Anyone who has watched a brilliant sunrise sour into grey knows the feeling Shakespeare starts from, and he builds a whole love poem on it. In the first eight lines the speaker recalls the many glorious mornings he has seen, with the sun lighting mountain tops, warming meadows and turning streams to gold, before clouds sweep in and the sun slips away westward. At line nine ("Even so") the poem turns personal. Someone the speaker calls "my sun" shone on him for a single hour and is now hidden. The final couplet insists that the speaker's love does not "disdain" him for it, because if heaven's sun can be stained, the "suns of the world" may be too.
The first quatrain makes the sun a monarch and a magician at once. He has a "sovereign eye" and "Flatter[s] the mountain tops", as if the peaks were courtiers. Then the language shifts to alchemy. The sun gilds "pale streams" with "heavenly alchemy", turning dull water into gold. The idea pays off in the second quatrain, where the clouds are "basest", a word that also means worthless metal, the opposite of gold. So the sky becomes a contest between precious and base, and the base wins. The clouds "ride / With ugly rack" on the sun's "celestial face", and the sun hides his "visage" and steals "unseen to west". The verbs are those of a person caught out, a thief or someone embarrassed who sneaks off the stage.
That detail complicates the poem. The sun does not simply get covered up. He "permit[s]" the clouds to ride across his face, which makes him partly responsible. The speaker may not intend this, but the word hangs over the rest of the poem, because a sun that allows his own disgrace is a more troubling beloved than one who is just unlucky.
The third quatrain echoes the first, with "early morn" for "glorious morning" and the brow in place of the meadows, and it makes the shift from general to personal. The sounds change as well. "But out! alack!" breaks the smooth iambic pentameter that has run along comfortably until now, and the punctuation chops the line into gasps. "One hour mine" sets a very short time against the long, expansive sweep of the opening, and "now" snaps the poem back to the present loss. Even the "region cloud" that has "mask'd him" suggests something theatrical, a disguise rather than a mere accident.
The couplet is where the poem's argument and its feeling pull against each other. Rhyming "disdaineth" with "staineth" in the soft, extended endings of lines thirteen and fourteen gives it a gentle, almost soothing close. The logic is neat: if the real sun can be stained, no earthly "sun" can be blamed for it. But "no whit", meaning not at all, protests a little too firmly, and "disgrace" in line eight has already shown that the speaker does feel the hurt. One reading is that he forgives. Another is that he talks himself into forgiving, borrowing the grandeur of nature to excuse something he cannot quite let go. The poem never settles which, and that unsettled quality is much of its charm.
The poem is significant because it shows a sonnet doing what the form does best, which is to turn a single comparison into an argument. The structure is the standard Shakespearean one, three quatrains rhyming ABAB CDCD EFEF and a closing couplet, with each part doing a job: describe, apply, conclude. The conceit is carried through so thoroughly that every image, from gilding to masking, belongs to one extended comparison. The poem also lets its language say more than the speaker does. He claims a calm, loving forgiveness, but words like "permit", "stealing" and "disgrace" suggest a more complicated hurt. That tension between what is declared and what is felt keeps the poem alive, and it is a good reason to read it more than once.