September has arrived, and that means stargazers have a final few weeks to see the long, starry arc of the Milky Way and all its attendant splendor. The rich constellations of Scorpius and Sagittarius are moving westward, but the lengthening nights keep these stars accessible for a little longer, at least for observers in the northern hemisphere. Jupiter and Saturn linger in the southwest to liven up the sky, and the planet Neptune reaches opposition near a relatively bright star that makes it easy to see this most distant major planet. Here’s what’s happening in the night sky this month…