A 2nd-century astrologer in Alexandria placed two birth charts side by side, looking for the angle between Venus and Mars. He was not the first to compare charts, but his method became the foundation of what we now call synastry. Synastry is the comparison of two natal charts to assess the dynamics between individuals. A free synastry chart, generated online, lists the aspects between each person’s planets and shows which houses each planet falls into. The interpretation rests on a few core principles: planetary meaning, aspect type, and house overlay.
Where the Practice of Comparing Charts Began
The earliest surviving treatise on synastry appears in Claudius Ptolemy’s Tetrabiblos (c. 150 CE). In Book IV, Ptolemy discusses how to compare the charts of a married couple by examining the positions of the Sun, Moon, and Venus. He argued that harmony depended on the Moon of one person being in a favorable aspect to the Venus of the other. The method was not speculative: Ptolemy drew on centuries of Babylonian and Egyptian observation of planetary cycles.
Vettius Valens, a 2nd-century astrologer whose Anthologies survive in Greek, gave detailed examples of synastry for lovers and business partners. He used the same tools we do today: aspects, house placements, and planetary dignity. The difference was that Valens worked with a tropical zodiac and whole-sign houses, while modern free charts typically use Placidus houses.
The practice faded in Europe during the Middle Ages but survived in Arabic astrology. Al-Biruni, in his 11th-century Book of Instruction in the Elements of the Art of Astrology, devoted a chapter to “the comparison of nativities.” He emphasized the role of the Lot of Fortune and the nodes of the Moon.
Modern synastry was revived in the early 20th century by Alan Leo and later systematized by Robert Hand in his 1980 book Synastry: Understanding the Astrology of Relationships. Hand’s work made synastry accessible to amateurs. Free online charts, which appeared with the rise of astrological software in the 1990s, put the same calculations into anyone’s hands.
What Each Planet Means in a Comparison
The planets carry the same meanings they do in a natal chart, but in synastry they describe how two people interact. We look first at the personal planets: Sun, Moon, Mercury, Venus, Mars. These govern daily life and immediate emotional exchange.
The Sun represents the core identity, ego, and life purpose. When one person’s Sun aspects the other’s Sun, it shows how their fundamental selves align. A Sun-Moon aspect is often considered the most important: it connects the conscious self with the other’s emotional needs. A Sun-Venus aspect suggests natural affection. A Sun-Mars aspect can bring passion or conflict.
The Moon rules emotions, habits, and the unconscious. Moon-Moon aspects show emotional resonance. Moon-Venus aspects indicate nurturing love. Moon-Mars aspects can stir up quick reactions.
Mercury governs communication. Mercury-Mercury aspects show how two people exchange ideas. Mercury-Venus aspects make conversation easy and pleasant. Mercury-Mars aspects can lead to arguments or stimulating debate.
Venus is love, attraction, and values. Venus-Venus aspects indicate shared tastes. Venus-Mars aspects are the classic “chemistry” aspect: desire, magnetism, sometimes friction.
Mars is drive, anger, and sexuality. Mars-Mars aspects show how two people assert themselves. Mars-Saturn aspects can create frustration or endurance.
Jupiter expands whatever it touches. A Jupiter-Venus aspect brings generosity and luck in love. Jupiter-Saturn aspects can indicate a relationship that grows slowly but steadily.
Saturn brings responsibility, limitation, and time. Saturn-Venus aspects often indicate a karmic bond or a relationship that requires work. Saturn-Moon aspects can feel heavy or restrictive.
Uranus, Neptune, and Pluto are generational planets, but in synastry they can indicate long-term themes: Uranus for sudden change, Neptune for illusion or spiritual connection, Pluto for power dynamics and transformation. Their aspects to personal planets are significant but should not be overemphasized.
Synastry Aspects, Explained
The angle between two planets determines the quality of the connection. Free synastry charts list these aspects with their orbs (the allowed deviation from the exact angle). Standard orbs are 8 degrees for the Sun and Moon, 6 degrees for personal planets, and 5 degrees for outer planets.
Conjunction (0°): The most intense aspect. The two planets merge their energies. A Venus-Mars conjunction can be magnetic; a Saturn-Moon conjunction can feel heavy. The closer the orb, the stronger the effect.
Opposition (180°): Tension and attraction. The planets are in complementary signs, creating a push-pull dynamic. An opposition between the Sun and Moon often means two people are drawn together but have very different core needs.
