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:

The Zodiac band — the 12 signs, each 30° of arc
The Houses — 12 sectors of life experience, anchored to the local horizon
The Planets (+ luminaries + points) — placed at their exact ecliptic longitudes
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 <tspan> 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

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.
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).
Equal vs. unequal slices: Do not assume all 12 houses are 30°. With Placidus, use the actual cusp degrees to draw each wedge.
Planet glyph overlap: Implement collision avoidance; spread glyphs and draw connecting lines to the true degree.
Retrograde direction: Rx does NOT change the planet's position on the chart — it is only a label.
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.

astrological_chart_rendering_guide.md 23 KB Historie Surový