北师大版高中英语必修1 Unit2 Lesson1 Yang Liwei returns safely to earth after 21 hours in space. (P22) 中earth前为什么没有定冠词the？

解释：

earth在上句中意为“陆地；地面（the surface of the world as distinct from the sky or the sea）”，可不加冠词。例如：

It plummeted back to earth at 60 mph.

它以每小时60英里的速度向地面垂直下落。《新牛津英汉双解大词典》

The astronauts were able to send the information back to earth.

宇航员能够把信息送回地面。《牛津英语搭配词典》