Trine (120°): Ease and flow. The planets are in the same element. A trine between Venus and Jupiter is effortless affection. Trines can be so comfortable that they are taken for granted.
Square (90°): Friction and growth. The planets are in signs of the same modality but different elements. A square between Mars and Saturn can mean frustration or discipline. Squares are not bad: they create the tension that drives change.
Sextile (60°): Opportunity. The planets are in complementary elements. A sextile between Mercury and Venus makes conversation easy. Sextiles require conscious effort to activate.
We look for patterns. A cluster of aspects to one planet (for example, three planets aspecting the other’s Moon) shows that person’s emotional life is deeply affected. A stellium in one house indicates a strong focus on that area of life.
How House Overlays Shape the Connection
When we overlay one chart onto the other, each planet falls into a house of the other person’s chart. That house shows the area of life where the connection is experienced.
A person’s Venus in the other’s 7th house (partnership) is a classic marker of romantic attraction. Venus in the 4th house suggests domestic comfort. Mars in the 5th house (romance, creativity) can indicate passionate dating. Saturn in the 1st house may mean the person feels responsible for or burdened by the other.
The house overlay is often more revealing than the aspect alone. A square between Sun and Mars might be challenging, but if the Sun is in the other’s 10th house (career), the friction may manifest as professional rivalry rather than personal conflict.
Free charts typically show the house positions in a table or on the wheel. We check which houses are activated by the other person’s planets. The more planets in the 1st, 5th, 7th, and 8th houses, the more intense the relationship.
Synastry vs. Davison and Transit Charts
Synastry is not the only way to analyze a relationship. The composite chart, developed by John Townley in the 1970s, calculates a single chart from the midpoints of both people’s planets. It represents the relationship itself as a separate entity. Synastry shows interaction; the composite shows the relationship’s identity and purpose.
The Davison relationship chart uses the midpoint of birth times and locations, creating a chart for the relationship’s “birth moment.” It is less common but useful for long-term partnerships.
Transit synastry compares current planetary transits to both natal charts. It shows what is happening in the relationship right now.
We use synastry as the first step. It answers the question “how do these two people affect each other?” The composite answers “what is this relationship for?”
Common Synastry Myths, Debunked
“Many squares mean the relationship is doomed.” Squares create friction, but they also create motivation. A relationship with only trines can stagnate. The balance of hard and soft aspects matters more than the count.
“Synastry predicts whether a relationship will last.” It does not. Synastry shows dynamics, not outcomes. Free will, timing, and life circumstances override any chart.
“Free synastry charts are inaccurate.” The calculations are identical to professional software. The only variable is the house system (most free sites use Placidus). The aspects are correct as long as birth times are accurate.
“Synastry only applies to romantic partners.” It works for friendships, family relationships, and business partnerships. The same principles apply: look at the relevant houses (3rd for siblings, 10th for colleagues) and the planets involved.
“The Sun sign compatibility is enough.” Sun sign compatibility is a single aspect among dozens. Two people with incompatible Sun signs can have strong Moon, Venus, or Mars connections that override the Sun.
FAQ
What is the most important aspect in synastry?
The Sun-Moon aspect, because it connects core identity with emotional needs. A harmonious Sun-Moon aspect (trine or sextile) indicates natural understanding; a square or opposition requires conscious adjustment.
Can a synastry chart predict if a relationship will end in divorce?
No. Synastry shows strengths and challenges, but it cannot account for personal growth, external events, or choice. A chart with many Saturn aspects may indicate a long-term bond, but it could also mean a difficult one.
How do I know if a synastry chart is “good”?
Look for a mix of soft and hard aspects between personal planets. A few squares or oppositions provide energy; several trines and sextiles provide ease. Avoid charts with no aspects between personal planets or with only outer planet aspects.
Do I need exact birth times for synastry?
For accurate house overlays and Moon sign, yes. Without birth time, the aspects between planets remain valid, but the house placements will be approximate. Many free charts allow you to enter “unknown” time and still generate aspects.
Synastry is a tool for understanding, not a verdict. Once you learn to read the aspects and house overlays, you can apply the same method to any relationship. For deeper study, explore our guide on composite charts and the glossary of astrological aspects. The full astrological transits hub offers more on timing and relationship dynamics.