Maybe it could refer to either the poet or the sun throughout the entire line, if you think of the mantle blue as being the actual sky that the sun is able to twitch over his shoulder.

或者两者皆可,从整句的行文看来，如果你们把蓝色的丝巾看作是真实的天空的话,那么太阳的确是能够在他的肩上轻轻颤动。

耶鲁公开课 - 弥尔顿课程节选

And so in Milton's verse here, sense or meaning is variously drawn out from one verse into another. Sense, the very spirit of meaning, is infused throughout an entire line rather than being singled out and separated or segregated to the end of the line in the form of the rhyme word.

弥尔顿的诗句里，感觉或意思在句与句,之间传递，感觉，意思的灵魂,贯穿于整句，而不是被分离出来,以韵脚词的形式放在句末。

耶鲁公开课 - 弥尔顿课程节选