Nobody, Shelley suggests, has ever improved a sunset by measuring it. "To Harriet" starts with a glorious evening and ends with a small domestic argument: if you can gaze at the sun and see only its blemishes, you might as well look at the person you love and see only their faults. The central idea is that love, like beauty, is spoiled by cold fault-finding, and that the right response to something wonderful is to take it in whole.
The first eight lines form one long, slow sentence addressed straight to the sun: "O thou bright Sun!" The sun doesn't simply set. It "descendest" with a "sublime" grandeur, and the old-fashioned "-est" endings (descendest, lendest, sheddest) give the speech a ceremonial, almost prayerlike tone. Those endings also make the rhymes in lines 2 and 4 feminine, ending on a soft, falling syllable, as if the voice were sinking along with the light. The sun is also generous. It lends its "million hues to every vapour", colouring even the most ordinary wisps of air. The paradox in "gleaming lovelier as thy beams decline" matters too, since the sun looks most beautiful just as it is fading.
The imagery grows more delicate as the octave goes on. The sun pours "the liquid magic of thy light" over "cobweb lawn and grove and stream". Light behaves like a liquid, and "lawn" may even carry a second meaning of fine, gauzy cloth, which would suit the cobweb. By line 8, "calm Earth" has become "the vision of a beauteous dream", so the real world is transformed into something half-imagined. The sounds help, because the long, flowing vowels and the many l's (liquid, lovelier, lendest) make the verse feel smooth.
At line 9 the poem turns, and the language turns with it. The form blends two sonnet traditions. The rhyme scheme runs ABAB CDDC EFEF GG, with an enclosed octave that feels Petrarchan and a closing couplet that feels Shakespearean. The turn arrives as a rhetorical question: "What gazer now with astronomic eye / Could coldly count the spots within thy sphere?" The gentle l's give way to hard, clipped sounds, and "coldly count" is clinical in a way the earlier lines never were. The sun does have spots, and the poem admits it. What it mocks is the person who would stare at the glow only to tally its imperfections, and that admission makes the argument more honest than a claim of perfection would.
The comparison follows. "Such were thy lover, Harriet" is a hypothetical, a portrait of the man Shelley says he is not. Notice that he speaks of himself in the third person, as "thy lover" and "he," as though holding himself at arm's length to test the idea. The pretence slips in the last line, where "he" becomes "our close-woven happiness". The word "woven" quietly echoes the earlier "cobweb": happiness is a fabric made by two people, delicate and made of threads. Seen that way, "pick flaws" is a precise verb, since picking at threads is how fabric comes apart. It also mirrors "count the spots", so the comparison is built from matching actions.
Temperature carries the contrast as well. The "coldly" counting astronomer is set against the "warm caress", and the man who would "turn senseless" from it is senseless in two ways, without feeling and without sense. The final rhyme, "caress" with "happiness", is a little loose, since a stressed syllable meets an unstressed one. That may be why the poem seems to settle at the end instead of snapping shut.
The poem is worth reading because it turns a sunset into an argument without making the argument feel forced. Its preference for wonder over measurement is a recognisably Romantic one, but the tone is warm and personal, closer to a promise than a lecture. Shelley doesn't tell Harriet she is flawless. He tells her that he has no wish to count.