Sources

This page lists the primary sources behind the calculator's calculation engine and its interpretive content. For a technical explanation of how these sources are applied in the calculation engine, see the methodology page.

Calculation engine

Bretagnon, P. & Francou, G. (1988). "Planetary theories in rectangular and spherical variables: VSOP87 solutions." Astronomy & Astrophysics 202, 309-315. The standard analytical planetary theory used in our ephemeris.

Chapront-Touzé, M. & Chapront, J. ELP-2000-82B lunar series. The standard analytical lunar theory used for the Moon's position.

Meeus, J. (1998). Astronomical Algorithms, 2nd edition, Willmann-Bell. The reference text for practical astronomical computation, including house system formulas and the obliquity of the ecliptic.

Astrodienst Swiss Ephemeris. Used as a validation reference. Our calculations are tested against Swiss Ephemeris output across thousands of dates.

JPL Horizons system. Used as a high-precision reference for testing Chiron and Pluto positions.

Indian Calendar Reform Committee (1955). The published Lahiri ayanamsa specification used in our sidereal mode.

IANA tzdata. Time zone database for historical local-time conversion.

Interpretive content

Jean Meeus, Astronomical Algorithms (2nd edition, 1998), Willmann-Bell. Primary source for our VSOP87 implementation, ascendant calculation, Placidus and Koch house cusp formulas, lunar phase calculations, and the obliquity of the ecliptic. Every spherical-trigonometry routine in the engine traces back to Meeus.

Chapront-Touzé & Chapront, ELP-2000-82B. Source for our lunar position theory. Used in truncated form retaining all terms above the 0.1-arcsecond amplitude threshold sufficient for natal-astrology accuracy across 1900-2100.

Simon, Bretagnon, Chapront, Chapront-Touzé, Francou, Laskar (1994), VSOP87. The published planetary theory underlying our heliocentric ephemeris. We use the VSOP87D spherical-coordinate variant referenced to the equinox of date.

Stückelberger & Grasshoff (eds.), Ptolemy's Almagest. Historical context for the development of the house systems and the original mathematical derivation of the ascendant. Informs our framing of why house systems disagree.

Robert Hand, Planets in Transit (1976). Standard modern reference on transit interpretation and planetary cycles. Informs the transit-related interpretive prose throughout the site, particularly on the planets-in-houses pages.

Liz Greene, The Astrology of Fate (1984). Foundational text on the psychological reading of difficult chart placements. Informs how we frame Saturn, Pluto, and the outer-planet content — emphasising integration over avoidance.

Stephen Arroyo, Chart Interpretation Handbook (1989). Practitioner-oriented guide to reading the natal chart as a whole rather than as a list of placements. Informs the structure of our birth-chart-reading guide and the elements/modalities sections.

Hart deFouw & Robert Svoboda, Light on Life (1996). Best modern English-language introduction to Vedic astrology. Informs our Vedic mode interpretive content, the nakshatra descriptions, and the framing of dasha periods.

B.V. Raman, A Manual of Hindu Astrology. Classical reference on Vimshottari dasha calculation, the planetary period ratios, and the structure of antardashas. Source for our dasha implementation.

Hart deFouw & Robert Svoboda, Light on Relationships (2000). Reference for Guna Milan (the Vedic compatibility scoring system) and the eight kuta factors. Informs our compatibility calculator's Vedic-side scoring.

Bernadette Brady, The Eagle and the Lark (1992). Reference on predictive techniques and the use of fixed stars and parans. Informs our advanced-features documentation.

Astrodienst Swiss Ephemeris. Used as a validation reference. Our calculations are tested against Swiss Ephemeris output across thousands of randomly chosen dates.

JPL Horizons. High-precision reference used for testing Chiron and Pluto, where the analytical theories are weakest.

Indian Calendar Reform Committee (1955). The published Lahiri ayanamsa specification used in our sidereal mode.

IANA tzdata. Time zone database for historical local-time conversion, including DST rules and political timezone changes.

Note on attribution

Interpretive prose on this site is original work informed by these sources. Where a specific phrase or framework is identifiably from one of these authors, we attribute it in context. The general vocabulary of astrology — sign meanings, planetary functions, aspect interpretations — is part of a common tradition that no single modern author owns. For an explanation of how these sources are applied in the calculator, read the methodology page.