Project 10: Synastry and Relationship Longevity

Source: bigastrologybook.com/2/research/19/project-10 Archive Date: 2026-03-21 Book: The Big Astrology Book of Research by Renay Oshop Dataset: 2,722 couples from Wikidata (SPARQL query, 2026); Swiss Ephemeris for chart calculations

Research Question

Does astrological synastry — the comparison of two birth charts to assess compatibility — predict whether a romantic relationship will endure? Specifically: do particular inter-chart planetary alignments distinguish long-lasting marriages from short ones?

Hypothesis

If synastry carries real information about compatibility, couples with "favorable" inter-chart aspects should show longer relationship duration on average than couples with "unfavorable" aspects, after controlling for demographic confounders.

The Dataset

Parameter Value
Source Wikidata (SPARQL query for spouse pairs with verified birth dates)
Valid couples analyzed 2,722
Duration metric Relationship duration in years (marriage date to divorce or death)
Long-term group Top 25% of duration: > 54.4 years
Short-term group Bottom 25% of duration: < 8.1 years

Data quality note: Wikidata relationship data has known quality issues. Date precision varies (some entries list year only), and "relationship end" conflates divorce with partner death — two very different demographic events. Survival-censored analysis (censoring for partner death) would be more rigorous but was not implemented here. These limitations are noted when interpreting results.

Methodology

Rather than using traditional binary aspect orbs (conjunction = within 8°, sextile = 60° ± 6°, etc.), this analysis used cosine similarity of inter-chart planetary angles — a continuous measure that avoids arbitrary orb cutoffs:

Cosine Value Aspect Angular Separation
+1.0 Conjunction 0°
0.0 Square 90°
−1.0 Opposition 180°

This produces a correlation coefficient for each planetary pair (e.g., "Partner A's Mars vs. Partner B's Mars") that can be compared across the long-term and short-term groups.

No assumptions were made in advance about which aspects are "good" or "bad." The analysis was agnostic about traditional astrological symbolism.

Results

The Critical Warning: Demographic Artifacts First

Before discussing any astrological results, the dominant finding must be stated clearly: the strongest signals in this dataset are not astrological — they are demographic artifacts of human mortality.

To be married for 60 years, both partners must survive 60 years from the wedding date. This creates a powerful survival bias: couples with large age gaps have different mortality curves, which systematically truncates their measured duration. Couples with similar outer planet placements (same generational positions) are close in age. Close-in-age couples are more likely to both survive to an extreme golden anniversary.

Synastry Pair Correlation (r) p-value Type
Pluto-Pluto +0.15 < 0.001 Demographic artifact (age matching)
Uranus-Uranus +0.14 < 0.001 Demographic artifact (age matching)
Uranus-Pluto −0.14 < 0.001 Artifact (generational contrast)
Neptune-Pluto +0.139 < 0.001 Artifact (age matching)

These outer-planet signals must be discarded before any astrological interpretation. A researcher who claimed "Pluto-Pluto conjunction predicts lasting marriages" would be publishing a demographic finding about age-similar couples, not an astrological one.

The Mars-Mars Signal

After setting aside all outer-planet signals, the most significant finding among personal planets is Mars-Mars Conjunction (p = 0.0006).

Synastry Pair Mean Cosine Difference (Long − Short) p-value Interpretation
Mars-Mars +0.133 0.0006 Long-term couples share Mars sign more often
Pluto-Saturn −0.123 0.0016 Generational/Saturnian mix
Neptune-Saturn −0.104 0.0067 Generational/Saturnian mix

The Mars-Mars result: couples in the long-duration group showed an average cosine similarity 0.133 higher than short-duration couples. In practical terms, this means long-term couples are more likely to share the same Mars sign (Conjunction) than short-term couples.

Why Mars? Mars governs action style, libido, conflict rhythm, and how anger is expressed. When one partner explodes (Mars in Aries) and the other withdraws (Mars in Pisces), the asymmetry accumulates friction. When both escalate similarly and de-escalate similarly — even if their conflict style is individually intense — the synchronicity of the pattern allows repair. The data supports "Eros fades, but friction kills" as a relationship dynamic.

