Question

What does the speaker talk about the sun ? How does he know whether it is day or night

Answer 1

The speaker of this poem is classic John Donne. That doesn't mean that this is how we would expect John Donne to speak if we saw him walking down the street (that would just be awkward), but this voice is entirely consistent with the voice we hear in much of his work.
Whether he was addressing God, Death, the sun, some lady he'd just met, or his wife, Donne's voice tends to take on a clever, argumentative, even haughty tone. We mean, in line 23, he says that princes are only pretending to be as great as him. It's as if Donne knows he's being clever, or as if he's performing a magic trick and saying, lookee here!
This sort of voice makes sense. Donne's poetry wasn't published in his lifetime; it only circulated among his friends. And his buddies were all upper-middle-class lawyers and writers who hung around coffeehouses trying to one-up each other. The speaker of this poem seems to intentionally go out of his way to make outlandish boasts and bizarre images and then try to make it all work out by the end of the poem.

Answer 2

Day and night is determined by many factors.

one can determine day and time through time, normally daytime starts ant dawn and lasts till the end of evening.

Day and night could also be determined through presence of sun or moon.

When there is the moon, then that is night and when there is the sun then that is day time.

In this context then  John Donne knows it is day or night through the sun, the presence of the sun confirms to him about the approach of day.