We can never expect ______ bluer sky unless we create a less polluted word.
It was a cold winter night and the moon was shining brightly across ______ night sky.
第一空a，第二空the。都是名词SKY为何冠词不同呢?
▲sky是自然界独一无于的事物（类似的有sun, moon, earth等），其前一般连用定冠词。如：
Lightning lit up the night sky. 闪电照亮了夜空。
The sun was low in the sky now. 太阳此刻低挂在天空。
Swallows darted about in the sky overhead. 燕子在头顶上飞来飞去。
A helicopter appeared in the sky above them. 一架直升机出目前他们上空。
The constellations in the night sky cannot be directly equated with the heroes of Greek mythology. 夜空中的星座不可以直接与希腊神话中的英雄相等同。
▲但，当它们遭到描绘性形容词的修饰时，假如不是特指，则其前要用不定冠词，此时的a含有 a kind of 的意思。如：
A cloudy sky is not always a sign of rain. 阴天可能不是下雨的征兆。
All I recollect is a grey sky. 我所记得的只不过灰色的天空。
The sun beat down all afternoon from a clear sky. 整个下午晴空当头, 骄阳似火。
The weather broke and thunder rumbled through a leaden sky. 天气骤变，雷声隆隆响彻乌云密布的天空。
The hues of the leaves in autumn are beautiful against a blue sky. 秋季，色彩丰富的树叶在蓝天的映照下很漂亮。
In sharp contrast to her mood, the clouds were breaking up to reveal a blue sky. 乌云日渐散开，露出了蓝天，这和她的情绪形成了鲜明的对照。
