Composite and Davison: a chart of the relationship itself
Synastry compares two people. The composite and the Davison build a third chart, the chart of the couple as a whole. The book has both, and explains how they differ.
Composite: midpoints
For each planet we take its position in both charts and find the middle of the shorter arc between them. The composite Sun sits halfway between the two Suns, Venus between the two Venuses. That moment never happened in the sky: it is a mathematical chart.
Davison: the midpoint in time and place
Ronald Davison’s method takes the moment exactly halfway between the two births and the point halfway between the two places, and casts an ordinary sky chart for it. Its planets are real, so they can sit in different signs than in the composite.
Why the signs differ
The Sun moves about a degree a day, so the composite Sun and the Davison Sun are usually close. Venus and Mars move unevenly, sometimes backwards, and there the two charts can be a sign apart. Change the dates below and see for yourself.
What houses need
The Ascendant and houses of both charts depend on the birth time. If it is not known for both of you, the book shows the composite and Davison without angles and houses.