Most people have glanced up on a bright afternoon and found the moon hanging in the sky, pale and oddly out of place. Longfellow starts from that ordinary sight and makes an argument from it. At noon the moon looks like "a schoolboy's paper kite," a flimsy toy bobbing about the sky. By night it is something else entirely. The poem is really about how much our surroundings and our mood decide what we are able to see and feel.
The comparison at the centre is between the moon and a poem. In the second stanza the speaker reads "a poet's mystic lay" in the same "broad daylight" and finds that it seems "at most / As a phantom, or a ghost." The repeated phrase "In broad daylight" links the two experiences, and so does "yesterday." Both the moon and the verse are there, but neither can make itself felt. Daylight is too loud and too bright for either to register, and the speaker is too caught up in it to notice.
The third stanza is the hinge. The day is called "feverish," and it dies away "Like a passion." Day is imagined as a kind of overheated emotion that burns itself out, and what follows is calm: "the night, serene and still, / Fell on village, vale, and hill." The soft, falling rhythm helps. The three landscape words, linked by the "v" sounds in "village" and "vale," settle over the scene like a blanket. The speaker doesn't go looking for inspiration. Quiet arrives, and the moon and the poem return with it.
The vocabulary shows how much has changed. In daylight the poem was a "phantom" or a "ghost," words for something thin and faintly unreal. At night the moon is "a spirit glorified." The poem draws on the same family of supernatural words but now raises them, so the ghost becomes something blessed. The moon also "Filled and overflowed the night," a liquid image that makes the light seem to spill past its limits. Its light brings "revelations," a word with religious weight, and the night itself becomes an "interpreter" that explains the poet's song to the speaker. The word "mystery" in the final line is not a problem to be solved. It is something the speaker is happy to receive along with "grace."
The form suits this quiet idea. Each line has seven syllables, with the stress falling on the first, third, fifth and seventh: "IN broad DAY-light, AND at NOON." This trochaic pattern gives a light, chiming, almost nursery-rhyme feel, and it stays the same throughout. The rhymes arrive in neat couplets (noon/moon, white/kite, day/away, still/hill), so the poem sounds simple and settled. The last pair, "me" and "mystery," only half-rhymes, because the second word ends on an unstressed syllable. The poem ends on a slight blur, which suits a poem about mystery. Longfellow's plain words and short sentences also mean the poem never competes with its own subject. It stays quiet while it describes quiet.
There is a structural balance as well. The first two stanzas show failure in the day, with the moon faint and the poem ghostly. The last three, the larger share, show recovery at night. The extra space is a quiet way of saying that this second state matters more. Even the capital letter in "the Poet's song" in the last stanza suggests the poet has grown in stature since the earlier "a poet's mystic lay."
What makes the poem worth reading is that it explains something real without lecturing. Anyone who has failed to enjoy a book in a noisy room, or loved the same book in a quiet one, knows what it describes. By tying that experience to the moon, Longfellow suggests that a poem, like moonlight, has not changed when it fails to move us. What has changed is the state we are in when we meet it. The moon was in the sky all afternoon, and the poem was on the page. The speaker had to wait for the night.