The center of mass of the moon is not at the geometric center . instead , it is closer to the earth . together with the mutual gravity , this shift of the center locks the revolution and rotational rates 同步自转的出现绝不是意外巧合，由于月球的质心并不位于月球的中心位置，而是较月心更接近地球，结果在相互的引力作用下，最终把月球的自转和公转周期锁定成一比一。
Most of them pass over the same place a couple of times a day . as they operate at a distance closer to the earth , they are only capable of taking cloud images of a limited area of the earth each time 它们大部份一天内经过同一地点一两次。由于较接近地球，它们每次只能拍摄到有限区域的云图。相对地球同步气象卫星，极地轨道气象卫星图像的数目较少，范围亦较小。
On january 13 , 2008 , beginning 30 hours before messenger ' s closest approach to mercury , the wide angle camera , part of the mercury dual imaging system ( mdis ) , began snapping images as it approached the planet 在2008年1月13日，开始30小时前信使号以最亲密的方式利用他携带的广角相机拍摄水星，不久后第一部分的水星双成像系统（ mdis ） ，就开始接收图像，因为它开始接近地球。
The earths atmosphere contains so - called greenhouse gases such as carbon dioxide co 2 and methane that trap the suns heat like a blanket near the earths surface . this keeps the earth warm rather like the way the glass traps the heat in a greenhouse 地球的大气层含有温室气体如二氧化碳和甲烷等，功能像毛毯一样把太阳的热力覆盖在接近地球表面的范围，就像玻璃把热力保存在温室内一样，保持地球温暖。
When the air pressure in a region is lower than its surrounding , the feature is called a depression , a cyclone or an area of low pressure . in the northern ( southern ) hemisphere , the air around a depression moves in a counterclockwise ( clockwise ) direction ( looking from above ) . near the earth s surface , however , friction tends to cause the air to move slightly inward across the isobars 当地区的大气压力比其外围低时，这个系统便称为低气压或气旋。在北（南）半球，低气压附近的空气循逆时针（顺时针）方向运行（从上面往下望） 。接近地球表面，摩擦力使空气稍微向内越过等压线。
