If understanding the character and potential of an individual person through his or her horoscope can be a demanding task, requiring considerable training and practice on the part of the astrologer, must it not be the case that determining the character and potential of a relationship through Astrology would be even more demanding? After all, when considering a relationship between two people, one would presumably have to understand the capacity for relationship, (of whatever type, be it romantic, business, parental etc.), in each participant, and one would then have to consider how well the individual charts cooperate and conflict with each other. Or so one might imagine.
I believe that if an astrologer has a reasonably sound grasp of ordinary natal astrology, (the study of a horoscope for an individual), it is relatively simple for him or her to extend this understanding to encompass the analysis of a relationship between two people. Indeed the astrological analysis of the quality of a relationship between two people can sometimes be considerably easier than the analysis of an individual!
The reason for this lies in the supreme ability of the human psyche to indulge in all sorts of conscious and unconscious maneuvers in defence of the self. If an individual experiences psychological conflicts, (and most of us do in one way or another), there is a tendency for considerable energy to be expended in trying to project some of the problems onto other people. This may give rise to considerable distortion of that part of the personality that is conscious, with the result that an average person, (when faced with an analysis of his or her chart), might accept a portion of the interpretation as being true, while the rest would be firmly denied - "Everybody else has these good/bad qualities, but definitely not me!" Much of course would depend on the skills and sensitivities of the astrologer, and the level of self-awareness of the owner of the horoscope.
Within a relationship, projections and other psychological defence mechanisms still occur, but to an outside observer they are much more visible, since they tend to be played out between the two partners, and both players are visible.
Capacity for Relationship
Many astrologers, who in my humble opinion should know better, make rather sweeping value judgments about Sun signs, Moon signs and, for that matter "Other signs". Some examples: "All Virgos are critical", "Pisces is a lazy sign", "Neptune in Scorpio shows drug addiction," "Moon in Scorpio or Capricorn is a terrible position for a woman", "Gemini men are two-faced".
When it comes to relationships, the value judgments become even more ridiculous. Some examples: "If Uranus in a man's chart aspects the Moon or Venus in a woman's chart, the relationship will be short-lived", "Sagittarius and Capricorn together are a recipe for disaster in a marriage", "Saturn aspects give unhappy relationships", "Saturn aspects give lasting relationships".
While the above statements are sometimes true, they are also sometimes false! They are sometimes also fairly irrelevant. If, say, there are 12 important points in a horoscope, (e.g. Sun, Moon Ascendant and 9 planets), there will be 144 interactions which have to be considered between two horoscopes in a comparison. Certainly the matter is more complicated than this, since the planetary positions are not completely independent. The inner planets tend to group near the Sun, while the outermost planets move so slowly that one is unlikely to come across a relationship in which, for example, one person has Pluto in opposition to the other person's Pluto. There are, nevertheless, many factors to be considered, and I believe it is wrong to make a hasty judgment based on one or two factors alone.
Traditionally, planets in Libra and planets in the 7th house, together with the sign , house and aspects of Venus indicate relationships. To the extent that Venus indicates our attempts to achieve objective understanding of the outside world, and that these attempts are often best achieved through interactions with other people, and that these interactions are called relationships, Venus in our chart may indeed be described as an indicator of our capacity to relate. Similarly planets in Libra tend to operate in a way which is cooperative rather than assertive, and thus may give an indication of how an individual cooperates with others. Planets in the 7th House often represent qualities that are obviously projected onto people and circumstances in the outside world, and to that extent the individual finds himself or herself involved with other people from an early age.
However, it would be overly simplistic to ignore the other planets, signs and houses. There are plenty of people with no planets in Libra or the 7th house, and who lack a strongly aspected Venus. They are not all hermits!
It could be argued that everyone needs relationships. People strong in Air signs/planets/houses need to understand the world through interactions with others; people strong in Water need to relate emotionally, so they need people to "e-mote" onto; strong Earth types need people to organise and build with, while strong Fire types need people to compete with.
Synastry in Relationships
Synastry involves the comparison of the horoscopes of two (or more) people. It is far more important that the horoscopes are "compatible" according to the wishes, beliefs and needs of the two people, (as shown by their individual charts), than that they are "compatible" according to some abstract theory, or collection of aphorisms.
One can learn about the likely meaning of planetary aspects between the horoscope by first understanding what such aspects would mean in an individual chart, and then allowing for the fact that one person will usually express one planet, while the second planet will usually express the other.
For example, Alison has her Sun in Cancer in conjunction with Bill's Saturn (also in Cancer). In a natal chart, a Sun-Saturn conjunction indicates that the conscious will is closely tied up with and influenced, for better or worse, by the conscience structure. Depending on other factors, (as always!), Sun-Saturn can indicate a person whose spirit is crippled by an overbearing self-criticism, or it can indicate a person with great patience, resourcefulness and perseverance in working constructively towards goals. Now what is likely to happen in the relationship? Other factors left aside for the moment, it is likely that Alison will display a relatively spontaneous tendency to express herself caringly, while Bill will also be caring, but in a much more structured and less spontaneous way. The interaction of Alison's Sun and Bill's Saturn could have very positive or very negative results, (much as may a Sun-Saturn conjunction in an individual chart). Much might depend on what kind of relationship was involved - the energies would express themselves quite differently depending on whether we were considering a marriage, a father-daughter relationship, a business relationship and so on.
The Relationship Chart
There is a method, as far as I know first described by Ronald Davison in "Synastry" (1977) in which a single chart is drawn up to represent the potential in a relationship. The idea is to calculate a horoscope for a point half way in time between the two birth dates, and half way in space between the two birthplaces.
On the assumption that time is linear, one can easily calculate the time half way between two birth dates and times, (remember to take into account leap years). Ronald Davison calculates the place for the Relationship Chart by taking the average of the Latitudes and Longitudes, while I believe it is more logical to take the midpoint of the great circle joining the two birth places. It must be said that when the two birth places are reasonably close, (within a few hundred miles of each other), using Davison's method yields adequate results, and involves no spherical trigonometry, (or a globe and a piece of string!).
A simple example - Person A born 1st January 1963, at 8 am GMT, Person B born 2nd February 1967, at 10 am GMT. 1964 was a leap year, so there are 1493 days and 2 hours between the two birth times. Half of this amount is 746 days and 13 hours, giving a date for the Relationship Chart of 16th January 1965 at 9 pm GMT. Person A born 40 degrees North, 90 degrees West, Person B born 42 degrees North, 92 degrees West, gives a Relationship Chart place of 41 degrees North, 91 degrees West, to a high degree of approximation.
In my experience, the Relationship Chart can be used to describe the nature and quality of the relationship itself, irrespective of the contributions made by the participants, (though information about this may be discovered by comparing the Relationship Chart with the horoscopes of the individuals). In particular, methods used in Natal Astrology, such as progressions and transits, can be applied to the Relationship Chart.
The Relationship Chart can be relocated to wherever the relationship is based, in much the same way as an individual horoscope may be recalculated to take into account a change in residence to a location significantly distant from the birthplace.