«A and an are sometimes used with singular nouns such as sun, moon, and sky that refer to just one thing. You normally use the with these nouns, but you use a or an when you are drawing attention to some special feature by adding a modifier or a following phrase or clause to the noun. This use is particularly common in literature.

We drove under a gloomy sky.»

Отрывок из книги: Cobuild, Collins. «Collins Cobuild English Grammar.» HarperCollins Publishers, 2012.