Suns of the world may stain when heaven's sun staineth.

"Suns of the world may stain when heaven's sun staineth." is the last line from Shakespear's sonnet 33.
I think I understand the rest of the poem, on a superficial level at least, but this last line confuses me. I'm not sure what he means. Is it a pun?
please lend me your good minds.

Thanks,
C

I'm putting the rest of the poem below for reference.

Full many a glorious morning have I seen
Flatter the mountain-tops with sovereign eye,
Kissing with golden face the meadows green,
Gilding pale streams with heavenly alchemy;
Anon permit the basest clouds to ride
With ugly rack on his celestial face,
And from the forlorn world his visage hide,
Stealing unseen to west with this disgrace:
Even so my sun one early morn did shine
With all triumphant splendor on my brow;
But out, alack! he was but one hour mine;
The region cloud hath mask'd him from me now.
Yet him for this my love no whit disdaineth;
Suns of the world may stain when heaven's sun staineth.