Most poets who describe the moonrise reach for silver and serenity. Shelley gives us a sickroom. In these six lines, a waning moon climbs into the sky looking like "a dying lady, lean and pale," who has stumbled out of her bedchamber in a daze. The central idea is that the natural world is as frail, confused and mortal as a human being. The poem asks us to see decline not as peaceful fading but as something unsteady and faintly unsettling.
The construction of the sentence matters as much as the picture. Shelley delays his subject. The poem opens on the word "And" and on the comparison, and we read through four full lines of the lady before we learn what she stands for. "The moon arose" does not arrive until line five. This holds back the moon in the same way the lady is held back: slowly, with effort. The syntax itself seems to totter before it gets anywhere.
The rhythm supports this. The lines are iambic pentameter in rhyming couplets (pale/veil, insane/brain), a form that usually sounds balanced and orderly. Here that order is under strain. The sentence runs on past the ends of the lines ("led by the insane / And feeble wanderings"), so the verse stumbles forward the way the lady does. The rhymes are also telling. "Insane" and "brain" are chimed together so that madness and mind lock into a single sound, and the pairing feels both neat and uncomfortable.
The imagery is carefully chosen. The "gauzy veil" is the first of several double meanings. It is the thin cloth a sick woman might wrap around herself, and it is also the haze or thin cloud through which a low moon looks dim. Likewise "murky East" gives us a dirty, clouded sky, and the lady's pallor is the moon's pallor. Each detail works on both levels, so the comparison never feels forced. The sound of the words helps as well. The "w" sounds in "wrapped," "wanderings," "white" and the title's "waning" give the passage a breathy, hushed texture, as if it were being said in a sickroom.
The most interesting word choice is probably "led." We expect a person to lead the way, but here the lady is "led by the insane / And feeble wanderings of her fading brain." Her mind is the guide, and it is a failing one. The moon has long been linked with madness (the word "lunatic" comes from the Latin for moon), and Shelley uses that link without making a point of it. The moon's wandering path across the sky becomes the aimless wandering of delirium. "Arose up" also seems a little redundant, since "arose" already means "went up." That extra word reads like extra effort, a body forcing itself to rise.
Then comes the last phrase: "A white and shapeless mass." The personification falls away. The lady, with her veil and her chamber, is gone, and what remains is a lump of pale matter. "Shapeless" is a harsh word for the moon, which we normally prize for its form, and "mass" has the dead weight of a body rather than a living person. The line is also short. It has only six syllables where the pattern calls for ten, and it ends on a dash with no rhyme for "East." Nothing in the text tells us why it stops, so we can only note the effect. The poem breaks off at its most formless image, and the unfinished couplet seems to enact the fading it describes.
What makes the poem significant is how much it does in so little space. It takes a familiar Romantic habit, reading human feeling into nature, and turns it toward illness, confusion and decay instead of comfort. The moon is not a calm witness to human life. It is a fellow sufferer, and a worrying one. A reader who has only ever seen the moon as a pretty ornament may find it hard to look at the sky the same way afterwards.