We do those shadows tread

As Virginia Woolf noted, so many of Donne's poems begin abruptly, with peremptory commands. “Batter my heart!”, “Hold your tongue!”, "Stand still!" This last begins a little poem both sweet and bleak: “A lecture upon the shadow.” The lovers walk together all one morning. He doesn't linger on that scene, but it echoes another of his love poems, “The Ecstasy” in which the lovers “like sepulchral statues lay...And we said nothing all the day.” Some of his most erotic poetry is not about bed but hours of quiet communion - lying on a violet-strewn river bank, walking together on a sunny morning. But the morning ends, and the shadowless moment of noon gives rise to this lecture, really a warning, about the phases of love. In its infancy, love casts shadows in order to hide from others; Donne's clandestine courtship of his employer's niece, Anne More, would have been full of such shadows. But after the noon of “brave clearness” in which the love is acknowledged and blessed, the shadows fall and lengthen the other way. Left to themselves at last, he predicts, the lovers disguise and mislead one another, and love declines into a night of mutual blindness and deceit. The power of this poem is the chill of dark it casts over the bright meridian of day; the sudden eclipse of love's morning. Like Hamlet grabbing Ophelia's elbow and muttering “Get thee to a nunnery: why wouldst thou be a breeder of sinners?" Donne here stands in front of his lover - in her face, probably in her light - to issue the doom of their love. Love's only possible moment of truth, he says, is impossible to keep. With the last word, now like Othello, direfully he puts out the light. 

Stand still, and I will read to thee 
A lecture, love, in love's philosophy. 
         These three hours that we have spent, 
         Walking here, two shadows went 
Along with us, which we ourselves produc'd. 
But, now the sun is just above our head, 
         We do those shadows tread, 
         And to brave clearness all things are reduc'd. 
So whilst our infant loves did grow, 
Disguises did, and shadows, flow 
From us, and our cares; but now 'tis not so. 
That love has not attain'd the high'st degree, 
Which is still diligent lest others see.
Except our loves at this noon stay, 
We shall new shadows make the other way. 
         As the first were made to blind 
         Others, these which come behind 
Will work upon ourselves, and blind our eyes. 
If our loves faint, and westwardly decline, 
         To me thou, falsely, thine, 
         And I to thee mine actions shall disguise. 
The morning shadows wear away, 
But these grow longer all the day; 
But oh, love's day is short, if love decay. 
Love is a growing, or full constant light, 
And his first minute, after noon, is night.