But of course it's much more complicated than that. To begin, here'sa 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 sign 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
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