"Declination arc is a predictive tool that was developed by Charles Jayne and Eleanor Heseltine. Declination arc is calculated by determining the absolute value of the difference between the declination of the natal and the progressed Suns. If the natal Sun is 20 north, and the progressed declination is 15 south, then one must calculate the distance from 20 north to its maximum northerly declination, and add it to the difference from that max northerly declination down to the progressed position at 15 south. This value is then added to or deducted (for converse progressions) from the natal positions of all planets.
Charles surprised me when he said that this was the most important tool that they had devised to rectify horoscopes during a study of rectification techniques in the 1940s. I asked him why this technique might show events more powerfully than did other more traditional techniques. An answer came from further studies of predictive techniques. He also taught us ascendant arc and vertex arc.
Ascendant arc is calculated by subtracting the natal ascendant from the progressed ascendant. Vertex arc is also computed by deducting the natal from the progressed vertex. I became confused when I was analyzing a chart that had transiting Saturn square Venus and ascendant arc Saturn trine Venus. After some discussion with Charles and more analysis, I concluded that these techniques must have their own unique qualitative flavor."