If you can find space for a second suggestion, I think the question will be cleared up by the following extract from the valuable work which I cited before (the Dictionary of Greek and Roman Biography and Mythology, edited by Dr. W. Smith):
"Eos, Ἠώς, in Latin Aurora, the goddess of the morning red, who brings up the light of day from the east. At the close of night she ascended up to the heaven from the river Oceanus to announce the coming light of the sun to the gods as well as to men. In the Homeric poems, Eos not only announces the coming Helios (the sun), but accompanies him throughout the day, and her career is not complete till the evening: hence she is sometimes mentioned when one would have expected Helios (Od. v. 390. x. 144.); and the tragic writers completely identify her with Hemera (the day), of whom, in later times the same mythes are related as of Eos."
As Aurora rises from the river Oceanus, he may be called her father, and as she sinks into the same, he may be called her grave. The expression then will mean neither more nor less than this, "We returned home before the close of day."
Perhaps Mr. Tennyson had a line of Lycidas running in his mind:
"So sinks the day-star in the ocean bed."
Milton's day-star, however, I take to be the sun himself.
Another of your correspondents, I see, suggests a different interpretation of the "crimson-circled star."
I hope I shall not be considered as taking too great a liberty if I avail myself of the medium of your pages to request Mr. Tennyson (deus ex machinâ) to descend and settle the question.
X. Z.