Comparative astrology analyses the relationship between two or more horoscopes. It usually involves the natal charts of two people, but in theory any charts that mark a birth, event, or initiation of a process can be compared to each other.
Birth chart comparison for two people involved in an intimate relationship is the most common type. However, the comparison of any two natal charts - such as those of parents and children, work colleagues or employees and their supervisors - can throw light on the underlying dynamics of a relationship. Multiple charts might be interpreted for a family. A comparison of different natal charts can reveal a great deal about the character of a relationship: its potentials, possibilities and dangers.
The astrologer might also compare event charts, such as of a nation's founding date and a natural disaster; a Horary Chart with a client's natal chart; or a worker's natal chart with the moment of a new job offer. In proposing an electional chart for a client's new business, an astrologer would compare it with her natal chart.
Comparative astrology generally makes use of one of three methods:
- Synastry, in which two charts are compared side-by-side or are overlaid.
- Midpoint composite charts, which show the midpoints of planets and sensitive points between the two charts.
- Davison relationship charts, based on the mean values of the birth locations and birth dates.