Natal Navigator computes planetary positions from analytical ephemerides of the VSOP87 class, derives the Ascendant and Midheaven with the true obliquity of the ecliptic, divides houses by the Porphyry method, weighs aspects with classical moiety orbs, and projects astrocartography lines both in mundo and zodiacally — the two conventions the field has used since Jim Lewis formalised it in the 1970s. The astronomical calculation is reproducible within the documented model accuracy; the interpretation applies traditional principles and is offered for reflection, not prediction.
1 · The astronomical layer
All positions come from an open-source ephemeris engine implementing truncated analytical planetary theories of the VSOP87 family (Bretagnon & Francou, 1988), together with a dedicated lunar theory and a separate model for Pluto. Accuracy for the relevant epochs is on the order of an arcminute — far below anything a chart wheel or a world map can visually resolve, and consistent with the standard ephemerides used across serious astrological software.
Two details matter more than they look. First, the Ascendant and Midheaven are derived with the true obliquity of the ecliptic, including nutation, rather than a fixed mean value. Second, the birth moment is converted through the IANA time-zone database from the birth coordinates, which resolves historical offsets and daylight-saving edge cases — a common source of incorrect rising signs in older tools. The result still depends on the accuracy of the entered birth time, location and historical time-zone record.
Charts are computed in the tropical zodiac, the convention of Western natal astrology since Ptolemy’s Tetrabiblos (2nd century CE), in which 0° Aries is anchored to the March equinox.
2 · Houses: the Porphyry method
Natal Navigator divides houses by the Porphyry method, one of the oldest documented quadrant systems (attributed to Porphyry of Tyre, 3rd century CE): each quadrant between the four angles is split into three equal parts along the ecliptic. It keeps the angles as true house cusps, behaves gracefully at higher latitudes where time-based systems distort, and matches the app’s premise that the four angles — Ascendant, Midheaven and their opposites — carry the structural weight of the chart.
3 · Aspects and moiety orbs
Aspects are computed between all ten planets plus the angles, covering the five Ptolemaic aspects and the standard minor ones. For the allowed deviation from exactness, the app uses moiety orbs: the classical convention, systematised in William Lilly’s Christian Astrology (1647), in which each body carries its own orb and an aspect’s effective orb is half the sum of the two. A Sun–Moon aspect therefore tolerates more deviation than a Mercury–Venus one — which is how the tradition weighted them, and measurably different from the flat per-aspect orbs many modern tools default to. Tighter aspects rank higher throughout the app.
4 · Planetary condition
Behind every reading sits a condition model drawn from the traditional toolkit: essential dignities (domicile, exaltation and their debilities, per the Ptolemaic rulership scheme), sect — the hellenistic distinction between day and night births that changes which planets act as allies — combustion (proximity to the Sun) and speed relative to the planet’s mean motion, including retrograde state. These are documented concepts with primary sources, not house rules we invented; their exact weighting into the app’s scores is part of the engine we keep to ourselves.
5 · Astrocartography: lines, both ways
Astrocartography, formalised by Jim Lewis as Astro*Carto*Graphy® in the 1970s, asks a precise question: from which places on Earth was a given planet exactly on one of the four angles at your birth moment? Each planet yields four lines — MC, IC, ASC, DC — ten planets, forty lines.
The app computes them in both accepted conventions, and lets you switch. In mundo: the planet’s actual body stands on the angle, derived from its equatorial coordinates and the local hour angle — MC/IC lines fall on meridians of longitude, ASC/DC lines are curves given by spherical trigonometry of the local horizon. Zodiacal: the planet’s ecliptic degree meets the angle, the projection Lewis’s original maps used. The two disagree most where a planet stands far from the ecliptic — which is precisely why the choice is exposed instead of hidden.
Line influence is treated as a band, not a hairline: cities score by their angular distance to the nearest line, fading with distance, and the app additionally computes parans — latitudes where two planets stand on angles simultaneously, a technique with roots in the hellenistic paranatellonta. How distance, condition and parans combine into a single city score is the part of the recipe we do not publish.
6 · Transits, cycles and the Moon
The daily layer runs the same ephemerides forward in real time. Transits to natal positions are tracked with the same moiety-based orb logic, phase-tagged as applying or separating, and windowed — a transit is presented as a period with a build-up, a peak and a fade, never as a point prediction. Retrograde windows and sign ingresses come straight from the ephemeris; lunar phase and illumination are computed from the Sun–Moon elongation. The day-score you see is an aggregate of these active windows; its exact weights, like the city score, stay in-house.
7 · Where the computing happens
The entire astronomical layer — chart, houses, aspects, lines, parans, transits — runs on your device. Birth data does not need to leave it for a chart to exist. When you ask the in-app Guide a question, the app sends the already-computed chart facts along with your question, and the Guide is constrained to interpret exactly those facts; it is explicitly forbidden from inventing degrees, dates or placements.
8 · What this is, and what it is not
The astronomical output is reproducible and can be compared with independent ephemerides within the stated model accuracy of roughly one arcminute for supported epochs. The interpretation applies documented principles from the astrological tradition — Ptolemy’s Tetrabiblos, the hellenistic sect doctrine, Lilly’s orb conventions, Lewis’s astrocartography — consistently and transparently. Astrology itself is not an empirically validated predictive science, and Natal Navigator does not present it as one: readings are offered for orientation and self-reflection, not as medical, legal or financial advice, and never as certainty about the future.
Which ephemeris does Natal Navigator use?
An open-source engine implementing truncated VSOP87-class analytical theories with a dedicated lunar model, accurate to roughly an arcminute for the supported epochs — the same family of models behind most professional astrology software.
Why Porphyry houses instead of Placidus?
Porphyry keeps the four angles as real cusps, stays well-defined at high latitudes where Placidus degenerates, and matches the app’s angle-centred method. House systems are conventions; the app names its choice instead of hiding it.
Why do my lines differ slightly from other astrocartography sites?
Usually one of three reasons: the other tool uses zodiacal projection while you are viewing in mundo (or vice versa), it uses a different house or orb convention, or it resolves the birth time zone differently. Switch the line method in the app to compare like with like.