“glorious morning have I seen / Flatter the mountain tops with sovereign eye”
The opening eight lines build an extended metaphor — the classical 'solar conceit' — comparing the sun to an all-powerful ruler whose 'sovereign eye' bestows and withdraws favour. 'Sovereign' means supreme, kingly; the sun gazes down like a monarch granting his countenance. The poem holds this metaphor in place before revealing, at line 9, that the 'sun' is actually the person the poet loves.
historical The sonnet formhistorical Petrarch and the Petrarchan lover“Gilding pale streams with heavenly alchemy”
Alchemy was the medieval and Renaissance science that claimed to transmute base metals into gold. Here the sun literally gilds — covers with a layer of gold — the pale streams it shines on, so sunlight is 'heavenly alchemy': the one natural process that actually turns things golden. The word also carries a hint of illusion, since alchemical gold was notoriously deceptive.
“Even so my sun one early morn did shine”
Line 9 — 'Even so my sun' — is the sonnet's volta, the turn where the argument pivots. For eight lines the poem seemed to describe an actual sunrise; this line reveals that the whole description was a vehicle for the speaker's situation: his beloved ('my sun') shone on him briefly, then withdrew into clouds. The Shakespearean volta typically arrives at line 9 or in the closing couplet; here it arrives exactly at the standard turn.
historical The sonnet form“Suns of the world may stain when heaven's sun staineth”
'Stain' carries two meanings simultaneously: the literal discolouration of the sun when clouds pass across it (its light is tainted), and moral staining — dishonour, shame. The couplet offers a forgiving argument: if the real sun in heaven is sometimes obscured and 'stained,' it is no surprise that a human 'sun' (the beloved) sometimes does the same. The forgiveness is genuine but tinged with sadness, since 'stain' is not a neutral word.
historical The Fair Youth, the Dark Lady, and the Rival Poet