Striking bells for particular schedules had poor social scalability: the more people with different schedules in a city, the greater the cacophony. Striking the time greatly increased the temporal coordination in a city: socially scalable schedules. http://www.fon.hum.uva.nl/rob/Courses/InformationInSpeech/CDROM/Literature/LOTwinterschool2006/szabo.best.vwh.net/synch.html …
vs. reflected emission (e.g. reflected local sun/star light off of artificial surfaces such as solar energy collectors). All of these would be very likely to look very artificial, and quite stand out from the natural galactic phenomena.