What does Lux mean by constellation in "The Voice You Hear When You Read Silently"?

When the poet uses the word "constellation" he is using it
to mean a whole array of things -- not just one.  He is saying that when you read
silently to yourself, you are exposed to a variety of different sensations (because he
says that it is a "sensory constellation) not just one
sensation.

When you read to yourself, you experience many
things.  You "see" things, of course, but you also engage your other senses, he says. 
Lux says that you can feel things and smell things.  You can hear the oats as they pour
into the feed troughs.  You can see how dirty the cow's haunches are, and you can
probably smell it as well.

The whole point of saying that
there is a "sensory constellation" is to tell you that reading can be a very vivid
experience that impacts all of your senses (at least in your
mind).