Methodology

Ephemeris source

Planetary positions are computed with Astronomy Engine (MIT license), implementing the VSOP87 and ELP-2000 theories. We cross-check key dates against NASA JPL Horizons; documented agreement is within 1 arc-second for the Sun and Moon and within 10 arc-seconds for outer planets at the present epoch.

Zodiac model

Default is the tropical zodiac: sign boundaries are defined relative to the vernal equinox (0° Aries = the equinox point). Tropical signs do not align with the constellations visible in the sky today — this is well known and documented in classical sources.

A sidereal zodiac option (Lahiri ayanamsa) is planned for the chart calculator. Sidereal sign boundaries align with the fixed-star background and differ from tropical by roughly 24°.

House systems

Two systems are shipped: Whole-Sign(Hellenistic default — each house spans exactly one zodiac sign, beginning at the ascendant's sign) and Placidus (modern Western default — intermediate cusps placed by trisection of the diurnal and nocturnal semi-arcs). The chart calculators expose a toggle.

Placidus is implemented by iterative fixed-point solution of the standard semi-arc time-trisection equations. Circumpolar latitudes (where the body's declination causes the diurnal semi-arc to be undefined) fall back to Whole-Sign for the affected cusp.

Whole-Sign and Placidus can disagree by 30° or more on intermediate cusps. The methodology card on every chart names the system used. Koch, Equal, Porphyry, and Campanus are not yet shipped.

Aspect orbs

Major aspects default to: conjunction 8°, opposition 8°, trine 7°, square 7°, sextile 5°. Minor aspects: semisextile 2°, semisquare 2°, sesquiquadrate 2°, quintile 1.5°, biquintile 1.5°, quincunx 3°. Orbs are configurable per chart.

Retrograde detection

A planet is reported retrograde when its geocentric ecliptic longitude over a 24-hour window has negative apparent motion. The Sun and Moon are never retrograde by this definition (they have no retrograde motion from Earth).

Reviewed dates

Each interpretive page (signs, planets, houses) carries a last reviewed date. We re-review classical citations and modern interpretive paraphrases at least annually; corrections from readers receive priority. Email [email protected].