In this article, I present a summary of different approaches to synastry astrology - the astrology of human relationships. There are certainly others, but rest assured that the ones described below are the most important and common in western astrology.
The classic synastric overlay is the most widely used method of horoscope comparison. In my experience it is also the most reliable and insightful approach when it comes to the base matter of interaction between two people. It must be stressed that synastry astrology is by no means reserved only for romantic relationships. It works equally well for friendships, collegial/professional relationships, students, siblings, children to their parents, etc. What more is, for a profound understanding of synastric compatibility, one must be aware of the levels involved. The synastric overlay investigates relationships from the standpoint of each individual. It shows how the interaction affects each person according to his or her given psychology. Ideally, synastry astrology describes the important physical, mental and spiritual ramifications of two human beings meeting and connecting. On the other hand, nothing guarantees that any one single emotion or attitude experienced from one side will be reciprocated.
How does synastry work? The position of the planets in chart one is "tossed" into or laid over the birth chart of person number two, resulting in a number of new aspects between the planets of person one and the planets of person two. And also in number one's planets falling into some particular houses of chart/person two. Then the procedure is reversed. Sounds complicated? At first it is. Practise makes perfect, though. Example: My Venus is at 16 degrees Pisces. If you would compare me synastrically to someone whose Moon was at 16 degrees Virgo, it would be revealed that we had a synastric Venus-Moon opposition. Why? because if you'd put my Venus in the other horoscope, it would be exactly opposite that Moon. (Since Pisces and Virgo are opposite zodiac signs). Likewise, if you'd put the Virgo Moon into my horoscope, it would indeed oppose my Venus at 16 degrees Pisces. The bottom line of synastric comparisons is that each partner reacts according to the planet that is involved in the aspect. In the example above, my response to the other person would be in terms of the love-experience nature of Venus (or in terms of some of its related qualities), and person number two would respond in terms of the Moon's essence of instinctive, receptive feeling.
The solar chart is derived from old sources, yet its use in contemporary synastry astrology could be considered quite controversial. It is closely connected with the so called "arabic parts", and holds to the idea that the most important initial contact between two people comes from the projected persona of one individual to the natal planet positions of the other. Astrologer E.W Neville claims that first attractions are shown most importantly by how someone's solar chart "captures" someone else's planets by house placement and aspects. The natal person is (more or less) drawn to the solar person's projection of his or her planetary energies. The solar chart is created by using mathematical formulas to assign new zodiac sign placements for all the planets. It is the natal ascendant that does most of the work here; hence the strong sense of outer projection and radiation that defines the solar chart. To me, the solar chart is a very interesting concept, and Neville presents a convincing argument for it in his book Planets in synastry. I am not ready, though, to confess to its ultimate value and truth.
Following synastric overlays, the composite horoscope is the most common method of analyzing relationships. It rests on a different premise compared to classic synastry astrology. More correctly, it is a continuation, an attempt to deepen the understanding of a relationship. The composite horoscope describes the relationship itself, the union of the energies of each individual. In this respect it illustrates the outcome of a relationship, after two people have moved beyond the initial stage of relating and formed an actual relationship. So, in contrast to the synastric overlay, the composite doesn't say much about each person's expectations, but rather treats the association as an entity of its own. Surely, it is reasonable to claim that every single human relationship is unique, and comes to acquire an essence different from any other. To analyze this is the aim of the composite horoscope.
The composite horoscope is created by calculating the midpoints between the planets in horoscope number one and horoscope number two, resulting in a completely new horoscope to interpret. Example: If my Sun is at 5 degrees Aries and your Sun at 5 degrees Leo, then our shared composite Sun will be at 5 degrees Gemini, which is the point in the zodiac exactly between our two Suns. The same procedure goes for the rest of the planets, and for the angles such as the ascendant, and then the Composite horoscope is done and ready for analysis.
Also known as the time/space chart, this relationship chart method was created by astrologer Ronald Davison. In theory it is similar to the composite horoscope. But in the Davison horoscope, the relationship chart is calculated for a different kind of "midpoint" - the midpoint between the moments in time and geographical locations of the births of two people. Because of this it has an actual time and location to consider. A simplified example: If I was born in New York, 1975, on the second day of april at noon, and you were born at noon on the fourth of April, 1975 in Boston - the Davison chart between us two is erected for noon, third of april, 1975, at a location exactly between Boston and N.Y. Usually things are not so simple however, and a computer is almost a requirement for the calculations. I cannot make any statement about the usefulness of the Davison chart since my experience with it is close to none, but it's an interesting potential to investigate within synastry astrology.
This is a very interesting approach that basically belongs in the category of "Horary astrology", the astrology of questions. Horary is an ancient horoscope method of answering questions, and a first meeting chart can be seen as the attempt to create "the question to which a relationship is the answer". If this is too cryptic, you mya see the first-meeting chart as a horoscope that shows the potentiality of a relationship, the seed of an interaction, which in itself can lead pretty much anywhere. The only requirement of the first meeting chart is, perhaps obviously, to know the time and place where two people first met at all. An exact time is desired, but even a less then exact time can reveal fascinating expectations and still unformed energies.
Erected for the time a wedding took place, or used as a predictive tool to determine the best possible future time for the wedding to happen. Wedding charts are old and worn examples of Horary astrology, but could certainly be used with good results if one is clear about what such a horoscope would be able to answer.
Individual needs and desires are of course essential also in comparison between horoscopes. This is shown by each partner's natal horoscope, where the needs and approach to love and partnership is mirrored in sign placements, house placements and aspects. Interestingly, the natal chart of one partner could also be compared to a Composite chart, Davison chart or First meeting chart, to reveal the impact of the relationship upon this partner. Or to show his or her experience of that particular relationship. The potential levels of activity and significance in synastry astrology, as mentioned in the beginning, almost endless. As always, it is important to discriminate the essential from the barely noticeable, in order to accurately observe and form judgements about how two people get along.