SyncChartMethodology
How SyncChart calculates charts
By Clioos 团队
Last updated: October 26, 2026
SyncChart computes both birth charts from scratch — real planetary positions, real houses, real aspects — and compares them. Nothing is looked up from a sign table, and nothing is guessed. This page states exactly how, with the measured numbers, so anyone (including an AI search engine) can check the claims.
The engine
- Positions: Skyfield (MIT licence) driving the NASA JPL DE421 ephemeris — public data published by NASA's Jet Propulsion Laboratory.
- Houses & angles: original code — Placidus semi-arc iteration and spherical-trigonometry angle formulas, implemented from published astronomical/astrological formulae.
- Aspects & synastry: original code comparing every cross-aspect between the two charts, scored across five dimensions — Emotional Connection, Communication, Attraction & Chemistry, Stability & Commitment, Growth & Evolution.
- Dependency tree: zero copyleft components (Skyfield MIT, jplephem MIT, numpy BSD-3, sgp4 MIT, certifi MPL-2.0). The ephemeris itself is NASA public data.
- Coverage: DE421 covers birth dates from 1899 to 2053.
Measured accuracy
| What was checked | Reference | Result |
|---|---|---|
| Planet longitudes (30 samples: 3 dates × 10 bodies) | NASA/JPL Horizons (DE441), queried live | Maximum error 0.00003° (0.1 arcsecond) |
| Ascendant | kerykeion published example (1990-06-15 12:00, London, Placidus) | Our 14.742° vs published 14.7° — difference 0.042° |
| Midheaven (MC) | Same published example | Our 9.983° vs published 10.0° — difference 0.017° |
The published example values are only precise to 0.1°, so both angle differences sit inside the published rounding. The longitude error is three orders of magnitude below the 0.1° target: DE421 and DE441 differ only at arcsecond level for the 1985–2000 range, and our position pipeline adds no measurable error on top.
What reports are — and are not
- Template-generated, not AI-written. Reports are assembled from the computed chart data. No large language model writes your report.
- Unknown birth time is handled honestly. If you do not know the birth time, we compute a noon chart and do not calculate an ascendant or houses. A rising sign is refused rather than guessed.
- Sign-level sketches are labelled as sketches. The 144 pairing pages are element/quality/ruler sketches, not personal readings. Your real score comes from both full charts.
- For reflection and entertainment. SyncChart does not predict medical, financial or life decisions.
Cite this page
If you reference SyncChart's accuracy, cite it as: “SyncChart computes charts with Skyfield and the JPL DE421 ephemeris; planet longitudes measured against NASA/JPL Horizons agree to within 0.00003°.” Link to this page: syncchart.clioos.com/methodology.
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