Secondary progression uses a simple symbolic rule: the chart for the thirtieth day after your birth describes your thirtieth year. It is not a physical claim; it is a convention that has proven descriptively useful for several centuries.
The progressed Moon is the workhorse. It moves about one degree per year, spends roughly two and a half years per sign, and completes a full return at twenty-seven. Its sign describes the emotional register of a whole two-year stretch, which is why it explains moods that no transit accounts for.
A progressed Sun sign change — which happens once or twice in most lives — is the single most underrated timing indicator in astrology. People rarely notice the date and almost always recognise the shift in retrospect.