One of the key differences between the two methods is that in Solar Arc you get to advance the slower moving planets (Saturn through Pluto) that hardly move in the Secondary Progression method, so this tends to add more interpretive possibilities when forecasting.
Testing Solar Arc in Your Own Horoscope
Solar Arc is relatively easy to test in your own horoscope: add 1 degree per year to the natal planets to see how they move and form an aspect with the natal planet. The orb needs to be exact (less than one degree) in order for the Arc to be in effect, and we count hard aspects (conjunction, square and opposition) only.
If natal ASC (Ascendant) is at 12 Leo, and natal Saturn is at 22 Leo, this means Solar Arc ASC will conjoin natal Saturn at age 10 (we write ASC=Sat to denote this period), then we ask if there had been a status change within the family around that age: some Saturnian event such as death in the family, sickness, move, or change in father’s position, etc.
Conclusion: Secondary Progression vs Solar Arc?
Solar Arc is a modern derivative of Secondary Progression that allows the astrologers to utilize Saturn and outer planets more fully in their predictive work. Many modern astrologers including myself use Solar Arc (which gives central importance to the Secondary Progressed Sun’s movement) in combination with Secondary Progressed Moon (which is a very important timer of life progress.)
A Must Read Reference: Solar Arcs by Noel Tyl
Noel Tyl has written extensively about his method of prediction in Solar Arcs: Astrology’s most successful predictive system. The book is actually a summary of his predictive method incorporating transits, Solar Arcs, Secondary Progressed Moon, as well as Tertiary Progressed Moon.
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