The distance the Sun travels along the ecliptic within a specified period of time.
The Solar Arc Direction is a predictive method based on the sun's diurnal movement. All the planets and axes are moved by the same amount as the sun moves in one day, which symbolises the passing of one year in the life on an individual. The movement of the Medium Coeli is also based on the movement of the Sun, which means: the distance between the Sun and the Medium Coeli always remains the same. The movement of all the other house cusps is then based on the movement of the Medium Coeli.
It is possible to use either the actual distance travelled by the Sun on any particular day (the true solar arc) or to use an average value: the mean solar arc that can be found using the Naibod key which is the Sun's average diurnal movement of 59 minutes and 8.33 seconds of arc.
Interpolation can be used to calculate the true solar arc. The resulting value is only slightly different from the average solar arc.
Consequently an easy way to calculate the solar arc is to consider the progressed sun as moving a year for a day, but then rotating the other planets the same number of degrees. If a solar arc calculation is desired for thirty years after a birth date, for example, simply move the sun and all the other planets along 30 degrees from their natal positions.
Solar arcs are interpreted similar to other secondary and tertiary progressions. The following points in particular should be considered:
- A directed planet or axis that forms an aspect with a horoscope factor in the natal chart will act as a trigger, i.e. it will activate the factor in the natal chart. A tight orb should be used, generally under one degree, which is valid for a period of several months
- A directed planet or axis that moves into a new sign or house indicates the beginning of a new phase in life
- Axes: A very accurate time of birth is needed to work with directed axes because the inaccuracies in prediction are otherwise so great as to make them virtually meaningless
Some astrologers find solar arcs to be extremely accurate. Others have less success with them.
- The Power Degrees of the Zodiac (by Frank C. Clifford, using both natal charts and solar arc (SA) directions; 2016, Astrodienst)
- Tyl, Noel, 2001. Solar Arcs: Astrology's Most Successful Predictive System, Llewellyn Publications