# Astrological Chart Rendering — Complete Technical Guide A reference for developers building a natal (birth) chart renderer. Covers geometry, coordinate systems, layers, data tables, glyphs, and the differences between traditional and modern layouts. --- ## 1. What a Chart Is A natal (birth) chart is a circular map of the sky at the exact moment and place of someone's birth. It answers: *where was every planet, from the viewpoint of Earth, at that instant?* The chart encodes that snapshot into a standardised circular diagram — called a **chart wheel** or **horoscope wheel** — that can be read at a glance. A complete chart has four interlocking layers: 1. **The Zodiac band** — the 12 signs, each 30° of arc 2. **The Houses** — 12 sectors of life experience, anchored to the local horizon 3. **The Planets** (+ luminaries + points) — placed at their exact ecliptic longitudes 4. **Aspects** — angular relationships between planets, drawn as lines across the centre --- ## 2. Coordinate System & Orientation ### 2.1 The Circle - The wheel is a full **360° circle** - It is divided into **12 houses** and **12 zodiac signs** (these two grids do not have to align, depending on the house system) - The viewer is conceptually at the **centre**, looking outward — like standing inside a globe ### 2.2 Cardinal Directions — IMPORTANT: East and West are flipped Because we look *outward from the centre*, the chart mirrors a compass: | Compass direction | Chart position | Angle on SVG canvas | |---|---|---| | **East** (sunrise) | **Left** (9 o'clock) | 180° | | **West** (sunset) | **Right** (3 o'clock) | 0° | | **South** (midday sun, highest point) | **Top** (12 o'clock) | 90° | | **North** (nadir, underground) | **Bottom** (6 o'clock) | 270° | > **Rule #1:** The Ascendant (ASC) is always on the **left**, on the horizontal axis. This is non-negotiable and universal across all Western chart styles. ### 2.3 Degree Placement Ecliptic longitude runs **counter-clockwise** from 0° Aries: - 0° Aries starts at the ASC position (left, 9 o'clock) and increases counter-clockwise - To convert ecliptic longitude `λ` to an SVG angle (clockwise from 3 o'clock): `svgAngle = (180 - λ) mod 360` - Or equivalently, place the ASC at the left, and increase counter-clockwise from there --- ## 3. The Four Angles — The Skeleton of the Chart These four points are the most important structural elements. They form a **cross** (the "Cross of Matter") and divide the chart into four quadrants. ``` MC (Medium Coeli) Top / 12 o'clock "Midheaven" — career, public life | | West ——— DSC ————————————+———————————— ASC ——— East (Descendant) | (Ascendant) Right / 3 o'clock | Left / 9 o'clock Partnerships, others | Self, body, appearance | IC (Imum Coeli) Bottom / 6 o'clock "Nadir" — home, roots, private life ``` | Angle | Abbrev | Position | House cusp | Meaning | |---|---|---|---|---| | Ascendant | ASC / AC | Left, horizon | 1st house | Eastern horizon; rising sign; self, appearance | | Descendant | DSC / DC | Right, horizon | 7th house | Western horizon; partnerships, "the other" | | Midheaven | MC | Top, meridian | 10th house | Highest point; career, public status, ambitions | | Imum Coeli | IC | Bottom, meridian | 4th house | Lowest point; home, roots, ancestry | > The ASC–DSC axis is the **horizon line** (horizontal). > The MC–IC axis is the **meridian line** (vertical). > In most (but not all) house systems, these axes align with house cusps 1/7 and 10/4 respectively. --- ## 4. The 12 Houses Houses are numbered **1 through 12**, starting from the ASC and going **counter-clockwise**. ``` 10 11 12 9 1 ← ASC (left) 8 2 7 6 5 4 3 ↑ IC (bottom) ``` ### House Number Layout (clock