What Is a Birth Chart?
The definition, the symbols decoded, and one real chart read out number by number.
A birth chart — a natal chart — is a diagram of where the bodies of the solar system appeared to be, seen from the exact place on Earth where you were born, at the exact moment you were born. That is the whole definition. Everything astrology then does with a chart is interpretation layered on top of that diagram, and the two are worth keeping apart: the positions are astronomy and can be checked against any ephemeris in the world; the meanings are tradition and cannot.
What the diagram actually contains
Strip the symbolism away and a birth chart is ten numbers plus one more. Each of the ten is the position of a body — Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto — expressed as an angle around a circle. The eleventh is the Ascendant, which is not a body at all but a direction: the point of that circle climbing over the eastern horizon at the birth moment.
The circle is the ecliptic, the plane of Earth's orbit, which from down here is simply the line the Sun traces through the year. Positions along it run 0° to 360°. Zodiux reports apparent geocentric ecliptic longitude in the true ecliptic of date: where a body appears from the centre of the Earth, corrected for the time its light took to arrive and for aberration. That is a measurement, and it is the same measurement an observatory would give you.
Cut that 360° circle into twelve equal 30° arcs beginning at 0° Aries — the March equinox point — and you have the tropical zodiac that Western astrology uses. A body at 40.9° of ecliptic longitude is at 10.9° of Taurus. Divide by 30, take the quotient for the sign and the remainder for the degree. That arithmetic is the entire relationship between a number and a sign; nothing more interesting happens in the conversion.
Two things follow immediately. The signs are a division of the seasons, not of the constellations — the constellation Taurus has drifted well away from the tropical sign of the same name, and Zodiux is not sidereal. And a sign boundary is a hard edge in arithmetic that nothing physical marks in the sky: 29.99° Aries and 0.01° Taurus are 0.02° apart, about 1.2 arcminutes, and read as different signs.
The three coordinates that fix a chart
A chart needs a date, a time and a place. That is not a form-design convention, it is the minimum information required to point at one instant and one observer. Each coordinate does a different job, and each one fails differently when it is wrong.
The figures below come from a single real date, 1 May 1994, computed with the site's own ephemeris rather than quoted from a rule of thumb — the rates change slightly with the season and with latitude.
How Zodiux turns those three coordinates into a chart
- Look the birth city up in a bundled dataset of about 36,000 places and read off its latitude, longitude and IANA time zone.
- Convert the wall-clock birth time to UTC through that zone, using the browser's own historical time-zone database. Daylight saving and pre-1970 offset changes are handled here — this step, not the astronomy, is where real-world error concentrates.
- Ask Astronomy Engine for the apparent geocentric ecliptic longitude of each of the ten bodies at that UTC instant.
- Divide each longitude by 30: quotient gives the sign, remainder gives the degree within it.
- Compare each body's longitude with its value 24 hours earlier. If the longitude is decreasing, mark it retrograde.
- Compute local sidereal time at the birth longitude and solve for the ecliptic degree standing on the eastern horizon — the Ascendant. With no birth time, this step is skipped rather than guessed.
- Compare every pair of longitudes and flag the ones falling within orb of 0°, 60°, 90°, 120° or 180°. Those are the aspects.
- Draw the wheel with the Ascendant at the nine o'clock position — or 0° Aries there, when no birth time was given — and list the eight tightest aspects.
| Coordinate | What it fixes | How wrong it goes when it is wrong |
|---|---|---|
| Date | The sign and degree of the Sun and of every planet. On 1 May 1994 the Sun sat at 10.9° Taurus. | The Sun covered 0.97° that day, so a one-day error is under a degree — invisible unless the birth falls within about a day of a sign change. Jupiter, Saturn and everything beyond barely register a day at all. |
| Time of day | The Moon's exact degree, and the rising sign. The Moon travelled 13.1° over that same 24 hours; the Ascendant at Chicago's latitude advanced 1.14° every four minutes that morning. | Two hours of error moves the Ascendant by roughly a whole sign and the Moon by about one degree. In practice the error is almost never in the astronomy — it is a mis-remembered hour or a daylight-saving rule. |
| Place | The horizon the wheel is hung on. Longitude sets local sidereal time; latitude sets how fast each sign climbs. | The same instant produces a completely different Ascendant in two cities. Sun, Moon and planet signs do not move at all — those are geocentric, measured from the centre of the Earth, so they are the same for everyone alive at that moment. |
Only step six needs the clock and the map. That is the reason a chart with no birth time is still worth running and still incomplete: everything else survives, and that one step is impossible. The full arithmetic of what a missing hour costs is in what a rising sign is.
