One can get a good glimpse of one's needs and desires in relationships through the Moon, Sun, Venus, Mars, and Ascendant. In general, the Sun and Moon primarily indicate true needs, Venus and Mars primarily indicate desires or attractions, and the Ascendant indicates some general needs and desires.
Obviously, the best relationship is one where one's needs (Sun and Moon) and desires (Venus and Mars) are both met. One way to do a general compatibility test between two people is to look at the relationship between each of these five placements across the two charts: Sun-Sun, Moon-Moon, Venus-Venus, Mars-Mars, and Asc-Asc. When the placements are in the same gender (fire and air are masculine, earth and water are feminine) or element--the same element or a close sextile is best--these placements are basically compatible. But of course it's much more complicated than that. To begin, here's a short discussion of the Moon, Sun, Ascendant, Venus, and Mars in terms of compatibility issues, listed in order of their importance and weight in synastry.
The Moon is the most important 'planet' in relationships. The Moon represents deep security needs, and it is difficult to be with with someone who does not offer us what our Moon needs. The element of one's Moon sign and house (I recommend using Porphyry House System) are elements that one's partner must have in a significant way if we are to be compatible with them, or at least one of these two elements (if there are two). If one's partner is not strongly made up of the same element of one's Moon sign and/or house, then strong harmonious aspects between one's Moon and the partner's chart could also create a harmonious Moon connection.
Positive Moon compatibility will show two people who feel "at home" with one another, and instinctively care for one another. It's like being family. That's why it's the most important planet in synastry. If you want to spend the rest of your life sleeping in the same bed as someone else, you obviously need to feel like family to one another.
In particular, since the Moon is feminine, the Moon shows what a man needs in a woman. But the Moon definitely shows very vital needs for women, as well.
Since the Sun is masculine, the Sun particularly shows what a woman needs from a man. But also, for men, it shows an element that must be present in their partners for compatibility to be present. If a man does not have the qualities of the element of a woman's Sun, it will be difficult for her to ultimately trust and feel respect for him. At the very least, she must feel the basic energy as a masculine/feminine polarity, i.e. an Earth or Water Sun sign needs strong feminine energy, and a Fire or Air Sun sign needs strong masculine energy.
The Ascendant shows our view of ourselves and of life, and how we bring ourselves into the world. So being with someone who has some major personal placements in harmony with our Ascendant is also very important. It's fairly important for two people to have their Ascendants of the same gender (fire and air signs are masculine, water and earth signs are feminine), and it's great if they are of the same element or a close sextile, but if not, this can be balanced by having a conjunction or opposition between the Sun, Moon, or Venus of one person and the Ascendant of the other. It's better if each person 'receives' one of these conjunctions or oppositions if the Ascendants are in a more conflictual sign relationship (square, semi-sextile, or quincunx), and especially if the Ascendants are square to one another.
Venus shows what a man is attracted to in a woman. Being with a woman who is strong in his Venus element, or who makes close sextiles to his Venus makes the relationship much easier, and increases sexual desire and compatibility. For a woman, her Venus is part of her feminine nature, so it is definitely best when her partner also relates well with this element. When people have positive Venus synastry, they naturally like and love each other. That, of course, is what makes positive Venus synastry so great.
Mars shows what a woman is attracted to in a man. Being with a man who is strong in her Mars element, or who makes close sextiles with her Mars, makes the relationship much easier, and increases sexual desire and compatibility. For a man, his Mars is part of his masculine nature, so it is best when his partner also relates well with this element.
However, too much Mars is really not a good thing for synastry and compatibility, and most Western astrological literature will not tell you this. Mars is not always the "pretty face" we've made him to be. Major conjunctions, squares, oppositions, and quincunxes with Mars are not really desirable for people looking for the most enjoyable relationship possible, but again, "a part is not the whole," so we still have to look at a lot more. It is certainly possible to have a rewarding connection with someone, even if you have the sort of Mars synastry discussed above, but that still doesn't mean that I would particularly recommend it. I believe that research and direct experience will show you the truth of what I'm saying. Sure, Mars contacts are great for sex, but how many hours a day can you have sex? And how many hours a day do you just spend time talking with a partner, not grunting like animals? In short, if you are the sort of person who can't get enough of conflict, major and hard Mars synastry may be just what the doctor ordered.
Connections that seem fated and karmic often include one or more of the following interplays between the charts: Conjunctions to the Vertex (especially personal planets or the Ascendant or Descendant), IC (cusp of the 4th house), South Node, and the progressed Vertex or Descendant.
Attraction does not guarantee longevity, of course. It generally does precede a significant relationship, and in some cases, is important simply for what it is, as we may not have a current interest in establishing binding ties.
Romantic attraction is often indicated when the Ascendant of one person conjuncts or opposes another person's Sun, Moon, Venus, or Mars. Most potent when it comes to attraction are interplays between personal planets that are yin and yang (feminine and masculine), as these can arouse much intrigue. If we consider the Ascendant as a yang energy, and exclude the neutral (unisex) personal planet Mercury, then we have the following interplays:
Of the planetary pairs in the list above, Sun-Venus seems to be the least sexual and romantic (it generally inspires a strong affection).
You might want to consider determining parallels and contra-parallels between planets in synastry. I generally do consider them, although I don't always use them if I am doing a quick analysis. If you want to know how to determine parallels and contra-parallels in declination.
Parallels are likened to conjunctions, and contra-parallels to oppositions. When planets form an aspect to one another in synastry and are also parallel to each other, the parallel acts to reinforce that aspect.
