Synastry is the astrological technique of comparing two natal charts to assess the dynamics between two people. It examines the aspects — geometric angles — that form between one person's planets and another's, and reads those aspects as indicators of attraction, friction, growth, and challenge in the relationship.
Synastry is chart comparison. You take two complete natal charts, overlay them, and examine every aspect between one person's planets and the other's. The resulting aspect grid tells you where the connection is easy, where it is difficult, where it is electric, and where it is invisible. It is the closest thing astrology has to a relationship X-ray.
Synastry is not sun-sign compatibility. The magazine version of astrology ("Aries and Libra: a passionate match!") uses one variable out of dozens. Real synastry uses every planet in both charts. Two people with incompatible Sun signs can have extraordinary Venus-Mars chemistry that more than compensates. Two people with compatible Sun signs can have a Saturn-Moon square that makes the relationship feel like a slow siege. The Sun sign tells you almost nothing about the full picture.
Synastry is also not a verdict. A chart with difficult aspects does not mean the relationship is doomed. It means the relationship will require conscious work in specific areas. Some of the most enduring relationships have challenging synastry — the challenge creates depth that easy connections do not always produce.
Both natal charts are calculated independently using the standard method: birth date, time, and location produce the exact planetary positions and house cusps for each person. The two charts are then compared by examining every possible pairing of planets: Person A's Sun to Person B's Moon, Person A's Venus to Person B's Mars, and so on through all combinations.
For each pair, the angular distance between the two planets is measured. If the angle matches one of the major aspect patterns — conjunction (0°), opposition (180°), trine (120°), square (90°), or sextile (60°) — within an acceptable margin (called an "orb"), the aspect is recorded. The orb is typically 6-8° for major aspects involving the Sun and Moon, narrower for outer planets.
The result is an aspect grid: a matrix showing every inter-chart connection. A thorough synastry reading examines all of these, but the most influential aspects are those involving the personal planets (Sun, Moon, Mercury, Venus, Mars) and the angles (ascendant, midheaven). These produce the dynamics that both people actually feel in daily interaction.
Venus-Mars aspects are the classic indicators of physical and romantic attraction. Venus conjunct Mars between two charts is one of the most reliable markers of mutual desire. The person whose Venus is involved tends to be the one who is desired; the person whose Mars is involved tends to be the pursuer. Venus-Mars squares create attraction with friction — the desire is strong but the styles conflict.
Moon aspects determine emotional compatibility. The Moon in synastry describes how two people feel together when nobody else is watching. Moon conjunct Moon creates intuitive emotional understanding. Moon square Saturn can create a dynamic where one person's emotional needs are chronically unmet by the other's instinct toward control or distance. Moon contacts are often more important for long-term viability than Venus contacts.
Sun-Moon contacts are the classic marriage aspects in traditional astrology. One person's Sun conjunct the other's Moon creates a natural polarity where one person's identity and will aligns with the other's emotional needs. This is a stabilizing aspect that provides a sense of familiarity and belonging.
Saturn aspects are the structural load-bearing walls. Saturn contacts create commitment, staying power, and the sense that the relationship matters. They also create restriction, obligation, and the possibility of resentment. A relationship with no Saturn contacts tends to feel light and uncommitted. A relationship dominated by Saturn contacts can feel heavy and inescapable. The right amount of Saturn is what makes a relationship last; too much is what makes it unbearable.
Synastry compares two separate charts. A composite chart creates a single chart for the relationship itself by finding the midpoint of each planet pair. If Person A's Sun is at 10° Aries and Person B's Sun is at 10° Leo, the composite Sun is at 10° Gemini — the midpoint of the two positions.
The composite chart describes the relationship as its own entity — what it looks like from outside, what it is "for," what challenges it will face as a unit. Synastry describes the dynamic between the two people. Both are useful. Synastry tells you how two people interact. The composite tells you what they build together.
In practice, experienced astrologers use both. The synastry shows whether the connection has enough attraction, emotional compatibility, and structural support to function. The composite shows what the relationship becomes once it exists — its purpose, its public face, its internal tensions.
Synastry can identify the areas of natural resonance and natural friction between two people with significant accuracy. The technique has been refined over centuries and the aspect interpretations are among the most consistently reliable in astrology. If two charts show strong Venus-Mars contacts and supportive Moon aspects, the attraction and emotional ease are genuinely likely to be present.
Synastry cannot tell you whether a relationship will last. Lasting requires choices, effort, context, and circumstances that no chart can fully predict. A chart with beautiful synastry can fail because of timing, geography, personal readiness, or a hundred other variables. A chart with difficult synastry can succeed because both people decide the relationship is worth the work.
Synastry cannot tell you how to feel. Two people can have powerful chart connections and one of them simply is not interested. Chemistry in the chart is potential, not guarantee. The chart shows what is possible between two people. What actually happens is still up to them.
Sun-sign compatibility uses one planet in one chart compared to one planet in another chart. A full synastry reading uses ten planets in two charts compared across all combinations. The difference in information density is roughly 100x.
The Sun represents identity and will. It is important. But a relationship runs on more than identity alignment. It runs on desire (Venus), drive (Mars), emotional attunement (Moon), communication style (Mercury), growth patterns (Jupiter), and structural compatibility (Saturn). Reducing all of this to "Aries and Leo: great match!" is not wrong about the Sun-Sun dynamic. It is just missing everything else.
The most common experience people have with sun-sign compatibility is that it works about half the time — which is roughly what you would expect from using one variable out of a system that contains dozens. Synastry uses the full system, and the accuracy improves accordingly.
ZODIA reads your real natal chart and tracks how today's sky touches it. Every morning. On WhatsApp. Built from the same astronomical foundations described on this page.
ZODIA’s interpretations draw on traditional Hellenistic astrology and verified astronomical data. Key references: