Most moon poems are about fullness: silver light, romance, glamour. Hopkins gives us a moon almost used up, and that choice is the key to the poem. The speaker wakes at midsummer, when the night is so short that it is "not to call night", and sees the last sliver of the moon slipping behind a mountain. Nothing dramatic happens. A man wakes up and sees something beautiful, and the poem is about how that feels.
The first image is a precise one. The moon is "the fringe of a finger-nail held to the candle", and anyone who has held a nail up to a flame knows how it glows, thin and slightly translucent. The second image, a "paring of paradisaical fruit", is more surprising. A paring is a peeled-off strip, a scrap, yet this one comes from the fruit of paradise. The moon is therefore both leftover and heavenly, "lovely in waning but lustreless". Hopkins admits the loss of brightness and still insists on the loveliness. The word "paradisaical" quietly brings Eden into the scene, and the poem's last lines suggest why.
The second half of the opening stanza turns the moon into a creature. It "stepped from the stool, drew back from the barrow" of the mountain Maenefa. A barrow is a mound, and the word also carries a sense of burial, so the moon seems to rise or retreat from something like a grave. Then the verbs get rougher: "A cusp still clasped him, a fluke yet fanged him, entangled him, not quite utterly." Judging by the imagery, the moon is snagged on the mountain's edge like an animal in a trap or an anchor caught on a rock. A fluke can be the barbed tip of an anchor, and "fanged" makes the hold feel predatory. "Cusp" is also a word for the points of a crescent, so the clinging horn may be the moon's own. The pronoun "him" gives the moon a body, and "not quite utterly" leaves him half-free, a struggle caught at the very last moment.
The sound of the language is as physical as the images. The lines are long and move by stresses rather than by a fixed count of syllables, in the manner Hopkins called sprung rhythm, so you have to slow down and lean on the beats. Consonants pile up: "white and the walk of the morning", "stepped from the stool", "clasped... cusp... fluke... fanged". There is no regular end-rhyme, but "utterly" and "easily" chime across the break between the stanzas and link the moon's struggle to the speaker's ease. The contrast is deliberate. The moon is caught and the watcher is not, and the sight comes to him with no effort at all.
The last three lines turn from the sky to the speaker. The sight was "prized" and "desirable", yet "unsought, presented so easily". He didn't go looking for it; he simply woke up to it. The final line is the strangest and most intimate: it "parted me leaf and leaf, divided me, eyelid and eyelid of slumber." The odd "me" makes the speaker the thing being opened, like a bud, or a book whose pages are turned. Sleep comes in layers, and the moon peels them back one at a time, just as the moon itself was a peeled paring. The gentleness of that gesture is the poem's emotional high point.
The poem is significant because it shows how much Hopkins could do in so little space. The ideas are not stated. Beauty is a gift that comes without being earned, and it can be most moving when it is fading. The poem never says this outright. We feel it through a nail in candlelight, a trapped animal, and eyelids opening like leaves. The word "paradisaical" and the sense of something freely given suggest that Hopkins, a Jesuit priest, may have heard something more than natural beauty in the moment. That is an inference from the text, not something the poem declares. Even without it, the poem rewards slow reading: the waning moon is no consolation prize, and it is the right thing to be looking at when you first open your eyes.