Planets in the first house synastry overlays
The first house represents our physical body, the image and personality we show to the world and the first impression we give. It represents the first `shield` we put up in front of strangers, before we feel comfortable enough to show our Sun and lastly our Moon (except when the Sun and Moon are in the public houses). The first house cusp represents the Ascendant aka Rising. The first house also shows how we approach life, especially new events, and the Ascendant may be a point of receptivity and attraction of these events. Example if you have a Scorpio Ascendant you will attract crisis like life events that enter your life in Scorpio manner (sudden, transforming, life-altering, intense) and through these events you will shape the rest of your planets/ points in the birth chart. The first house also indicates what kind of environment we grew up in and what traits we were forced to develop to survive in that environment.
In synastry, planets of the partner that fall into your first house (and especially those that conjunct the Ascendant) are of crucial importance for the first house person, because the interaction with the planet person is triggering life events meant to be experienced in order to develop all the elements of the first house's natal chart.
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What does it mean when you have the Ascendant in the last degrees of a sign, and your first house is mostly in the next sign (I am of course talking about the Placidus system). I am mentioning this, because, in my observation, people that have the Ascendant in the last degrees of a sign usually have dominant first house synastry (this may be also explained through the anaretic degree theory). This means you have mastered the sign of the Ascendant in previous lives, or in the first part of your life and you will also have to develop through the matters of the next sign. I have noticed that these people in their childhood and youth, when having first house synastry overlays, they get Ascendant prevalent synastry. But, in the more mature stages of their life, if the first house synastry overlay shows up, it always involves the next sign from the Ascendant.
SUN in the FIRST HOUSE synastry overlay
When your partner's Sun falls into your first house, your partner's ego, external identity, career and life mission will help you on your life path. Your personality and even your physical body could be impacted by your partner's Sun. These people may work together to achieve great things in life, depending also on the first house's placement of the Sun. The Sun may have an energizing effect on the first house person. The life mission of the Sun person depends on integrating the characteristics the first house person is displaying into their conscious identity and even career. In time, because the Sun may gain more expertise in matters of the sign they share in common with the first house, Sun person may consider they should have authority over the first house person and may try to reshape the first house person's image.
Example: Sagittarius Sun and Sagittarius first house.
The person with Sagittarius Sun may try to develop their knowledge and life experiences by going to University, reading and traveling long-distance. The Sagittarius first house was born in a family of University professors and was exposed to Sagittarius like experiences since childhood. They have traveled, are well read, have taken courses on philosophy and religion, etc. What the Sun person is trying to be and how they are trying to shine in the world, the first house already knows because of their exposure to it throughout life. When they meet, the Sun person immediately notices that the first house person is very knowledgeable and that what they are trying to become in this lifetime the first house has already experienced.
MOON in the FIRST HOUSE synastry overlay
VENUS in the FIRST HOUSE synastry overlay
Venus is the planet of relationships and finances. When one person's Venus falls into your first house, your partner's self esteem, way of relating, and finances is influenced by your physical presence. The Venus partner's money, resources and admiration can assist the life path of the first house person.
Usually the physical presence of the first house person is seen as an valuable asset or a resource by the Venus person. Beauty and grace can also be projected on the first house person. The first house person can be the Venus person's muse. Venus is a superficial planet. It has more to do with beauty on the materialistic and mental plane. The values of Venus can also be projected onto the personality and the behavior of the house person. Conforming to these projections will result in an affectionate response from the Venus person. If the house person doesn't have Sun, Moon, Mars, Venus also located in the first house, or if personal planets don't have aspects or house placements that reinforce traits of the sign that occupies the first house, this will be a very superficial liking. In this case, as soon as the first house person expresses their Sun, Moon, Mars, the affectionate response of Venus will disappear and Venus may try to condition the first house to act as the sign of their first house all the time.
The gifts and compliments that the Venus person gives are well received by the first house.
Venus can also be envious of the first house person, because, depending on the Venus person's rising, Sun and Moon, they may not be, or are unable to become what they value and admire (Venus), but the first house person has innately the traits that Venus wants.