The moon is the earth s only natural satellite . its average distance from the earth is 30 times the earth s diameter . its radius is only about one - fourth the earth s and its mass is only 1 81 the mass of the earth 月球是地球唯一的天然卫星,地月平均距离为地球直径的30倍,月球直径只及地球的四分之一,质量则为八十一分之一。