How our natal chart is calculated

The Sun's apparent longitude comes from the Earth's VSOP87 heliocentric orbit (Bretagnon & Francou, Bureau des Longitudes, Paris). The Moon's position uses the ELP2000-82B lunar theory (Chapront) with its principal perturbation terms. Mercury through Neptune use the full VSOP87 planetary series.

Pluto uses the analytical theory from Meeus, 'Astronomical Algorithms' chapter 40, precessed from the J2000 frame to the ecliptic of date. It agrees with JPL to about 0.1 arcminute across 1900-2100.

All positions are geocentric, corrected for light-time (planetary distances up to ~5 light-hours), annual aberration (up to 20.5 arcseconds) and nutation in longitude (IAU 1980 series, up to 17.2 arcseconds) - i.e. true apparent positions, the convention used in astrology.

Longitudes are referred to the true equinox and ecliptic of date - the tropical zodiac used in Western astrology. 0 degrees Aries is defined as the ascending intersection of the ecliptic and the celestial equator at your birth moment.

The obliquity of the ecliptic uses the IAU mean obliquity polynomial plus the nutation correction in obliquity (Delta-epsilon).

We deliberately do not apply the sidereal (Vedic) ayanamsha; the two systems currently differ by about 24 degrees.

Your local birth time is converted to UT using the IANA time zone database (the same tzdata that keeps every computer's clock honest), so historical daylight-saving rules are handled correctly.

Ephemeris computations require Terrestrial Time: we add Delta-T from the NASA/GSFC polynomial approximations (Espenak & Meeus). For 1961 this correction is about 38 seconds; ignoring it would shift the Moon by up to 20 arcseconds.

Supported range: 1900-2100. Outside this window the Delta-T approximation and ephemeris accuracy degrade, so we refuse the calculation rather than return silently wrong numbers.

The local sidereal time is derived from Greenwich apparent sidereal time (IAU 1982 series) plus your geographic longitude. The Midheaven is the ecliptic point culminating on your meridian; the Ascendant is the ecliptic point rising on your eastern horizon.

Intermediate house cusps follow the Placidus system: each cusp is the ecliptic point that has completed one third (or two thirds) of its semi-diurnal or semi-nocturnal arc. We solve these iteratively to sub-arcsecond convergence.

Placidus is mathematically undefined for circumpolar latitudes (roughly beyond 66 degrees north or south). There we fall back to the Porphyry system and say so explicitly in your results. A Whole Sign option is always available.

We compute the five classical major aspects - conjunction (0 degrees), sextile (60), square (90), trine (120) and opposition (180) - among the ten classical bodies.

Orbs follow common professional practice: 8 degrees, extended to 10 when the Sun or Moon participates; 6 degrees for the sextile, extended to 8 with a luminary. The Node is displayed but excluded from aspect computation.

We validated every body against the JPL Horizons ephemeris service (observer ecliptic-of-date longitudes, IAU76/80 frame) at two epochs: 2000-01-01 12:00 UT and 1961-07-01 18:45 UT (the AA-rated birth data of Diana, Princess of Wales).

Maximum deviation across all ten bodies and both epochs: 0.65 arcminutes (Moon). The Sun, all planets and Pluto agree to 0.25 arcminutes or better. The Ascendant is additionally verified by an independent horizon-intersection solver agreeing to under 0.001 arcseconds.

In practice this means planet positions agree with professional Swiss-Ephemeris-based software to within rounding of the printed arcminute.