In the poem, the speaker is meant to be an anonymous observer of the universe.  Literally, the poem starts by applying theoretical science to an observation of the sun by looking at the sky from Earth, then zooming out on an astronomical scale over the course of four rhyming couplets, and then abruptly switching back to a trivial human problem, to show how small humans are in whatever kind of universe or multiverse we live in.  By the fourth couplet, the scale is large enough that the problem of light-years of distance preventing places in the farthest observable reaches of the universe from being observed in real time is being discussed.  The fifth couplet illustrates the triviality of a single human by pointing out an annoying but not really important issue in contrast with the grand scale and incomprehensible magnitude of the cosmos.  The end-of-line rhyme within each couplet helps to convey the four distinct stages of scale or “zoom” presented, without the ambiguity that would arise of a certain line potentially applying to the preceding or following set of scale, without the rhyme.  The first couplet talks about the Sun, the second talks about stars in general, the third talks about the Milky Way being comparatively tiny, the fourth talks about the far reaches of the universe where even light takes millennia to reflect back to us from, and the fifth talks about an annoying episode in one classroom on Earth.  Due to the scalar progression of size throughout the first four couplets, the perspective is dramatically and slowly made broader, so the sudden switch to a comparatively minute scale is more of a shock to the reader.