September is a month of change. One change is that the days grow noticeably shorter. This month the full moon is on Sep 20 and is called the Harvest Moon because it is the closest full moon to the equinox. Another important astronomical event transpiring in September is the autumn equinox on Sep 22. The fall equinox denotes the shift from summer to autumn. https://www.timeanddate.com/astronomy/sights-to-see.html