Reading the notation you are actually looking at
Most people meet a birth chart before they meet an explanation of one, and the first obstacle is not the astrology — it is that the output is written in symbols nobody taught them. Every chart calculator, Zodiux included, prints roughly the same set of marks. Here is the whole vocabulary in one place.
A single line of chart output reads left to right in the same order every time: body, sign, degree, and a retrograde mark if there is one. ☉ Sun in ♉ Taurus 10° is the Sun, in Taurus, ten whole degrees into that sign. Nothing else is hiding in it.
The same key, stripped of the reading order and arranged for looking one symbol up, is in the glossary. Use this version to learn the notation and that one to check a mark you have already met.
What Zodiux prints, precisely
The free chart gives you the Sun, Moon and rising sign at the top, then all ten bodies by sign and floored degree with retrograde marks, then the wheel, then the eight tightest aspects it found. It draws no house cusps, and it does not compute the Midheaven, the lunar nodes, Chiron, asteroids or the Part of Fortune — so none of those appear.
Houses are the one thing a reader most often comes looking for and does not get from the free tool. They are in the paid report, which uses whole-sign houses counted from the rising sign and names that choice on the page, and which leaves the house section out entirely rather than deriving it from a chart with no birth time.
| What you see | What it is | How to read it |
|---|---|---|
| ☉ ☽ ☿ ♀ ♂ ♃ ♄ ♅ ♆ ♇ | The ten bodies, always in this order: Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto | The moving parts. What each one is traditionally read for, and how fast it goes, is in the planets in astrology |
| ♈ ♉ ♊ ♋ ♌ ♍ ♎ ♏ ♐ ♑ ♒ ♓ | The twelve signs: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, Pisces | Which fixed 30° arc of the circle the body occupied. A coordinate, not a description of the body |
| A number followed by ° | How far into that sign the body is, from 0 to 29 | Zodiux floors it — 21.65° of Cancer prints as 21°, not 22°. A calculator that rounds will disagree with us on roughly half your placements without either being wrong |
| ℞ | Retrograde: the body's ecliptic longitude is decreasing rather than increasing | A viewing angle from a moving Earth, not a reversal in space. Sun and Moon never carry it. Transits and retrogrades has the geometry |
| ASC, or a tick on the rim | The Ascendant — the rising degree. On the Zodiux wheel it is a gold tick at nine o'clock, and the whole wheel is rotated to put it there | The chart's starting point. Absent entirely when no birth time was given; the wheel then puts 0° Aries in that slot instead |
| ☌ ✶ □ △ ☍ | The five classical aspects: conjunction 0°, sextile 60°, square 90°, trine 120°, opposition 180° | The angular gap between two bodies. Zodiux allows 7° either side for a conjunction, trine and opposition, 6° for a square, 5° for a sextile — see aspects explained |
| Lines across the middle of the wheel | Those same aspects, drawn as chords between the two bodies | Red for the squares and oppositions, periwinkle for the rest. A drawing of the aspect list, not extra information |
| Spokes and a numbered ring inside the signs | House cusps, on charts that draw them | Zodiux's free chart draws none. Why, and what the systems disagree about, is in the twelve houses explained |
Anatomy of the wheel
The wheel drawing is a display convention, not the data. It exists because the underlying quantity is an angle, and angles are easier to compare on a circle than in a column of numbers. Four different kinds of thing get drawn on it, and confusing them is the single most common beginner error.
Signs are the band; planets are what sits in it
The twelve signs are a fixed 360° ruler. They do not move, they do not depend on where or when you were born, and they carry no information on their own. The planets are the moving parts, and a planet's sign is just a statement about which 30° arc of the ruler it happened to occupy. "Sun in Taurus" is a coordinate, in the same sense that "in the 40s" is a coordinate on a thermometer.
