Shelley opens with a question, and it is a tender one. "Art thou pale for weariness / Of climbing heaven and gazing on the earth?" He speaks to the moon as if it were a tired traveller, and the whole stanza imagines a life for it. The moon climbs, wanders, watches, and never settles. By the end the question has almost become an answer. The moon is pale because it is lonely and unsatisfied, and the real subject is a state of mind that the moon happens to display.
The details of that loneliness are carefully chosen. The moon is "companionless" even among the stars, which have "a different birth". Shelley doesn't say how they differ, and the vagueness helps. The moon is surrounded by lights and still belongs to none of them. Then comes the most striking image: the moon is "ever changing, like a joyless eye / That finds no object worth its constancy." Two ideas are fused here. The changing moon, with its waxing and waning, becomes a restless, roving eye. The word "constancy" turns a fact of astronomy into a moral judgment. The moon is not fickle because it is shallow. It shifts because nothing it sees deserves its loyalty. Inconstancy, usually a fault, becomes a sign of high standards and disappointment.
The form supports this feeling. The first stanza alternates short lines with long ones. "Art thou pale for weariness" and "Wandering companionless" are brief and almost breathless, while the lines between them stretch to a full ten syllables, as if the moon's path were being traced up and across the sky. The short lines also share a trailing ending, since "weariness" and "companionless" both sink onto the soft sound of *-ness*, which is almost a sigh. The long lines, "earth" and "birth," land on a firmer rhyme. The stanza closes on a pair, "eye" and "constancy," that rhymes only loosely to a modern ear. That slight looseness suits a thought that ends unresolved.
The second stanza shifts from the sky to the human viewer: "Thou chosen sister of the Spirit, / That grazes on thee till in thee it pities..." The capital "S" suggests something larger than an ordinary person, perhaps the soul or the imagination, and "sister" makes the moon a kin figure rather than an object. The line as printed here reads "grazes", which may be a slip for "gazes". Either way the idea is clear. The Spirit looks at the moon so long and so intently that it begins to feel pity, though the sentence breaks off before we learn for whom. We might guess the pity is for the moon, or for the Spirit itself reflected in it. That is an inference from the text, since the lines stop. The moment is still revealing. Pity is where looking turns into identification, and the observer discovers that the weary, wandering, unsatisfied thing in the sky is a portrait of the self.
This is the poem's central idea. Nature is a mirror, and the moon is "chosen" as the Spirit's sister because it displays the Spirit's own restlessness. Many poems describe the moon as beautiful or calm. Shelley makes it exhausted, which makes it a companion in unhappiness and not just a decoration. The moral vocabulary reinforces this: "weariness," "joyless," "worth," "constancy," "pities." These are words for human feelings and judgments, laid over a distant rock in space, and the effect is to make the sky feel strangely intimate.
The poem is also worth reading for what it leaves undone. A school reader might be frustrated that it stops mid-thought, but the broken ending suits its mood, because a poem about unfulfilled searching ends before it can find an object worth its constancy. What remains is brief and concentrated. A single sustained image, the moon as a tired and disappointed eye, carries the whole feeling, and a few lines are enough to show how looking at the sky can become a way of looking at ourselves.