"Sometimes too hot, the eye of heaven beams" indicates that it is possible for it to get too hot. The sun is Heaven's eye. The sun might be too bright at times. "And frequently is his gold completeness darkened," which says that the "his," the sun, is frequently obscured. His gold color becomes duller. His gold completeness is darkened - that is, made less brilliant- many times each day. When you look at the sky on a clear night, you are seeing darkness with stars and planets reflected from beyond our galaxy. But over there, out in space, there are other galaxies with their own stars and planets, some of which are far more massive than ours. If they were all placed end to end, they would still not reach across our galaxy. And yet we know that these other galaxies exist because light from them reaches us even though they are so far away. Light travels at near the speed of sound, so this means that some of the objects in these other galaxies are closer to us than anything else in our universe. Our galaxy is almost infinitely large, but even so, there are places within it where conditions are so extreme that no life can survive. It's likely that somewhere else in this galaxy, if it had the right kind of planet, life could have started. Perhaps it has already started, without us knowing about it. Stars are very powerful sources of energy at their core temperatures.