Past Orientation of the Lunar Spin Axis
Past Orientation of the Lunar Spin Axis
复制标题
月球自转轴过去的方向
作者：
W. Ward
月球自转轴的方向是从地月系统的早期历史追溯到现在的。月球在地球上掀起的潮汐造成了月球轨道的扩张。地球在月球上掀起的潮汐使月球脱离了同步自转，并将其自转轴驱动到卡西尼状态-即在共凹构形中，与月球轨道法线和法线共面拉普拉斯平面(目前与法线到黄道面重合)。这些事件的结合导致了月球自转轴的复杂历史。在其轨道半长轴在地球半径30到40之间扩展的大部分时间里，月球的倾角大约在25�到50�之间。事实上，在很短的一段时间内，倾角周期性地达到高达77�的值；也就是说，月球的自转轴距离其轨道平面只有13�。
The orientation of the lunar spin axis is traced from the early history of the earth-moon system to the present day. Tides raised on the earth by the moon have caused an expansion of the lunar orbit. Tides raised on the moon by the earth have de-spun the moon to synchronous rotation and driven its spin axis to a Cassini state—that is, in a coprecessing configuration, coplanar with the lunar orbit normal and the normal to the Laplacian plane (which is at present coincident with the normal to the ecliptic). This combination of events has resulted in a complex history for the lunar spin axis. For much of the period during which its orbital semimajor axis expanded between 30 and 40 earth radii, the obliquity of the moon was of order 25� to 50�. In fact, for a brief period the obliquity periodically attained a value as high as 77�; that is, the spin axis of the moon was only 13� from lying in its orbit plane.