Statistical caveat: The Bonferroni-corrected significance threshold for 100 planetary pairs × multiple aspects is approximately p < 0.0005. Mars-Mars at p = 0.0006 barely misses this threshold. The result is robust against individual tests but would not survive the most conservative multiple-testing correction. It should be treated as highly suggestive, requiring replication.

The Love Planets: Null Results

The traditional "compatibility" planets — Venus and Moon — showed no significant association with relationship duration.

Synastry Pair Correlation p-value Significance
Venus-Mars ~0 > 0.05 None
Sun-Moon ~0 > 0.05 None
Venus-Venus ~0 > 0.05 None
Moon-Moon ~0 > 0.05 None

These null results are potentially the most practically interesting finding. Romantic chemistry — commonly attributed to Venus-Mars and Sun-Moon connections — does not predict whether a relationship lasts 50 years. Venus may govern who attracts us; Mars may govern whether we can tolerate each other day-to-day over decades.

Summary Table: Signal Hierarchy

Priority Synastry Factor Status
1 (discard) Outer planet conjunctions (Pluto, Neptune, Uranus same sign) Demographic artifact — age matching
2 (genuine?) Mars-Mars Conjunction p = 0.0006; strongest personal-planet signal
3 (weak) Saturn-outer planet oppositions Ambiguous: age mix or Saturnian stability
4 (null) Venus-Mars, Sun-Moon, Venus-Venus, Moon-Moon No significant association with duration

Limitations

  1. Survival analysis not implemented: Partner deaths should be treated as censored events in proper survival analysis (Cox proportional hazards model). Simply measuring duration from marriage to death treats death as "relationship failure," inflating duration for couples where one partner dies young.

  2. Wikidata quality: Relationship dates for historical figures may be approximate. The "marriage" start date may conflate legal marriage with the actual start of a relationship.

  3. Selection bias: Couples notable enough to appear in Wikidata may not represent typical relationships. Long marriages may be systematically over-represented among famous people (marital stability is itself a form of social reputation).

  4. Quartile splitting: Comparing only the top and bottom 25% inflates effect sizes. A continuous regression of duration on cosine similarity for all 2,722 couples would be a more conservative test.

Conclusion

The 2,722-couple Wikidata dataset reveals a two-tier structure in synastry analysis:

Tier 1 — Demographics dominate: Outer planet conjunctions predict marriage duration, but this is entirely explained by age similarity and survival statistics, not astrology. Recognizing and discarding this artifact is essential for methodological integrity.

Tier 2 — Mars emerges: After removing outer-planet confounds, Mars-Mars Conjunction shows the strongest personal-planet signal (p = 0.0006, mean difference +0.133). Couples who share action style and conflict rhythm appear to last longer. This is consistent with the hypothesis that long-term relationship success depends more on conflict compatibility than romantic chemistry.

Venus, Moon, and Sun connections — the traditional pillars of astrological compatibility analysis — show no statistical association with relationship duration in this dataset. They may matter for attraction and early relationship quality, but they do not predict which couples will still be together after fifty years.

Archived code and raw data outputs preserved in backup/.

Synastry and Relationship Longevity

The Question

Can astrological synastry—the comparison of two birth charts—predict whether a relationship will last? Astrologers have long claimed that certain planetary alignments between partners indicate compatibility or conflict. We put these claims to a rigorous statistical test using a massive dataset of historical couples.

The Data

Methodology

We adopted a strictly agnostic methodology. Unlike traditional astrology, we made no a priori assumptions about which aspects are "Good" (Trines, Sextiles) or "Bad" (Squares, Oppositions).

Key Findings

1. The "Mortality Bias" (Artifact)

The strongest predictors of extreme longevity were Outer Planet Conjunctions (Pluto-Pluto, Uranus-Uranus).

2. The Mars-Mars Connection (Astrological Signal)

After filtering out the generational noise, the most significant "Personal Planet" signal was Mars-Mars.

Conclusion

While demographic factors (Age Matching) dominate the statistics of "Golden Anniversaries," the astrological data suggests that Mars Compatibility (Syncronized Action) is a critical factor for relationship endurance, more so than the traditional "Love" planets of Venus or Moon (which is consistent with the idea that Eros fades, but friction kills).

Data Provenance

Relationship Data