”的區(qū)別。

公轉(zhuǎn)和自轉(zhuǎn)之間，沒有任何內(nèi)在的必然聯(lián)系，在宇宙中，雖然天體無數(shù)，但是要做到自轉(zhuǎn)和公轉(zhuǎn)同步，幾乎是不可能的，只有人為機(jī)械控制，才可以演示出來。

在宇宙中，物體不自轉(zhuǎn)的標(biāo)準(zhǔn)：其幾何特征是物體有一面始終面向公轉(zhuǎn)中心，另一面始終背向公轉(zhuǎn)中心；其物理特征是自轉(zhuǎn)力矩和自轉(zhuǎn)角動(dòng)量（角速度）均為零，而公轉(zhuǎn)角動(dòng)量（角速度）不為零。“天”上的和地上的不自轉(zhuǎn)物體都是如此。平動(dòng)只是此標(biāo)準(zhǔn)中，在公轉(zhuǎn)半徑為無窮大條件下的特例或者叫做一種理想狀態(tài)，我們不能用此標(biāo)準(zhǔn)，來判斷自然界中的物體，是否自轉(zhuǎn)。

月球在公轉(zhuǎn)的過程中，只發(fā)生了來回?cái)[動(dòng)（物理天平動(dòng)，彗星最明顯）現(xiàn)象，而沒有發(fā)生連續(xù)自轉(zhuǎn)現(xiàn)象，連續(xù)自轉(zhuǎn)的物體只有自轉(zhuǎn)的快慢之分，是不可能出現(xiàn)物理天平動(dòng)的，也并不是所有的天體都在自轉(zhuǎn)。在太陽(yáng)系中，如果我們能發(fā)現(xiàn)一個(gè)同步逆向自轉(zhuǎn)的天體，那真是奇跡！

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.

圖1 月球繞地球公轉(zhuǎn)，圖2 小球圓周運(yùn)動(dòng)，小球無法自轉(zhuǎn)。

圖3 同步順向自轉(zhuǎn)，圖4 只公轉(zhuǎn)不自轉(zhuǎn)，圖5 同步逆向自轉(zhuǎn)（假平動(dòng)）。