Keep planets and angles separate
Secondary progressions place planets at a symbolic date: one day after birth for each tropical year of age. A 30-year interval therefore samples the planetary sky roughly 30 days after birth. The birth moment and target date determine this symbolic moment. Longitude, latitude, and the birth clock time are additionally needed for angles and houses.
There is no single, inevitable way to progress Earth’s rotation. Its standard Naibod setting directs the birth sidereal angle by the Sun’s mean daily motion, 0.98564733° times years of life. It also offers a setting that applies the one-day-per-year analogy to Earth’s roughly 361° daily sidereal rotation.
| Step | Naibod | Progressed day |
|---|---|---|
| Progressed planets | Sky at symbolic ephemeris moment | Same planetary sky |
| Angle input | Natal ARMC + age × 0.98564733° | Sidereal angle at symbolic moment |
| Ascendant and MC | Recomputed from directed ARMC, latitude, and obliquity | Recomputed from symbolic-day ARMC, latitude, and obliquity |
| House cusps | Porphyry quadrant trisections | Porphyry quadrant trisections |
Why the difference matters
Sidereal rotation makes nearly one full turn in a solar day. After removing whole turns, its net change is near one zodiac degree per symbolic day, but it is not exactly the constant Naibod key. The symbolic date also carries the actual sidereal-time details used by the astronomy library. The resulting Ascendant and intermediate cusps may diverge enough to move a progressed planet into a neighboring house. Compare the displayed ARMC and cusp positions before interpreting a house change.
Porphyry houses and limits
The Ascendant sets cusp 1 and the Midheaven sets cusp 10. Opposite axes set cusps 7 and 4. Each intervening ecliptic quadrant is divided into three equal arcs. This explicit geometry keeps the house convention the same between the two methods. Placidus or another quadrant system can yield different intermediate cusps even with identical angles.
The calculator requires a recorded birth time and coordinates. It accepts a manually entered historical UTC offset because modern zone rules cannot safely be assumed for an older birth. It rounds the symbolic moment to the nearest UTC minute, supports 1800–2100, and withholds positions at extreme latitude or a collapsed quadrant.