The poem starts with a quiet admission: when we look at the night sky, we only see "a convenient part." The constellation Ursa Major, the Great Bear, is a shape we have picked out and named, and that shape leaves out almost everything around it. The speaker seems aware that our way of seeing is partial and a little self-serving.
Then the poem shifts to an odd, tender image. The bear "has nowhere to fall," so his dying is strange: he is fixed in the sky, with no ground to land on. Yet a moth is there to witness it. The moth is tiny and fragile, and it is drawn to light, so it makes sense that it would hover near a star-bear. The mood is hushed and gently melancholy, but not bleak. Something huge ends, something small keeps it company, and the poem leaves us wondering who the real witness is.