28 (solar cycle) × 19 (lunar cycle) × 15 (indiction cycle) = 7980 years https://en.wikipedia.org/wiki/Julian_day https://en.wikipedia.org/wiki/Julian_day indi

28 (solar cycle) × 19 (lunar cycle) × 15 (indiction cycle) = 7980 years https://en.wikipedia.org/wiki/Julian_day https://en.wikipedia.org/wiki/Julian_day indication cycle seems to be like a Roman Egypt property tax assessment cycle, used to date medieval documents throughout Europe. I guess Joseph Scaliger must have been looking for notes about astronomical events in archival documents, and wanted to get them all on a consistent calendar.