Parallels in synastry are as easy, natural, and spontaneous connections between the two energies represented by the planets involved. The interplay is easier than the conjunction, and the affinity is spontaneous. Saturn parallels to personal planets are a little easier than the conjunctions in synastry. The Saturn person is not as inclined to criticism or control as is often the case when it is an interaspect in synastry. The stabilizing influence of Saturn is more prominent. When the planets involved are yin and yang (such as Sun with Moon, Moon with Mars, Venus with Mars, etc.), we have some wonderful connections.
Some astrologers use Solstice Points (Antiscia) in Synastry work, where the Antiscion degrees of one person's planets and points are compared to the other person's natal positions.
Look for close conjunctions and oppositions from the planets in one person's natal chart to the other person's Antiscion points.
The woman's solstice Venus exactly conjunct the man's Descendant in the synastry of a couple who both felt they fell in love at first sight (no other Ascendant-Descendant connections in regular synastry); a man whose Venus-Moon conjunction did nothing but square his partner's Saturn—the anticision degrees of Venus-Moon tightly conjunct her Sun/Moon midpoint; in two cases of married couples with no Sun-Moon interaspects nor Sun/Moon midpoint contacts, solstice Venus and solstice Mars, respectively made tight conjunctions to Sun/Moon midpoints; tight solstice Vertex conjunction to the Moon; exact solstice North Node conjunct Venus in synastry; a man's solstice Sun tightly conjunct a woman's Moon in a relationship in which proposal of marriage occurred within the first two weeks of meeting (they did marry).
With midpoint work, it is customary to consider the following aspects: conjunction, opposition, square, semi-square, and sesquiquadrature. When a planet makes any of these aspects to a midpoint, the notation looks like the following (where, for example, Venus is the planet forming one of these aspects with the Sun/Moon midpoint):
In synastry, the Sun/Moon midpoint is very often activated by the other person's personal planet in significant relationships. The Sun/Moon midpoint represents a point of "inner marriage" and symbolizes close personal relationship. If another person's personal planet, and especially the Sun or Moon, forms an aspect such as the ones referred to above, that person represents some kind of promise of integration to the Sun/Moon midpoint owner. Whether or not integration actually happens, this synastry connection offers a promise which can be difficult to resist. Therefore, it is a strong indicator of attraction.
The Sun/Moon midpoint is often activated by transit and by progression when a person forms a significant relationship, and is often seriously challenged when there are disruptions in significant relationships, such as divorce itself. An example:
Progressed Mercury=Natal Sun/Moon
Transiting Pluto=Natal Sun/Moon
Transiting Uranus=Natal Sun/Moon
Transiting Saturn=Natal Sun/Moon
The more aspects, the more likely a significant event will occur. When the first overt signs of challenges to the marriage occurred, Progressed Mars=Natal Sun/Moon. When separated, Uranus progressed to an exact square to the natal Sun/Moon. At the time of the legal divorce, no directions or transits to the Sun/Moon were present. The divorce was simply a legality. The disruptions that meant something were represented in the transits and directions to the Sun/Moon midpoint.
The Sun/Moon midpoint is a highly sensitive point in synastry. Inherent in its symbolism is the desire for companionship. Another midpoint that you might want to consider in relationship analysis is Venus/Mars, which Reinhold Ebertin in his most influential book, The Combination of Stellar influences, referred to as "the impulse to love" and "passion". I do not believe that if somebody's personal planet = my Venus/Mars that this would be suggestive of a powerful connection between us in general. In relationship, it may just as well signify a short-term affair or an impulsive attraction. Other strengthening factors in synastry should be present in order for this connection to mean more, of course. However, it can be interesting to construct astrological "statements" (such as her Venus/Mars= his Sun) in order to arrive at more details in a relationship analysis. Of course, it should also be revealing if we take a look at the midpoint statements made in the individual charts. If, natally, a person has Venus/Mars=Saturn, how might that play out in terms of relationship potential and expectations?
Some of us look to astrology to try to understand what happened to a relationship that has been dissolved.
Sometimes, relationships start off with so much promise and "oomph" that they leave individuals wondering "what happened?" when the relationship cools off. Most of the time, the answers are to be found in natal chart synastry. However, there are some answers in progressions as well.
An example is a relationship that started off with tremendous power. Both individuals were immediately taken with each other. After some months, the feelings seemed to be dying, particularly on the man's part. The synastry between the natives was good, but not spectacular. What may explain the fizzling out that occurred is the man's progressed Sun/Moon midpoint.
At the time of meeting, his Sun/Moon midpoint was directly opposite (exact) the woman's natal Moon. This is a powerful connection, as the Sun/Moon midpoint represents close relationship. The Sun/Moon midpoint is the "inner marriage", and it is a highly sensitive point in synastry.
The progressed Sun/Moon midpoint moves at an approximate rate of 6 or 7 degrees per year, which translates roughly to a little more than one degree every 2 months. Because this couple does not have interchart aspects between the Sun and Moon, nor Sun/Moon midpoints, in their natal charts, the progressed to natal contact seems to have been a transient thing. As the man's progressed Sun/Moon midpoint progressed beyond the woman's Natal Moon (out of range and no longer in aspect), the connection fizzled as well, considering there weren't strong, permanent natal connections in the first place. These types of situations can happen in synastry between other progressed points and natal points, some of which are longer-lasting. For example, if my progressed Mars conjuncts your natal Venus, the effects would last (depending on Mars' speed at the date that corresponds with my secondary progressions) quite possibly a good 2-3 years if we allow a small orb, and many more years if we accept a larger orb of influence.
The composite chart is a chart of the relationship itself and the dynamics of the relationship.
When interpreting interchart aspects, keep in mind: