In Shakespeare's Sonnet 14, the tone of the poem is one of tribute or acclaim for the woman he loves.
The speaker tells the subject of the poem that he is not well-versed in being an astronomer with the "talents" as one might have expected in that day, but he has enough to call himself an astronomer, and he will relay this information to his audience later. (This is somewhat paradoxical in that he says he is an astronomer, but can't do what astronomers can do: all will be clarified later.)
...methinks I have astronomy,
But not to tell of good or evil luck,
Of plagues, of dearths, or seasons' quality;
Nor can I fortune to brief minutes tell,
Pointing to each his thunder, rain and wind,
Or say with princes if it shall go well (lines 2-7)
Line nine, as is typical for Shakespeare's sonnets, starts the third quatrain with a shift of his focus from what he cannot do, to what he can do. Whereas the tone has seemed to indicate a lack of ability, the speaker has just been getting started—and here he changes the direction of his overall message. The "shift" is seen with the word "But."
In the third quatrain he alludes to being an astronomer again, but the stars he gazes at are not in the heavens, but in his sweetheart's eyes. Using a metaphor, he compares her eyes to stars:
But from thine eyes my knowledge I derive,
And, constant stars, in them I read... (lines 9-10)
The last two lines of the quatrain assert that in this woman, truth and beauty would live together in perfect harmony, but especially if she were to have a child and pass these characteristics on to that child.
As truth and beauty shall together thrive,
If from thyself to store thou wouldst convert; (lines 11-12)
The rhyming couplet (the poem's conclusion—the final two lines) sums his message up: that in his role of the "astronomer," if she does not have a child, that truth and beauty will die when she dies, for she epitomizes all that truth and beauty are, in his "astronomer's eyes."
Or else of thee this I prognosticate: [foretell]
Thy end is truth's and beauty's doom and date. (lines 13-14)