analogy) ``` [11] [10] [9] [12] [8] [1] ASC [7] DSC [2] [6] [3] [4] [5] IC ``` ### Quadrant Groupings | Quadrant | Houses | Axis between | Theme | |---|---|---|---| | 1st | 1, 2, 3 | ASC → IC | Personal Identity, awareness of self | | 2nd | 4, 5, 6 | IC → DSC | Personal expression, integration with environment | | 3rd | 7, 8, 9 | DSC → MC | Social identity, awareness of others | | 4th | 10, 11, 12 | MC → ASC | Social expression, integration with society | ### House Types | Type | Houses | Modality analog | Quality | |---|---|---|---| | Angular | 1, 4, 7, 10 | Cardinal | Most powerful; planets here are prominent | | Succedent | 2, 5, 8, 11 | Fixed | Stable; accumulate resources | | Cadent | 3, 6, 9, 12 | Mutable | Transitional; mental, service-oriented | ### The 12 Houses — Traditional Meanings | House | Traditional name | Life domain | |---|---|---| | 1 | House of Self | Identity, body, appearance, first impressions | | 2 | House of Value | Money, possessions, self-worth, material resources | | 3 | House of Communication | Siblings, short trips, writing, local environment | | 4 | House of Home | Family, roots, ancestry, private life, real estate | | 5 | House of Pleasure | Creativity, romance, children, play, speculation | | 6 | House of Health | Daily routines, work, health, service, employees | | 7 | House of Partnership | Marriage, contracts, open enemies, one-to-one relating | | 8 | House of Transformation | Death, shared resources, sexuality, taxes, occult | | 9 | House of Philosophy | Higher education, travel, religion, law, publishing | | 10 | House of Career | Status, reputation, authority, vocation, father | | 11 | House of Community | Friends, groups, hopes, humanitarian causes | | 12 | House of Undoing | Secrets, isolation, karma, hidden enemies, institutions | --- ## 5. House Systems The **zodiac** is fixed; the **houses** depend on birth time and location. Different house systems calculate house cusps differently. | System | Description | Common use | |---|---|---| | **Placidus** | Divides each quadrant by the time it takes a degree to travel from ASC to MC; produces unequal house sizes | Most common in modern Western astrology | | **Whole Sign** | Each house = one entire zodiac sign; ASC sign = House 1; very simple | Traditional / Hellenistic astrology; increasingly popular | | **Equal House** | All houses exactly 30°, starting from ASC degree | Clean rendering; used by some modern and Vedic astrologers | | **Koch** | Similar to Placidus but uses a different trisection method | Common in German-speaking countries | | **Campanus** | Divides the prime vertical; produces differently shaped houses | Rare, used by some traditional practitioners | | **Porphyry** | Divides each quadrant into three equal parts | Simple alternative to Placidus | > **Rendering implication:** Placidus (and Koch, Porphyry) produce **wedges of unequal angular width**. Whole Sign and Equal House produce **equal 30° slices**. The chart wheel must reflect the actual cusp degrees, not assume equal slices. --- ## 6. The Zodiac Band The outermost ring of the wheel displays the **12 zodiac signs** in counter-clockwise order, each occupying exactly 30°. Their order is always fixed: | # | Sign | Glyph | Element | Modality | Unicode | |---|---|---|---|---|---| | 1 | Aries | ♈ | Fire | Cardinal | U+2648 | | 2 | Taurus | ♉ | Earth | Fixed | U+2649 | | 3 | Gemini | ♊ | Air | Mutable | U+264A | | 4 | Cancer | ♋ | Water | Cardinal | U+264B | | 5 | Leo | ♌ | Fire | Fixed | U+264C | | 6 | Virgo | ♍ | Earth | Mutable | U+264D | | 7 | Libra | ♎ | Air | Cardinal | U+264E | | 8 | Scorpio | ♏ | Water | Fixed | U+264F | | 9 | Sagittarius | ♐ | Fire | Mutable | U+2650 | | 10 | Capricorn | ♑ | Earth | Cardinal | U+2651 | | 11 | Aquarius | ♒ | Air | Fixed | U+2652 | | 12 | Pisces | ♓ | Water | Mutable | U+2653 | The zodiac band starts at **0° Aries** (not necessarily at the 9 o'clock position — its starting point rotates based on the ASC). The Ascendant degree within a sign is placed at the 9 o'clock position, and the entire zodiac band rotates accordingly. ### Element Colour Coding (common convention) | Element | Signs | Colour | |---|---|---| | Fire | Aries, Leo, Sagittarius | Red / orange | | Earth | Taurus, Virgo, Capricorn | Green / brown | | Air | Gemini, Libra, Aquarius | Yellow / sky blue | | Water | Cancer, Scorpio, Pisces | Blue / teal | --- ## 7. The Planets & Points ### 7.1 Planet Glyphs | Body | Glyph | Unicode | Category | |---|---|---|---| | Sun ☉ | ☉ | U+2609 | Luminary | | Moon ☽ | ☽ | U+263D | Luminary | | Mercury ☿ | ☿ | U+263F | Inner planet | | Venus ♀ | ♀ | U+2640 | Inner planet | | Mars ♂ | ♂ | U+2642 | Inner planet | | Jupiter ♃ | ♃ | U+2643 | Social planet | | Saturn ♄ | ♄ | U+2644 | Social planet | | Uranus ♅ | ♅ | U+2645 | Outer / modern | | Neptune ♆ | ♆ | U+2646 | Outer / modern | | Pluto ♇ | ♇ | U+2647 | Outer / modern | | North Node ☊ | ☊ | U+260A | Point | | South Node ☋ | ☋ | U+260B | Point | | Chiron ⚷ | ⚷ | U+26B7 | Asteroid / modern | | Part of Fortune ⊕ | ⊕ | — | Arabic lot | > **Traditional astrology** uses only the 7 classical planets: Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn. > **Modern astrology** adds Uranus, Neptune, Pluto, Chiron, the Lunar Nodes, and sometimes asteroids (Ceres, Juno, Pallas, Vesta). ### 7.2 Planet Placement Each planet is placed at its **ecliptic longitude** (degrees + minutes within a sign). The planet glyph is rendered: - Inside the house ring (between the house cusp lines and the chart centre) - At the correct angular position around the wheel - With a small label showing degree and minutes: e.g., `♀ 14°37'` - A tick mark or line from the planet glyph to the exact degree on the zodiac ring is common **Collision avoidance:** Multiple planets in the same house or close degrees will overlap. Standard solutions: - Cluster glyphs and spread them slightly within the sector - Draw a fine line from each glyph to its exact tick mark on the zodiac ring - Use a minimum angular spacing (e.g. 5°) between glyphs ### 7.3 Retrograde A planet in retrograde motion is marked with **Rx** or **℞** next to its glyph. This is displayed on the wheel and in data tables. --- ## 8. Aspects Aspects are **angular relationships** between planets, drawn as lines across the centre of the wheel. ### 8.1 Major Aspects | Aspect | Symbol | Angle | Orb (modern) | Nature | Colour convention | |---|---|---|---|---|---| | Conjunction | ☌ | 0° | ±8° | Neutral / intense | Yellow or grey | | Opposition | ☍ | 180° | ±8° | Tense / polarising | Red | | Square | □ | 90° | ±8° | Tense / challenging | Red | | Trine | △ | 120° | ±8° | Harmonious | Blue | | Sextile | ✱ | 60° | ±6° | Harmonious / mild | Green | ### 8.2 Minor Aspects | Aspect | Symbol | Angle | Orb | Nature | |---|---|---|---|---| | Quincunx (Inconjunct) | ⚻ | 150° | ±3° | Mildly challenging | | Semi-sextile | ⚺ | 30° | ±2° | Mildly harmonious | | Semi-square | ∠ | 45° | ±2° | Mildly tense | | Sesquiquadrate | ⚼ | 135° | ±2° | Mildly tense | | Quintile | Q | 72° | ±2° | Creative | | Biquintile | bQ | 144° | ±2° | Creative | ### 8.3 Rendering Aspects - Lines are drawn **between the two planet positions**, straight across the chart centre - Colour-code by aspect type (see above) - Line style can vary: solid for major, dashed for minor - Optionally show an aspect grid table (see Section 11) - Traditional