Bonus Edna St. Vincent Millay Quiz by StevensDN

Question 2

In this excerpt from a Millay sonnet (“How innocent of me and my dark pain”):

The gods are patient; they are slaves of Time
No less than we, and longer, at whose call
Must Phoebus rise and mount his dewy ___,
And lift the reins and start the ancient climb…

What is the missing, seemingly anachronistic three-letter word? (Hint: the line does not rhyme with the other quoted lines.)