What is the birth chart?

I.The wheel frozen at the first breath

A birth chart —also called a natal map or nativity— is the drawing of the sky as it was seen from the precise place of birth, at the exact instant of the first breath. That instant is fixed in a wheel divided into twelve sectors, on which the planets, the signs of the zodiac and the angles they form with one another are arranged.

The underlying idea is ancient. Claudius Ptolemy formulated it in the 2nd century in the Tetrabiblos: the moment of birth contains symbolic information that describes tendencies, needs and tensions. It does not determine events. It draws a field of possibilities. The birth chart is therefore not a verdict, but an instrument of knowledge.

The Church preserved this instrument during the centuries when astronomy and astrology had not yet separated. Monasteries copied the tables of Ptolemy. Medieval universities taught the Quadrivium — arithmetic, geometry, music and astronomy — as a noble discipline. Without that patient conservation, the modern birth chart would not exist.

• ✦Natal map, nativity and birth chart are three names for the same object: the sky of birth fixed in a wheel.
• ✦The chart does not determine events: it draws a field of possibilities. It is an instrument of knowledge, not of divination.
• ✦The Church preserved astronomical tables in monasteries and universities during the centuries when astronomy and astrology were one and the same discipline.

II.The four pillars: planets, signs, houses and aspects

A birth chart rests on four elements. Reading them separately is the minimum condition for not getting lost in the whole.

The planets are the functions. The Sun is identity, the Moon the emotional world, Mercury the mind, Venus love and values, Mars will and action. Jupiter, Saturn and the transpersonal planets —Uranus, Neptune and Pluto— mark the collective rhythms that overflow individual biography.

The signs are the modes. A planet in Aries acts with impulse and beginning; the same planet in Taurus acts with slowness and permanence. The sign tinges the planet with a temperament. The houses are the stages: the first is the body and presence, the fourth the family of origin, the seventh the partner, the tenth the public vocation. The aspects, finally, are the conversations between planets: a trine flows, a square tenses, an opposition demands balance.

• ✦Planets — the functions: what acts (Sun=identity, Moon=emotion, Mercury=mind, Venus=love, Mars=will).
• ✦Signs — the modes: how it acts (Aries=impulse, Taurus=permanence, Gemini=dispersion…).
• ✦Houses — the stages: where it acts (I=body, IV=family, VII=partner, X=vocation).
• ✦Aspects — the conversations: trine flows, square tenses, opposition demands balance.
• ✦Four pillars. Function, mode, stage and dialogue. The chart gathers them in a single map.

III.How it is calculated: ephemerides, time and place

Calculating a birth chart requires three data: the date, the exact time with minutes and the place of birth. Without the time, the planetary positions in signs remain valid, but the ascendant and the houses are left unspecified. The ascendant shifts one degree every four minutes: an error of a quarter of an hour can change the rising sign and, with it, the entire house map.

The calculation is performed by astronomical software that consults the planetary ephemerides. Astrogoy uses Swiss Ephemeris, the same database used by professional astrologers and observatories. Positions are not estimated: they are calculated with the precision of celestial mechanics.

The most widespread house system is Placidus, which divides the celestial sphere according to the time each degree takes to rise. It is the modern standard. The classical alternative, Whole Sign, assigns a whole house to each sign. Both are legitimate; Placidus refines the intermediate houses, Whole Sign simplifies them.

• ✦Three non-negotiable data: date, time with minutes and place. Without the time, the ascendant and houses remain imprecise.
• ✦The ascendant advances one degree every four minutes. An error of fifteen minutes changes the rising sign and the entire house map.
• ✦Astrogoy calculates with Swiss Ephemeris: the same database as observatories. Calculated positions, not estimates.
• ✦Placidus house system (modern standard) by default. Whole Sign (classical) as a legitimate alternative.

«Stellæ quidem secundum se non habent influentiam in actus humanos secundum liberum arbitrium determinatos.»

The stars by themselves have no influence on human acts determined by free will.

— Thomas Aquinas, Summa Theologiae II-II, q.95, a.5

IV.What the birth chart is for

The birth chart does not predict the future. That is the first distinction to fix. Thomas Aquinas established it in the Summa Theologica (II-II, q.95): natural astrology, which observes the influence of celestial bodies on matter and character, is lawful; judicial astrology, which claims to read free acts and destiny in the stars, is condemned. The birth chart belongs to the first.

