Combined Charts
Contents
A combined chart is calculated by taking an "average" of two or more base charts, to produce a single representative chart. There are five methods for calculating combined charts in this program.
Composite - Midpoints
The midpoint of the shortest arc joining each chart point, angle and house cusp within the two base charts becomes the position of that point in the new chart. Note that it is sometimes possible for the house cusps to be out of expected order after this method is applied. See Composite Chart Houses.
When calculating any type of composite charts, Solar Fire first determines the composite longitudes, and then determines the composite right ascensions separately. Thus any right ascensions for a composite chart in Solar Fire are true composite right ascensions rather than right ascensions of composite longitudes.
Composite - Derived Ascendant
The midpoint of the shortest arc joining each planet, asteroid and the Midheaven within the two base charts becomes the position of that point in the new chart. The chart angles and house cusps are then derived from the Midheaven according the new chart's latitude.
Composite Group - Derived Ascendant
The average position of each planet, asteroid and the Midheaven along the shortest arc joining them within each of the base charts becomes the position of that point in the new chart. The chart angles and house cusps are then derived from the Midheaven according the new chart's latitude. Robert Hand describes this method in detail in his book "Planets in Composite".
Relationship - Davison
The new chart is calculated as if it was a natal chart for the midpoint in time, latitude and longitude of the two base charts.
Coalescent
This is a method which was developed fairly recently by Lawrence Grinnell and David Dukelow, and is published in a booklet “Coalescent Horoscopes - A New Method of Synastry” published by ACS Publications in 1992. To determine each planet’s coalescent position, first find the shortest arc between the pair of planets in the two base charts and divide this by 360 degrees to get the harmonic factor. Next, multiply the position of the planet which has the earliest zodiacal position by the harmonic factor. This must be repeated for each pair of planets and for the Midheavens. The authors of this technique recommend using the Meridian house system to derive the coalescent house cusps from the coalescent Midheaven.