How Shymea computes a chart.

Shymea keeps one order fixed: normalize the moment and place, compute the astronomical positions, derive the Western placements, and only then generate language. A fluent paragraph cannot repair a wrong time zone or an aspect that was never there, so the astronomy is settled before any interpretation begins. The short version of this page lives on About; the conceptual companion is How Shymea Computes a Western Astrology Chart.

Astronomical source: JPL DE442 via Skyfield

Planetary positions come from NASA Jet Propulsion Laboratory's DE442 planetary ephemeris, evaluated with the Skyfield astronomy library. Every chart in the app — natal, transit, synastry, horary, astrocartography — shares one Skyfield instance and one ephemeris file, so the same instant always yields the same coordinates. Skyfield returns geocentric apparent positions (Earth's position subtracted from the body's, with light-time applied), which we then rotate into ecliptic longitude and latitude for the chart.

A local birth time is not yet an astronomical timestamp. The engine resolves the birthplace's historical time-zone and daylight-saving rules, converts to UTC, and hands Skyfield the astronomical time scales the ephemeris requires. Uncertain or rounded birth times are the largest single source of error in any chart — see Limitations below.

Tropical zodiac

Shymea uses the tropical zodiac: 0° Aries is the northern-hemisphere vernal equinox point for the moment of the chart, and the circle is divided into twelve equal 30° signs from there. A sign label is derived arithmetically from the ecliptic longitude — a body at 103°14′ is 13°14′ Cancer because Cancer begins at 90°. We do not offer sidereal zodiacs; Shymea is a Western astrology app by design.

Houses: Placidus, with a whole-sign note

Natal, transit and synastry charts use Placidus houses. Our house calculator derives the Ascendant and Midheaven from local sidereal time, the obliquity of the ecliptic and the birth latitude, then finds the intermediate cusps by iterative trisection of the diurnal and nocturnal semi-arcs — the classical Placidus condition, not a table lookup. The same module can also compute Koch, Equal and Whole Sign cusps; whole-sign houses are described in the houses guide and the journal's house-systems explainer, but Placidus is what the app renders. Horary charts use Regiomontanus (see below).

Aspects and the orbs we use

Aspects are measured in ecliptic longitude. For two bodies at λ₁ and λ₂ the separation is min(|λ₁ − λ₂|, 360° − |λ₁ − λ₂|), so bodies at 359° and 1° are 2° apart. The separation is compared against each aspect angle and its orb. In natal and synastry charts the orbs are:

• Conjunction (0°) and opposition (180°): 8°
• Trine (120°) and square (90°): 6°
• Sextile (60°): 4°
• Minor aspects — semisextile (30°), semisquare (45°), sesquiquadrate (135°): 2°; quincunx (150°): 3°

Transit-to-natal aspects use deliberately tighter orbs so a daily reading is not crowded with contacts that are days from exact: 3° for conjunction, opposition, trine and square, and 2° for the sextile. Applying versus separating is decided from the transiting body's actual daily motion, not from a keyword rule. The aspects guide explains what each angle traditionally means.

Retrograde and stations

A planet's retrograde flag is not read from a table. The engine samples the body's geocentric ecliptic longitude at neighbouring instants, derives a signed speed in degrees per day, and marks the planet retrograde when that speed is negative. The Moon's True Node is checked the same way, one day apart. Stations — the moments the speed passes through zero — are located by scanning that speed over a date range, which is how the calendar pages and the Cosmos view know when Mercury turns.

Transits

Transits are the live sky, computed with the same ephemeris, compared against the natal longitudes with the tighter orbs above. Daily content is keyed to the user's own local date, so a reading never rolls over at somebody else's midnight. Void-of-course Moon is computed as the interval between the Moon's last Ptolemaic aspect in its current sign and its ingress into the next.

Progressions

Secondary progressions use the classical day-for-a-year mapping: the progressed chart for a given age is the real sky 1 tropical year → 1 day after birth, with the tropical year taken as 365.2422 days. The progressed positions are then run through the same aspect and house machinery as a natal chart. Nothing is approximated by adding a fixed degree per year.

Returns

A solar return is the instant the transiting Sun reaches the exact natal Sun longitude in a given year; lunar, Saturn and Jupiter returns are found the same way for their bodies. The engine solves for the return instant numerically against the ephemeris and casts a full chart for that moment, either at the birthplace or a chosen location. The Saturn return page and the solar return article describe how those charts are read.

Relocation and astrocartography

Astrocartography lines are computed in mundo — from right ascension and declination, not zodiac longitude. For each of the Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto the engine takes the apparent geocentric RA and declination at the birth instant and the Greenwich apparent sidereal time, then solves for the geographic longitude where the body culminates (MC line), anti-culminates (IC line), and for each latitude the longitude where it sits on the horizon rising (ASC line) or setting (DSC line). A relocated chart re-derives the Ascendant, Midheaven and house cusps for the new coordinates while keeping the planets where they were at birth. See our astrocartography hub for what the lines mean.

Horary

Horary charts are cast for the moment a question is received, at the querent's location, using Regiomontanus houses rather than Placidus — the system William Lilly worked in. Before judgment the engine applies the traditional considerations (early or late Ascendant, Moon void of course, Saturn in the seventh, and so on) and then evaluates the significators' aspects, receptions and essential dignities. The horary guide and the Lilly checklist document the rules, the horary overview explains what the feature does, and Study 05 tests the engine against 17 charts Lilly judged himself.

Where the AI comes in — after the calculation

Only once the chart exists as numbers — placements, cusps, aspects with their orbs, retrograde flags — is it written into a profile and handed to a language model. The model is asked to explain a chart it is given; it never produces a placement, an aspect or a date. If a number appears in a Shymea reading, it came from the ephemeris, not from the model. Names and raw birth data are tokenised before the profile leaves the engine.

Limitations

• Birth-time uncertainty dominates. Earth turns about one degree every four minutes, so a rounded time can move the Ascendant, Midheaven and house cusps materially while leaving most planetary sign placements intact.
• Historical time-zone rules can be incomplete for older records and some regions; an incorrectly recorded local time cannot be detected from the chart itself.
• Placidus houses are undefined inside the polar circles; charts for extreme latitudes should be read with that in mind.
• High-precision astronomy is not evidence that astrology is scientifically validated. We use DE442 so the astronomy is right; the interpretation is a tradition, offered for reflection and entertainment, not a prediction of fact.

We have measured this rather than asserted it. Almost all other astrology software computes positions with the Swiss Ephemeris; we compared 870 positions from both engines across 1900–2035 and published the full table in Study 01. The two never disagreed about a sign or a whole degree.

Corrections are welcome — if you find a technical statement on this page that the engine does not match, write to support@shymea.com. Our editorial policy covers how the written guides are produced.