What is the difference between a Secondary and Solar Arc Progression?

The Progressed Bi-Wheel Chart, similar to a Transit Bi-Wheel, shows a different way of looking at the present, past, or future position of the planets to the Natal chart. These progressed positions can be compared to the Natal positions or to each other for a more informed understanding of the quality of the period surrounding the events of a given period. The Progressed Chart can also be constructed from the progressed Midheaven and progressed Ascendant as a freestanding chart, either for the original birth location or for the individual’s current location. For example, if Progressed Sun was conjuncting natal Mars, then transits to natal Mars could be looked at more closely and would possess additional meaning at this time. The Progressed positions move slowly, even for the inner planets, and the orbs for their interaction are correspondingly very tight. It takes about a year for a progressed inner planet to change position by one degree. TimePassages provides two types of progressions in common use, Secondary Progressions and Solar Arc Progressions, also called Solar Arc Directions. 

The most common type of progression, Secondary Progression, is sometimes described as “a day for a year.” For example, if a planet moved 28 degrees from its natal position by the 30th day of your life, the program progresses the planet’s position 28 degrees for the 30th year of your life. This technique is based on equating two fundamental cycles, the earth’s 24-hour rotation and the Earth’s yearly motion around the Sun. While other forms of progressed positions are possible, the technique of Secondary Progression is by far the most commonly used. The Sun moves about a degree per year by secondary progression. The Progressed Ascendant and Progressed Midheaven are calculated from the Solar Arc Progression of the Midheaven. 77 

Another common type of progression, Solar Arc Progressions, or Directions, takes the progressed position of the Sun and moves all the other planets by the same number of degrees. For example, if the Sun moved 30 degrees from its natal position by the birth hour of the 30th day of your life, then Mercury’s Solar Arc progressed position at the end of the 30th year of your life would also be 30 degrees from its natal position. In fact, in this example, all the planets would be progressed by 30 degrees.