Secondary progressions & solar arc
Day-for-a-year progressions (traditional) and solar-arc directions (20th c.)
Secondary progressions advance the chart one day for each year of life, a measure with old scriptural rationale that Placidus used and that James Holden traces to a mention as early as Kepler (unverified against Kepler's own text). Solar arc, a 20th-century development, moves every point of the chart forward by the arc the Sun has travelled by progression, so the whole chart keeps its shape.
What it measures
Secondary: the real sky a few weeks after birth, re-read as decades of life — the progressed Moon moves about one degree per month of life. Solar arc: every body advanced by the Sun's progressed arc, roughly a degree a year, though the true arc diverges from a flat 1°/year by about a degree by age 30.
Sources
- Placidus de Titis, Primum Mobile (17th c.)
- James Holden, A History of Horoscopic Astrology