Re: Solar Arcs
"Solar Arc" means Solar Arc Directions. Direction means that all factors are moved together by the same rate. Solar Arc is the rate specified. There are other directions. Degree-Per-Year directions are directions whose rate is one degree per year. Degree-Per-Year and Solar Arc are close to the same rate.
Secondary Progressions show that the sp.Sun moves an average of 0.986 degrees a year, nearly a degree per year. Using the amount the Sun has moved to the future date by secondary progression is the solar arc specified. The "arc" is the number of degrees between birth and the future date. It amounts to about one degree per year of life. When this is found, one moves all chart factors by this arc to create the Solar Arc Directed planets for comparison to natal planets.
The Solar Arc positions are placed around the outside of the birth chart so one may compare the sa. positions to the natal positions. If you know the chemistry of planetary pairs as used to interpret transits, then you use the same chemistry with Solar Arc Directions, but the sa. directions will be slower and longer lasting than most transits. That is why they carry weight.