Poets usually treat the moon as the emblem of change. It swells, thins, vanishes and returns. Thoreau's moon refuses that role. She is "full-orbed" in the first line, her ray is "unchanged," and a few lines later we are told flatly that she "does not wane." What changes is the person looking up at her. Astronomically, of course, the moon does wane, so the claim is symbolic: the poem is interested in the idea of something steady, not in a lunar calendar.
The epigraph, credited to Raleigh, sets the terms. "Time wears her not," it says, and "Mortality below her orb is placed." The cosmos is split in two, with a changeless moon above and perishable human life beneath. Thoreau's poem borrows that picture and makes it personal. The epigraph's "orb" returns as "full-orbed," and its old-fashioned "doth" is echoed in the speaker's "declineth." The language stays slightly antique, as if the poem were speaking in the moon's own formal register.
The form carries the argument. Each stanza alternates a longer line of four beats with a shorter line of three, and the rhythm is mostly a smooth iambic walk: "Mounts up the eastern sky." The first and third stanzas rhyme neatly in an ABAB pattern, *ray / sky / aye / steadily* and *here / sphere / light / night*. The one loose joint is *sky* and *steadily*, which rhyme only if the final "-ly" is allowed to ring a little like "eye." The middle stanza is where the ground shifts. Only *bless* and *less* rhyme cleanly, while *fortune* and *soon* barely touch. The line "She does not wane, but my fortune," ends with an extra unstressed syllable and no verb. The sentence starts out about the moon, swerves to the speaker, and does not reach its verb until "declineth," two lines later, attached to "my wayward path." Judging by the effect, the stumbling grammar and imperfect rhymes enact the speaker's own unsteadiness, while the framing stanzas sound as settled as the moon.
The poem also leans on negatives. The moon is "not doomed," "does not wane," and her rays "do not bless," and in the end she "shines not the less." We learn who she is mostly by what never happens to her. That last double negative is the poem's quietest and most confident move, since it states constancy as an absence of reduction instead of a boast of brilliance. The verbs make a contrast too. The moon "mounts" while the speaker's path "declineth," so she rises as he falls. "Wayward" is a good word for that path, suggesting wandering as well as misfortune.
There is a chill in this steadiness. The speaker's fortune is one "which her rays do not bless," so the moon is not a kind patron, only a reliable fact. The final stanza turns that into something more consoling. "And if she faintly glimmers here, / And paled is her light" grants that, from where we stand, she can look dim, perhaps through cloud or haze. The key word is "here," which pins the dimness to the observer's position. Her real condition is described in the last two lines: "alway in her proper sphere / She's mistress of the night." "Sphere" works both as the moon's orbit and as a person's rightful place, and "proper" suggests that this place is her own and no one else's to grant or withdraw. One reasonable reading is that the poem offers a lesson about worth. A thing's value does not depend on how brightly it is seen from any one spot, or on whether its light happens to fall on you.
The poem is small, and its interest lies in the reversal of a stock symbol and in how closely the sound matches the sense. It is also a reminder that Thoreau, known above all for prose such as *Walden*, turned to verse as well. Here he does it in three plain quatrains, finding in the most changeable-looking object in the sky an argument for constancy.