1.220 nouns referring to one thing only

'A' and 'an' are sometimes used with singular nouns such as 'sun', 'moon', and 'sky' which 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 qualifier to the noun. This use is particularly common in literature.

• We drove under a gloomy sky.
• A weak sun shines on the promenade.

For more information on singular nouns, see paragraphs 1.35 to 1.41.