With context that it is getting dark, why say "日が暮れる" instead of just "暮れる"?

Question

X and Y are in a forest. They have been talking about it getting dark and cold. They both know that it is a long walk out of the forest.

　「これから長いことと歩かなければいかん」Xは指摘した。「２日間もです。もしかしたら３日かも。もう日が暮れかかっています」
　Yは空を見やった。「毎日このくらいの時間になれば日が暮れる。暗くなるのが怖い、X?」 　この挑発に、Xは乗らなかった。......

1. Thus far, the verb "暮れる" has not been used.
2. "暗くなっている" has been used.
3. "日が暗くなっている" has never been used.

Surely, #3 demonstrates that "暗くなっている" and "暮れている" are different in nuance. And, that is fine, a different issue.

Given context, can't you assume the "日" in "日が暮れている"?

We already know it is getting cold / dark / late.
It is said twice in a row.
And, Y says it while he is looking at the sky.

(wild guess): "日が" places emphasis on the sun. This emphasis allows you to capture the sun's practical, and symbolic, meanings?

Answer 1

暮れる alone can mean 'a day ends' or 'the sun sets' but 日 usually accompanies it. I don't think '日が' have any implications here.

暮れる actually means 'a day, a month, a season or a year ends'. We say '今年も暮れますね' in December.