come down to earth/down on earth/come back to earth
（a）Who on earth opened my letter?
(b) She was the most solitary person on earth.
on earth/on the earth 404
(c) He is very down on earth.
(d) Bill was sitting and daydreaming so his mother told him to come down to earth and to do his homework.
(e) After Jane met the movie star it was hard for her to come back to earth.
(f) In 1968 an Apollo spacecraft went round the moon and came back to earth.
(g) And it was one of the few man-made objects on earth that could be seen by the astronauts who landed on the moon.
我们在学习中已牢记，在名词 earth（地球）、sun（太阳）和 moon（月亮）之前必须加定冠词 the。但是，在上列 7 例中，在 earth 之前却不加定冠词，这是怎么回事呢？
对句(a)和(b)的解释,见 on earth/on the earth。句(c)中的 down on earth 是个成语,意思是“直率”“老实”(direct)。在 on earth 之间绝不可加定冠词 the。在句(d)和(e)中,come down to earth 和 come back to earth 都是成语,意思是“回到现实中来”“抛弃幻想”“不再胡思乱想”。在这两个成语中 earth 之前都不能使用定冠词 the。
对于句(f)可以作这样的解释:当 earth 与非成语的 come back to 连用时,常常省略定冠词 the。
对于句(g)似乎可以这样来解释: 当 earth 充当 on, with, from 等介词的宾语时, 有一种省略定冠词 the 的趋势。例如:
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Man's life on Earth(EED) 地球上人类的生活
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get away from Earth(ECD) 离开地球
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If the astronauts momentarily lost radio contact with earth, the whole mission ruined. 倘若宇航员短时间地与地面失去无线电联系，整个任务就会失败。