地球引力

1.The tides are caused by the attraction of the moon for the earth.

潮汐是由月亮对引力引起。

2.The shape and perigee of artificial satellite orbits shall continually variate due to caused by the atmospherical drag and the earth gravity.

人造卫星轨道由于大气阻力和引力场等因素摄动,其形状和近点位置将不断改变。