Houses are the third kind of thing, and the one the birth time is really for: the same 360° band rotated so that your Ascendant becomes its starting point. Signs answer "in what style", houses answer "in what area of life", and only houses require an exact time. Aspects are the fourth, and the only one that is not a position at all — pure subtraction between two of the numbers, with a tolerance.
Keeping those four apart resolves most beginner confusion in one move. A planet does not "have" a sign the way it has a mass; it is passing through one. A sign is not doing anything; it is a stretch of ruler. A house is not a place in the sky so much as a place in your sky. And an aspect is not a thing on the wheel at all — it is a relationship between two things, which is why it is the only element that disappears and reappears depending on how wide an orb someone allows.
Why the same birthday does not mean the same chart
This is the point where the popular idea of astrology and the actual object part company. Take two births on the same calendar date, 1 May 1994, and run both. All figures below are computed, not illustrative.
Chicago, 07:12 local time (12:12 UTC): Sun 10.9° Taurus, Moon 27.6° Capricorn, Ascendant 7.0° Gemini.
Lisbon, 21:40 local time (19:40 UTC): Sun 11.2° Taurus, Moon 1.7° Aquarius, Ascendant 14.4° Scorpio. Portugal ran on Central European Time from 1992 to 1996, so that evening was UTC+2 — exactly the sort of historical offset that a chart calculator gets wrong if its time-zone data is thin.
Same date. The Sun differs by three-tenths of a degree and lands in the same sign — and that agreement is the entire content of a shared "star sign". The Moon has crossed a boundary: it entered Aquarius at 11:34 that morning, Chicago time, so one of these charts has a Capricorn Moon and the other simply does not. The Ascendant is not even close. Gemini against Scorpio is five signs apart, and the only thing that changed was where the observer stood and how far the Earth had turned.
Hold the place fixed and move only the clock, and the same effect shows up more quietly. Push the Chicago birth from 07:12 to 09:12 and the Sun advances 0.08°, the Moon 1.11°, and the Ascendant travels from 7.0° Gemini to 6.7° Cancer — 29.7° in 120 minutes.
One chart, read out in prose
Here is the Chicago chart in full, read the way the wheel is meant to be read: fast bodies first, then the pattern between them, then the slow background last. Nothing in this section is a prediction. It is a description of a sky. The reading order itself, and where to stop, is how to read a birth chart.
The Sun is at 10.9° Taurus. That is the placement a birthday alone would give you, and it is one body out of ten. Mercury sits 1.27° away at 12.2° Taurus — inside the 7° conjunction orb, and unsurprising, because Mercury is never more than about 28° from the Sun and therefore always lands in the Sun's sign or one either side. Venus is one sign ahead at 6.4° Gemini, and Mars is at 12.9° Aries.
The Moon is at 27.6° Capricorn, near the end of the sign and about four hours from leaving it. Its sextile to Pluto at 27.1° Scorpio is 0.44° from exact, the tightest aspect in the chart, and it is 1.24° from a conjunction with Uranus at 26.3° Capricorn. Both involve the fastest-moving body, which is another way of saying both would have been different for a birth the same afternoon.
The Ascendant is 7.0° Gemini. It exists in this chart only because a time to the minute was supplied; it is the one figure here that a missing birth time would delete outright.
Jupiter is at 9.6° Scorpio and marked retrograde, 1.25° from an exact opposition to the Sun. Those two facts are the same fact: a planet opposite the Sun rises as the Sun sets and is near its closest approach to Earth, and outer planets always read as retrograde around that point. The retrograde mark is geometry, not an omen. Saturn at 10.2° Pisces makes the neatest configuration in the chart — 0.58° from a trine to Jupiter and 0.68° from a sextile to the Sun.
Uranus 26.3° Capricorn, Neptune 23.3° Capricorn, Pluto 27.1° Scorpio, all retrograde. These three move so slowly that everyone born within months of this date shares them almost exactly — Saturn takes about 29.5 years for one orbit and the three outer bodies take very much longer. That is why they are called generational, and why a chart is read from the fast bodies inwards rather than the other way round.
