“交替地”用英语怎么说

当伴星围绕着主星运转时, 它交替地朝着地球或背离地球运转.As a companion star orbits its primary , it alternately approaches and recedes from the Earth.为了先提出较简单的结果,我们采用交替地讨论一些问题的方式.We have chosen to alternate the topics in order to have the simpler ...