Synastry is the comparison of two natal horoscopes to determine the dynamics of the relationship, compatibility and any potential positive or negative influences within that relationship. It can be used accurately for any type of relationship, even casual ones, though it is most often used for romantic partners. There are two approaches to synastry: the comparison of aspects and signs between two charts and the imaginary placement of one person's planets in another's chart to see how it would affect the others chart. While the second is highly enlightening, the first method is the most common definition and most commonly used system of synastry.

The major problem with which astrologers contend is the client who says "I am a Taurus and my boyfriend is an Aquarius, so we can't get along." The use of the Sun sign alone in the astrology of relationships is everywhere you turn. This narrow view appears in magazines, articles and, unfortunately, many published books. If the Taurus and Aquarius suns happened to be within an 8 degree orb of each other, which is unlikely, they could indicate minor ego conflicts. But the rest of the chart could show all comparative trines, indicating an ease of interaction on all other levels. The first thing we need to get past in using synastry is that "sun sign only" mentality.

Before I go further, I will mention a newer technique for evaluating relationships, the Composite Chart. This type of chart uses the midpoints between A's planets and cusps and B's planets and cusps. The result is a single chart for the relationship itself. I have used this method also, but have found it to describe the relationship itself more as a separate entity, using a third person perspective of the relationship. While a composite chart is highly accurate for those purposes, it does not describe the dynamics of the relationship as well as synastry. It can advise of potential problems, but cannot pinpoint them regarding which person is the major player in those issues or why. It is invaluable in determining the purpose of a relationship or what the higher purpose might be. For instance, a composite chart of a married couple might find heavy emphasis on the fifth house (children, sex, pleasure) or in the 10th (business, career, etc.), thus describing the major thrust of the relationship. It is not a technique to be overlooked, but I feel personally that it does not tell as much about interpersonal dynamics as synastry.

I look for two major comparative influences first in a synastry chart, neither of which involves the traditional sun compared to sun aspects as I mentioned above. The first is A's sun conjunct B's ascendant. It indicates a powerful link on all levels, spiritual, emotional and physical. The second is A's sun conjunct B's moon. In a romantic relationship, it is described as better if the sun is the man's chart and the moon is the woman's. But as we are going into the 21st century, I assume we are past those stereotypes and have seen it work just as well with the charts of any two people. If either of those aspects is present, it is a good start, but it is still necessary to look at the rest of the chart's aspects.

I have a third aspect I look for that seems to me to be even more powerful, when A's chart ruler (ruler of the ascendant) is conjunct B's sun or moon. This aspect seems to create an immediate and automatic bond between the two charts and people. I think more research needs to be done on that comparative conjunction. Some other important aspects are any of "A's" planets conjunct "B's" planets or vice versa. Naturally, the condition of A's planet natally, as well as B's, has to be taken into account. For example, even A's sun conjunct B's moon could still indicate difficulty if both planets are afflicted in their respective natal charts.

Aspects between the two charts are interpreted the same way as in natal astrology, with the exception that two people are involved in the aspect. I use the same orbs that I use for natal astrology, 8 degrees orb, especially for sun or moon aspects. I lower the orb a bit for planets that are not highly important in the natal chart. At least six degrees orb is normal for all major aspects. The major aspects to look for are conjunction, opposition, trine, square and sextile. I do not intend to go into minor aspects here, but I would like to mention briefly the comparative inconjunct, or quincunx. It is important if there are comparative inconjunct aspects to the key relationship planets, for example, in a child/parent relationship, you might find this aspect with sun, moon, or rulers of the related houses. This would indicate either the child having to take care of the parent with problems, or depending upon the stronger planet, possibly a parent with a child that requires special health care.