come down to earth/down on earth/come back to earth

【问】 (a) Who on earth opened my letter?

(b) She was the most solitary person on earth.

(c) He is very down on earth.

(d) Bill was sitting and daydreaming so his mother told him to come down to earth and todo 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 astro-nauts 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?

【答】 对句(a)和(b)的解释，见on the earth/on 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，comedown 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：

1. Man's life on Earth(EED)

地球上人类的生活

2. get away from Earth(ECD)

离开地球

3. 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。