It serves to name. An individual who has spent decades not understanding why he reacts with fury to authority finds in a Mars-Saturn square a language for his experience. It is not that the chart explains the fury: it gives it a name, situates it, makes it workable.

It also serves to recognize tensions the subject intuits but cannot articulate. The chart imposes nothing. It offers a map on which the subject reads his own relief.

• ✦The chart does not predict: it describes tendencies and tensions. Free acts remain outside its reach (Thomas Aquinas, II-II q.95).
• ✦It serves to name: to give language to experiences the subject intuits but cannot articulate.
• ✦It serves to situate: to locate tensions (Mars-Saturn square) and talents (Sun-Jupiter trine) on a legible map.
• ✦It imposes nothing. It offers a map on which the subject reads his own relief.

"Natural astrology is lawful when it observes corporeal effects; it is condemned when it presumes to judge free acts." — Thomas Aquinas, Summa Theologica II-II, q.95, a.5

V.Birth chart, transits and progressions: three levels

The birth chart is the sky of birth. It does not move. It is the base on which everything else is read.

The transits are the planets in their current movement. When Jupiter passes over the position the natal Moon occupied, Jupiter is said to transit the Moon. The transit activates, for a time, the lunar archetype of the subject. Astrogoy calculates transits in real time with Swiss Ephemeris.

The progressions are a symbolic displacement: one day of life equals one year. The progressed Moon advances one degree per year, marking the slow displacement of the emotional world. Three levels, then: the fixed base, the current sky and the symbolic unfolding.

• ✦Birth chart — the sky of birth. Fixed. The base of all reading.
• ✦Transits — the planets in current movement aspecting the natal chart. The clock. Astrogoy calculates them in real time.
• ✦Progressions — symbolic displacement (1 day = 1 year). The slow unfolding of the inner world.
• ✦Three levels: fixed base, current sky, symbolic unfolding.

VI.Common myths about the birth chart

The first myth is that the chart predicts. It does not. It describes tendencies and tensions; free acts remain outside its reach. Whoever promises to read the future in a birth chart is lying or has not read Ptolemy.

The second myth is that the sun sign is enough. The sun sign is a tenth of the chart. Reducing a natal map to the sun sign is like reducing a biography to a surname. The ascendant, the Moon and the aspects weigh as much as the Sun.

The third myth is that the birth chart is esoteric. It is astronomy applied to a symbolic end. Positions are calculated with public ephemerides; the method is verifiable. What is done with the map — the interpretation — belongs to the order of the symbol, not of divination.

• ✦Myth 1: the chart predicts. False. It describes tendencies; free acts remain outside (Ptolemy, Tetrabiblos I.3).
• ✦Myth 2: the sun sign is enough. False. It is a tenth of the chart. Ascendant, Moon and aspects weigh as much as the Sun.
• ✦Myth 3: it is esoteric. False. It is astronomy applied to a symbolic end. Positions verifiable with public ephemerides.
• ✦The birth chart is not an oracle. It is a map. Whoever uses it as an oracle gets frustrated; whoever uses it as a map finds his place.

VII.Chronology of the birth chart

The birth chart was not born all at once. It was built over twenty centuries, on successive layers that the Church preserved and transmitted. Each milestone added an element: the planets in Antiquity, the houses in the Hellenistic world, astronomical rigor in the Renaissance, electronic computation in the 20th century.

VIII.How to read your first chart

A calculated chart has dozens of elements. Trying to read them all at once leads to bewilderment. It is best to start with the Big Three: Sun, Moon and ascendant. The first reading is built on those three. The other planets qualify; the aspects tense or harmonize; the houses locate. But the skeleton is the Big Three. Whoever reads it, reads the essential.

IX.Sources

The sources of this guide combine pre-conciliar texts (Ptolemy, Thomas Aquinas, Manilius) with modern reference works and the technical documentation of Swiss Ephemeris. Planetary positions are calculated with the same database used by astronomical observatories.

X.Frequently asked questions

The most common questions about the birth chart, answered with directness and with source.