01Ephemeris — NASA JPL DE440S
Planetary positions are computed from the NASA Jet Propulsion Laboratory DE440S planetary ephemeris. This is the same numerical ephemeris JPL provides to spacecraft mission planners, and it is the reference dataset behind most professional astronomy software.
DE440S covers the Sun, Moon, Mercury through Pluto, the lunar nodes, and Chiron at sub-arcsecond precision for any moment between the years 1550 and 2650. That is a thousand-year window — more than enough for historical charts (Newton, Bach, Joan of Arc) and forward planning. The "S" stands for "small": it is the compact version of the full DE440 file, trading a slightly narrower time window for a much smaller download, which matters because the ephemeris ships with the application and is consulted on every chart calculation.
We do not use averaged tables, synthetic generators, or copied lookup data. The same .bsp file the SPICE toolkit reads is the file we read.
02House systems
The default house system is Placidus, the most widely used system in modern Western astrology and the one most readers expect. We also support Whole-Sign (the dominant system in Hellenistic astrology and increasingly common in modern traditional revivals), Equal (often used when birth time is shaky), Koch (a popular German alternative to Placidus for psychological work), and Porphyry (the oldest known quadrant system, useful at extreme latitudes where Placidus breaks down).
House system is a setting per chart, not a sitewide preference. The system used is always shown on the rendered wheel — so you can take a printed Placidus chart and switch to Whole-Sign without losing track of which house any planet was originally placed in.
03Aspects and orbs
We compute the seven Ptolemaic aspects (conjunction, opposition, trine, square, sextile) plus the common minor aspects (quincunx, semi-sextile, semi-square, sesquisquare, quintile, biquintile). Orbs by default are eight degrees for the conjunction and opposition, six for the trine and square, four for the sextile, and two for the minor aspects.
The luminaries — Sun and Moon — get an additional ±2° latitude on their orbs versus the other planets, reflecting their greater interpretive weight in every astrological tradition we know of. This convention is editable in settings for users who prefer tighter Hellenistic-style orbs.
Aspect lines on the wheel are rendered with opacity scaled to orb tightness — so a 0.2° trine looks visibly more saturated than a 5.9° trine of the same color. This is a small detail with a big effect on legibility.
04Timezones and coordinates
Geocoding draws from a curated 100,000-city database with vetted historical coordinates. Each city carries its IANA timezone identifier (America/New_York, not "EST -5"), which is the only correct way to handle historical timezone changes — daylight saving experiments, wartime time, the regional carve-outs that make 1940s American charts notorious. The IANA tz database is the same dataset used by every major operating system.
For times before 1970 (where modern OS tz data sometimes gets thin) we fall back to a longer historical table. If a chart is cast for a date and place where the timezone is ambiguous, the chart page surfaces a warning rather than guessing silently.
05AI and interpretive prose — what AI does (and does not) do
No astrological claim on this site is generated by AI from scratch. Every interpretive line — what Mars in Aries means, what a Sun-Saturn square pulls toward, how a Venus return reads — was written first by a human author drawing on the studio’s reading of the classical and modern interpretive tradition.
AI is used only for refinement: tightening prose, smoothing transitions between sections, adapting tone to context. The AI is never asked to invent a delineation, predict an outcome, or fill a gap in the source material. If a planet-sign-house combination has no human-written delineation in our library, the chart shows the computed fact and links to the glossary — it does not synthesize a plausible-sounding reading on the fly.
This is the editorial line that matters most for trust. Astrology is hard enough without a language model freely guessing inside it.