Synastry compares two separate natal charts. A midpoint composite constructs a new set of symbolic planetary positions halfway between the two charts. They answer different calculation questions and can show different aspects for the same pair of people.
Try the synastry calculator and composite calculator with the same two sets of inputs. This article uses invented people, not a real couple or a relationship outcome study.
What is being compared?
| Question | Synastry | Midpoint composite |
|---|---|---|
| Positions | Two separate sets of natal longitudes | One short-arc midpoint for each corresponding planet |
| Aspects | Person A’s planets to person B’s planets | Between positions within the constructed chart |
| Physical sky? | Each natal chart represents its own birth instant | The combined positions need not occur in a real sky |
| This site’s output | Cross-chart contacts; Ascendant contacts when time is known | Planetary midpoints and aspects, without composite houses or angles |
The same pair, worked both ways
Invented reference birth: June 15, 1990, 14:30, Singapore (UTC+08:00), latitude 1.2897°, longitude 103.8501°. The UTC instant is 1990-06-15 06:30 UTC. All positions use the tropical zodiac and apparent geocentric longitude.
Person B: October 10, 1992, 12:00, New York (UTC−04:00), latitude 40.7128°, longitude -74.006°; 1992-10-10 16:00 UTC. The following table uses the ten bodies from Sun through Pluto, excluding nodes and Lilith.
| Body | Person A | Person B | Composite |
|---|---|---|---|
| Sun | 23°55′ Gemini | 17°36′ Libra | 20°45′ Leo |
| Moon | 12°18′ Pisces | 5°17′ Aries | 23°48′ Pisces |
| Mercury | 5°18′ Gemini | 4°51′ Scorpio | 20°05′ Leo |
| Venus | 18°30′ Taurus | 18°57′ Scorpio | 18°44′ Aquarius |
| Mars | 10°53′ Aries | 14°41′ Cancer | 27°47′ Taurus |
| Jupiter | 15°50′ Cancer | 0°01′ Libra | 22°56′ Leo |
| Saturn | 24°03′ Capricorn | 11°51′ Aquarius | 2°57′ Aquarius |
| Uranus | 8°10′ Capricorn | 14°11′ Capricorn | 11°10′ Capricorn |
| Neptune | 13°43′ Capricorn | 16°14′ Capricorn | 14°59′ Capricorn |
| Pluto | 15°24′ Scorpio | 21°31′ Scorpio | 18°28′ Scorpio |
| Person A | Aspect | Person B | Orb |
|---|---|---|---|
| Mercury | Sextile | Moon | 0.01° |
| Jupiter | Opposition | Neptune | 0.39° |
| Venus | Opposition | Venus | 0.45° |
| Neptune | Conjunction | Uranus | 0.45° |
| Pluto | Trine | Mars | 0.72° |
| Pluto | Sextile | Neptune | 0.82° |
| Neptune | Opposition | Mars | 0.97° |
| Mars | Sextile | Saturn | 0.97° |
The contact list is a reproducible subset in body order, not a ranking of relationship importance. It excludes angles here to keep the table focused on planetary contacts. Our full synastry tool also includes available Ascendants.
Why you cannot average zodiac numbers directly
Positions of 350° and 10° have a short-arc midpoint at 0°. An ordinary arithmetic average gives 180°, the opposite side of the circle. Our method wraps the separation into the shortest signed arc and adds half that separation to the first position. Exactly opposite positions have two equally valid midpoints; the tool flags them as ambiguous.
A planetary midpoint composite is also different from a Davison chart, which is based on midpoint birth time and location. Changing the construction method changes the result. A midpoint composite has no single physical event time, so our tool does not assign retrograde motion to its constructed points.
What unknown birth times change
Our tools use local noon for unknown times. The Moon and contacts near an orb boundary can vary during the day; angles require recorded times. Read the unknown-time guide before treating these contacts as settled. Our composite tool omits houses and angles even when both birth times are supplied.
Method and limits
Both methods use the same tropical natal longitudes. Synastry applies our published major-aspect orb rules; the methodology explains them. A contact, midpoint or score cannot establish whether a relationship will last. The comparison demonstrates calculations rather than validating relationship predictions.
Download the unrounded comparison data (JSON). To reproduce it, run node scripts/build-comparison-research.ts from the project repository. The script uses the same calculation modules and saved station table as our tools. This is reproducibility within one implementation, not an independent accuracy check.
Cite this comparison
Natal Chart Lab (2026). Synastry vs Composite Chart: Differences With One Example. Article and worked examples; supporting data. Calculated comparison output is available under CC BY 4.0: credit Natal Chart Lab, link to this article and the license, and indicate changes.
Prepared with AI assistance and checked against calculation code and saved examples. Astronomical calculations do not establish astrological claims about personality, relationships or future events.