The sun set below the horizon without my noticing.
Literal
Sun [topic-は] not-knowing [during-間] horizon-below [at-に] sank.
知らぬ間に ('without [me] noticing,' literally 'during the [time of] not knowing') uses the classical negative 知らぬ (vs. modern 知らない). A set poetic/literary expression. 地平線 ('horizon,' literally 'earth-flat-line') is the standard word.