the old way
所以我们在学校里强调天空只有一个,地球只有一个,太阳、月亮也只有一个。
这里还有一个:
Skies is a poetic/literary word used to mean heaven or "heavenly power." The first example sentence could mean "reach for heaven."
In some set phrases, the used word is skies, as in "he wrote to his sister praising Lizzie to the skies," where "to the skies" means very highly or enthusiastically.
https://english.stackexchange.com/questions/29302/using-skies-instead-of-sky