charts often omit minor aspects from the wheel to reduce clutter --- ## 9. Visual Layer Stack (outermost → innermost) ``` [Layer 5] Zodiac sign band — outermost ring; 12 coloured segments, sign glyphs [Layer 4] House cusp tick marks — radial lines at each cusp degree on zodiac ring [Layer 3] House ring — labelled with house numbers (1–12) [Layer 2] Planet ring — planet glyphs with degree labels + Rx markers [Layer 1] Aspect web — lines connecting planets across centre [Layer 0] Centre circle — sometimes blank, sometimes shows subject name/chart data ``` Typical radii (as percentage of total wheel radius, working outward from 0): | Region | Inner radius | Outer radius | |---|---|---| | Aspect web / centre | 0% | 35% | | Planet glyph ring | 35% | 60% | | House number labels | 60% | 68% | | House cusp lines | 35% | 78% | | Inner zodiac band edge | 78% | — | | Zodiac sign glyphs | 78% | 90% | | Outer zodiac band edge | 90% | — | | Degree tick marks | 90% | 100% | > These ratios can be adjusted; this is a common professional proportion. The key constraint is that the zodiac band is always outermost, and planets sit inside the house ring. --- ## 10. Chart Header / Subject Information Many (but not all) charts display a **data block** either inside the centre circle or in a box outside the wheel: - **Subject's name** (if personal chart) - **Date of birth**: day, month, year - **Time of birth**: HH:MM (+ timezone or UTC offset) - **Place of birth**: city, country - **Latitude / Longitude** - **House system used** (e.g. Placidus) - **Zodiac type** (Tropical or Sidereal) For anonymous or event charts, the name is omitted. Time and place are almost always shown. --- ## 11. Accompanying Data Tables Professional chart printouts (and most software) include one or more tables alongside the wheel. ### 11.1 Planet Positions Table The most essential table. Rows = planets, Columns = sign, degree/minute, house, retrograde status. ``` Planet | Sign | Degree | House | Rx ----------|-------------|----------|-------|---- ☉ Sun | ♌ Leo | 14° 22' | 5 | ☽ Moon | ♑ Capricorn | 07° 58' | 10 | ☿ Mercury | ♌ Leo | 28° 11' | 5 | Rx ♀ Venus | ♎ Libra | 02° 45' | 7 | ♂ Mars | ♊ Gemini | 19° 03' | 3 | ♃ Jupiter | ♐ Sagittarius| 11° 30' | 9 | ♄ Saturn | ♒ Aquarius | 06° 21' | 11 | Rx ♅ Uranus | ♉ Taurus | 14° 07' | 2 | ♆ Neptune | ♓ Pisces | 22° 55' | 12 | ♇ Pluto | ♑ Capricorn | 26° 17' | 10 | Rx ☊ N. Node | ♊ Gemini | 00° 48' | 3 | ASC | ♍ Virgo | 22° 15' | 1 | MC | ♊ Gemini | 20° 48' | 10 | ``` ### 11.2 House Cusps Table Shows the zodiac degree at the beginning (cusp) of each house. ``` House | Sign | Degree ------|-------------|-------- 1 | ♍ Virgo | 22° 15' (= ASC) 2 | ♎ Libra | 18° 42' 3 | ♏ Scorpio | 17° 30' 4 | ♐ Sagittarius| 19° 10' (= IC) 5 | ♑ Capricorn | 21° 48' 6 | ♒ Aquarius | 22° 30' 7 | ♓ Pisces | 22° 15' (= DSC) 8 | ♈ Aries | 18° 42' 9 | ♉ Taurus | 17° 30' 10 | ♊ Gemini | 19° 10' (= MC) 11 | ♋ Cancer | 21° 48' 12 | ♌ Leo | 22° 30' ``` ### 11.3 Aspects Grid (Aspect Matrix) A triangular grid showing all planet-to-planet aspects. Rows and columns are planets; intersecting cells show the aspect symbol (if one exists within orb) or blank. ``` ☉ ☽ ☿ ♀ ♂ ♃ ♄ ♅ ♆ ♇ ☉ – ☽ □ – ☿ ☌ △ – ♀ ✱ ☍ ✱ – ♂ △ □ △ ✱ – ♃ ☍ ✱ ☍ □ △ – ♄ □ ✱ □ △ ☍ △ – ... ``` Colour-code each symbol by aspect type for readability. ### 11.4 Elements & Modalities Balance Table Summary count of how many planets fall in each element and modality. Common in modern psychological astrology. ``` Elements: Modalities: Fire ♈♌♐ → 3 Cardinal ♈♋♎♑ → 4 Earth ♉♍♑ → 2 Fixed ♉♌♏♒ → 3 Air ♊♎♒ → 3 Mutable ♊♍♐♓ → 3 Water ♋♏♓ → 2 ``` ### 11.5 Dignities Table (Traditional charts) Shows each planet's **essential dignity** — how well or poorly placed it is in its sign: | Planet | Sign | Dignity | |---|---|---| | ☉ Sun | ♌ Leo | Domicile (home sign) | | ☽ Moon | ♉ Taurus | Exaltation | | ♂ Mars | ♑ Capricorn | Exaltation | | ♄ Saturn | ♓ Pisces | Fall | | ♀ Venus | ♏ Scorpio | Detriment | Dignity levels: **Domicile > Exaltation > Triplicity > Term > Face > Peregrine > Detriment > Fall** --- ## 12. Traditional vs. Modern Chart Layouts ### 12.1 Traditional Western (Hellenistic / Medieval) - **House system:** Whole Sign most common; also Alcabitius, Porphyry - **Planets used:** 7 classical (Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn) - **Outer planets:** Not used or shown separately as "modern additions" - **Aspects:** Primarily the 5 Ptolemaic aspects (conjunction, sextile, square, trine, opposition) - **Dignities table:** Central to interpretation; always shown - **Arabic lots:** Part of Fortune and others may be plotted on the wheel - **Wheel style:** Often simpler, less cluttered; may use bolder house division lines - **Sect:** Day/night birth distinction noted; planets classified as diurnal or nocturnal - **Colour:** Often black and white or minimal colour; functional over decorative ### 12.2 Modern Western (Psychological / Humanistic) - **House system:** Placidus (default), Koch, or Equal; houses of unequal size - **Planets used:** 10 (add Uranus, Neptune, Pluto) + Chiron + Nodes - **Aspects:** All 5 major + minor aspects (quincunx, semi-square, etc.) - **Dignities:** Often de-emphasised or omitted ("no planet is inherently bad") - **Arabic lots:** Rarely shown - **Wheel style:** More visual, colourful; aspect lines colour-coded; glyphs styled - **Elements/modalities table:** Almost always included - **Chart data block:** Usually shown prominently - **Outer ring:** May include a second ring for transits (biwheel) or progressions ### 12.3 Square / Table Chart (Vedic / Jyotish, some North Indian tradition) Not circular — uses a **diamond or grid** layout: - North Indian style: Fixed diamond grid; houses are boxes, signs rotate - South Indian style: Fixed signs in boxes; houses rotate - No aspect lines drawn; aspects calculated by sign and house - Not the focus here, but worth knowing this exists ### 12.4 Biwheel Chart (Transit / Synastry) Two concentric wheels: - **Inner wheel:** Natal chart (birth data) - **Outer wheel:** Transit positions (current sky), progressions, or partner's natal chart - Aspect lines drawn between inner and outer planets - Planet glyphs in the outer ring are often styled differently (lighter, or different colour) --- ## 13. What Goes Where — Spatial Summary ``` OUTSIDE the wheel: - Chart title / subject name (top or header area) - Birth data (date, time, place) — top or bottom margin - Optional: House system label OUTERMOST ring (zodiac band): - 12 sign glyphs, one per 30° sector - Degree tick marks (every 1° or every 5°) - Sign dividers (bold lines every 30°) - Element colour fill per sign sector NEXT ring inward (house cusps): - Cusp lines radiating from centre to zodiac ring - House number labels (1–12), placed in the middle of each house sector - Angular house cusps (1, 4, 7, 10) drawn as bolder/heavier lines PLANET ring: - Planet glyphs at their ecliptic longitude - Degree + minute label beside each glyph - Rx symbol for retrograde planets - Fine lines from glyph to exact tick mark on zodiac band (when glyphs are nudged apart for readability) CENTRE of wheel: - Aspect lines (coloured by type) - Sometimes: subject name, chart date, or decorative element - Four angles may be labelled (ASC, DSC, MC, IC) just inside the cusp lines OUTSIDE or BELOW the wheel (tables): - Planet positions table - House cusps table - Aspects grid / matrix - Elements & modalities table - Dignities table (traditional) ``` --- ## 14. The Angles on the Wheel — Rendering Detail The four angles must be visually prominent: - The **horizontal axis** (ASC ↔ DSC) is drawn as a solid line across the wheel at the 9 o'clock / 3 o'clock positions - The **vertical axis** (MC ↔ IC) is drawn as a solid line at 12 o'clock / 6 o'clock - Labels **ASC** (or AC), **DSC** (or DC), **MC**, **IC** are placed just outside the wheel at each axis endpoint - The degree and sign of each angle should be shown: e.g. `ASC 22° ♍` - The house 1 cusp line and ASC line are typically the same line --- ## 15. Typography & Glyph Notes - Astrological glyphs are available as Unicode characters (see tables above) and render in most modern fonts - Dedicated astrology fonts (e.g. *Astro*, *Starfont*, *Hamburg* symbol sets) provide better glyph shapes - Glyphs on the wheel should be **large enough to read at a glance** — typically 14–18px minimum - SVG `text` elements or `` can be rotated along the arc of each house sector for degree labels - **Never rotate planet glyphs** — they should always be upright, even when placed around a circular ring - Degree labels can be slightly rotated to follow the ring --- ## 16. Minimum Viable Chart vs. Full Chart ### Minimum Viable (wheel only) - Zodiac ring with 12 signs - House divisions (Whole Sign is simplest: equal 30° slices) - Planet glyphs at correct positions - ASC/MC axes labelled ### Standard (most software default) - All of the above - Placidus house cusps (unequal) - Aspects drawn as coloured lines - Degree labels on planets - Planet positions table alongside wheel - ASC, DSC, MC, IC labelled ### Full Professional - All of the above - Aspect grid table - Elements/modalities table - House cusps table - Dignities (optional) - Rx markers - North/South Node plotted - Chiron plotted - Part of Fortune plotted - Subject name + birth data block --- ## 17. Common Pitfalls 1. **Clockwise vs. counter-clockwise:** Houses are numbered counter-clockwise. Zodiac signs also increase counter-clockwise. On a standard SVG, Y increases downward, so convert carefully. 2. **ASC rotation:** The ASC degree must land at exactly 9 o'clock (left). Rotate the entire zodiac band so the ASC degree is at 180° on an SVG (pointing left). 3. **Equal vs. unequal slices:** Do not assume all 12 houses are 30°. With Placidus, use the actual cusp degrees to draw each wedge. 4. **Planet glyph overlap:** Implement collision avoidance; spread glyphs and draw connecting lines to the true degree. 5. **Retrograde direction:** Rx does NOT change the planet's position on the chart — it is only a label. 6. **Axis labelling confusion:** ASC is left (east), DSC is right (west) — opposite of a standard map. --- ## 18. Reference: Aspect Orb Defaults (Placidus / Modern Western) | Aspect | Degrees | Default orb | Tight orb | |---|---|---|---| | Conjunction | 0° | ±8° | ±3° | | Opposition | 180° | ±8° | ±3° | | Square | 90° | ±7° | ±3° | | Trine | 120° | ±7° | ±3° | | Sextile | 60° | ±6° | ±2° | | Quincunx | 150° | ±3° | ±1° | | Semi-square | 45° | ±2° | ±1° | | Semi-sextile | 30° | ±2° | ±1° | Traditional astrology uses tighter orbs, applies orbs to the **signs** rather than exact degrees in some systems (sign-based aspects), and considers whether a planet is "applying" (moving toward exact) or "separating" (moving away). --- *This document describes Western tropical astrology conventions. Vedic/Jyotish astrology uses different coordinate systems (sidereal zodiac), different house systems, and a different visual format — square charts rather than the circular wheel described here.*