Lines 1-5 ("The flame-red [...] doubloon") introduce the poem's main image: the "harvest moon" of the title. The harvest moon is the full moon that occurs closest to the fall equinox, either in September or October. Traditionally, it marks the transition from summer to autumn, which is also the harvest season in farming communities.
At first, the poem portrays the moon playfully, using vivid descriptions and figurative language to evoke its dazzling presence in the twilight sky. This moon is "flame-red" and "vast"; it seems to move like a huge "balloon," slowly "Roll[ing]" and "bouncing" along the hilly horizon. As night falls, it rises: "takes off, and sinks upward / To lie on the bottom of the sky, like a gold doubloon." A doubloon is an antique Spanish coin often associated with pirates and seafaring tales, so this simile (and oxymoron) plays on the familiar image of gold treasure sinking to the bottom of the sea.
Notice, too, how the "balloon" metaphor transitions into the "doubloon" simile within the space of a line, mirroring the rapid visual changes of the moon itself. The moon is an ancient symbol of change, in fact. And over the course of the poem, this harvest moon not only marks a seasonal change but also mysteriously transforms the landscape below. It may even be a warning sign of disaster or doom.
Though the loose, "bouncing" rhythms and gaudy rhymes ("moon"/"balloon"/"doubloon") of this first stanza sound like lighthearted children's verse, the poem's imagery will grow pretty disturbing by the end! An early hint of this shift comes in the poem's ominous first adjective: "flame-red."