Birth Chart Synastry

By Astrolivo editorial team

Birth chart synastry compares two individual natal charts—Person A and Person B—to examine how one person's calculated planetary placements align with the other's. Unlike general zodiac sign compatibility, which looks only at two Sun sign archetypes in the abstract, synastry calculates exact astronomical degrees and inter-chart aspects across the Sun, Moon, Mercury, Venus, Mars, and outer planets.

Astrolivo presents synastry as a transparent comparison of calculated chart factors and traditional symbolic interpretations. It does not produce a compatibility percentage, predict whether a relationship will last, or assign fixed roles to either person.

How Two-Chart Synastry Works

A synastry comparison keeps two separate birth records distinct—clearly identifying Person A and Person B throughout the results so you can always see whose placement is being referenced.

To calculate a two-chart comparison, each person's chart is computed independently from their birth date, birth time (if known), and birthplace, and then compared side by side:

1. Individual Placements (Person A and Person B): First, each person's own natal positions are established in the tropical, geocentric zodiac. Understanding how Person A and Person B each process emotion (Moon sign), communicate (Mercury sign), and express affection (Venus sign) provides the baseline context.
2. Inter-Chart Aspects: Next, the calculator measures the angular distance along the ecliptic between placements in Person A's chart and placements in Person B's chart. An inter-chart aspect describes the angular relationship between placements in the two charts. Common examples include conjunction (0°), sextile (60°), square (90°), trine (120°), and opposition (180°).
3. Houses and Angles (When Birth Time Is Known): When a verified birth time and birthplace are available, synastry can also include each person's houses and angles as chart context when birth time is reliable.

Inter-Chart Aspects vs. Individual Placements

A common point of confusion when reading synastry is separating what belongs to an individual chart from what happens between the two charts.

• Individual placements describe each person's own natal pattern. For example, if Person A has Venus in Virgo and Mars in Gemini forming a natal square in their own chart, that dynamic belongs to Person A regardless of who they are with.
• Inter-chart aspects describe the angular geometric relationship between a placement in Person A's chart and a placement in Person B's chart. They show how the two charts symbolically converse with one another.

A Hypothetical Synastry Example

Imagine a hypothetical comparison between Person A and Person B. Suppose Person A has their natal Sun at 14° Capricorn and natal Moon at 18° Taurus, while Person B has their natal Sun at 10° Aries and natal Venus at 16° Taurus.

If you looked only at a Sun-sign pairing (Capricorn and Aries), traditional commentary would focus entirely on the 90° tension between two Cardinal signs in different elements. In a two-chart synastry calculation, however, you see both layers clearly:

• Person A's Sun (14° Capricorn) square Person B's Sun (10° Aries): An inter-chart square (orb 4°), traditionally associated with differing approaches to initiative and pacing.
• Person A's Moon (18° Taurus) conjunct Person B's Venus (16° Taurus): An inter-chart conjunction (orb 2°), traditionally associated with shared appreciation for domestic comfort, steadiness, and warmth.

Keeping Person A's and Person B's labels explicit lets you see whose Moon and whose Venus form the conjunction, rather than blending both charts into a vague summary or claiming that Sun-sign pairing equals full synastry.

What Happens When Birth Time Is Unknown

Accurate synastry relies on being honest about what the input data can and cannot support. You do not need to guess a birth time to compare two charts, and Astrolivo never substitutes a silent default time that would fabricate sensitive points.

When either Person A or Person B has an unknown birth time, the synastry display adjusts automatically and states these limitations beside the results:

• Unreliable angles and houses are hidden: For any person whose birth time is unknown, the rising sign (Ascendant), Midheaven (MC), house cusps, and house divisions that depend on an exact clock time are omitted rather than guessed.
• Full-day sign possibilities are shown: Because the Earth's rotation and the Moon's rapid motion (roughly 12° to 15° per day) mean the Moon or a planet near a boundary can change signs during a 24-hour local calendar day, the comparison shows the sign possibilities across the entire local day for that birth record.
• Asymmetric data is handled per person: If Person A has an exact birth time and Person B does not, Person A's angles and houses remain visible for Person A's own chart, while Person B's time-dependent angles and houses are hidden and clearly marked.

If you want to understand how birth time affects chart calculations in detail, see how to read a birth chart and why charts on different sites can differ.

Why Synastry Is Different From a Relationship Score

Astrolivo does not calculate a single compatibility percentage or rank relationships on a numerical scale. Even in a full two-chart comparison, astrology is a symbolic framework for reflection, not a diagnostic test.

A pair of charts can contain both flowing aspects (such as trines and sextiles) and challenging aspects (such as squares and oppositions). Traditional astrology interprets flowing aspects as areas where two people may naturally understand each other's rhythm, and challenging aspects as areas that call for conscious discussion and mutual respect. Neither set of aspects guarantees harmony or conflict in real life.

When to Use Synastry vs. Sign Compatibility

• Use Birth Chart Synastry when you have birth dates and birthplaces for two specific people (Person A and Person B) and want to examine how their personal planets, inter-chart aspects, and known houses interact.
• Use Zodiac Sign Compatibility when you only want to explore the general symbolic themes of two Sun signs, or when you do not have full birth dates.
• Use the Birth Chart Calculator when you want to explore one person's natal placements in depth before comparing two charts.

Methodology and Calculation Basis

Astrolivo computes natal positions using the Skyfield astronomy library and NASA JPL DE440s ephemeris in the tropical, geocentric zodiac, with Placidus houses when birth time is known and a 6° orb for the five major aspects (conjunction, sextile, square, trine, and opposition). Above about 66° latitude, where Placidus houses are undefined, Astrolivo shows the houses in the Whole Sign system with a visible label. Calculations are strictly separated from symbolic interpretation; you can read full technical details on our methodology page.

Frequently Asked Questions

Is Sun-sign compatibility the same as synastry?

No. Sun-sign compatibility compares only the general seasonal archetypes of two zodiac signs. Birth chart synastry compares two complete birth records (Person A and Person B), including the Moon, Mercury, Venus, Mars, outer planets, inter-chart aspects, and—when birth time is known—the rising sign and houses.

Why are houses and the rising sign missing for one person in a comparison?

The rising sign, Midheaven, and houses depend on the Earth's daily rotation and shift through all twelve signs every 24 hours. If a person's birth time is marked as unknown, Astrolivo omits their unreliable angles and houses while noting uncertainty for time-sensitive placements rather than guessing a noon or midnight time that would produce misleading results.

Does synastry work for friendships, family members, or colleagues?

Yes. Because synastry compares how two people's symbolic placements communicate, collaborate, and handle tension, the chart geometry is the same regardless of whether Person A and Person B are romantic partners, friends, relatives, or creative collaborators.