Thirteen of the forty-five possible pairs fall inside Zodiux's orbs; the chart lists the eight tightest; six of those are worth a sentence, and they are the six above. That is the entire chart: eleven numbers, a handful of angles between them, and a horizon. Everything a reading adds is commentary on those.
A chart and a horoscope column are different objects
They get the same shelf in a bookshop and they are not the same kind of thing at all. The differences are worth being exact about, because most scepticism about astrology is really scepticism about the column, aimed at the chart by accident.
Three differences that actually matter
- Scope. A column works from one number — the Sun's sign, a 30° bucket shared by roughly a twelfth of everyone alive. A chart is the other ten placements plus the horizon, and the combination is close to unrepeatable outside of near-simultaneous births in the same city.
- Time. A chart is fixed. It describes one instant that has already happened and is not coming back. A column is about a period ahead of you, which makes it a forecast; a chart is not, and Zodiux does not turn it into one.
- Checkability. Every position in a chart is a falsifiable statement about the sky and can be verified against any independent ephemeris. Nothing in a column can be checked against anything.
What is not in a chart, and what a chart does not determine
The precision of the astronomy does not transfer to the interpretation, and printing both in the same typeface makes it easy to forget that — "the Moon stood at 27.58° Capricorn" and "a Capricorn Moon is reserved about what it needs" are not the same kind of sentence. Using astrology responsibly takes that apart properly. What belongs here is the narrower question of what is simply not in the file.
Four things people expect to find, and where they actually stand
- Houses. A standard part of the general technique, absent from the free Zodiux chart because drawing them forces a silent choice between Placidus, Whole Sign and Koch that moves planets between houses. The paid report makes the choice out loud instead.
- Lunar nodes, Chiron, asteroids, the Part of Fortune, the Midheaven. Not computed anywhere on Zodiux, so not displayed anywhere. Their absence is a scope decision, not an oversight.
- A different zodiac. Zodiux is tropical. A sidereal or Vedic chart of the same birth currently reads about 24° back, which moves most placements a whole sign. Both are computed correctly; they measure from different zero points.
- Anything about your life. A chart does not decide your character, your relationships, your work or your future, and it is not a source of guidance on health, on money or on the law. It also cannot tell you your birth time — if the time is uncertain, the chart inherits that uncertainty rather than resolving it.
And a chart does not settle what astrologers themselves disagree on. Which house system, how wide an orb, whether Pluto and the outer planets belong in a tradition that predates their discovery, whether to use the tropical or sidereal zodiac at all — these are open questions with real consequences for what a chart says. Zodiux picks the tropical zodiac and a fixed orb table, declines to pick a house system for the free chart, and prefers to tell you that rather than let a clean-looking wheel imply the question was never there.
Method. Every position quoted on this page — the 1 May 1994 charts, the daily rates, the aspect orbs, the noon-fallback comparison, the obliquity error figures — was computed with the same code the site ships, Astronomy Engine via the site's vendored ephemeris module, and not taken from a table or from memory. Positions are apparent geocentric ecliptic longitude in the true ecliptic of date, corrected for light-travel time and aberration. Signs are tropical: twelve equal 30° arcs from 0° Aries. The Ascendant comes from local sidereal time at the birth longitude. Birth cities carry their own IANA time zone from a bundled dataset of about 36,000 places, and the wall clock is converted to UTC before any astronomy runs. Statements about what Zodiux displays were read off the shipped chart code rather than described from intent. Interpretive statements are labelled as traditional convention wherever they appear, and are kept separate from the computed figures.
Limits. This page defines a birth chart, decodes its notation and demonstrates one; it does not read yours, and it deliberately stops short of telling you what any placement means about your life. The free Zodiux chart does not draw house cusps, so it takes no position on Placidus versus Whole Sign versus Koch, and it does not compute the Midheaven, the lunar nodes, Chiron or asteroids — where houses or those points matter to a technique, that technique is outside what the free tool supports. Displayed degrees are floored, not rounded, so a comparison against another calculator will differ by up to one degree on presentation alone. The obliquity of the ecliptic is held fixed at 23.4367° in the rising-sign calculation. The realistic source of error in any chart is not the astronomy but the birth time and its historical time zone, particularly for births before 1970. Astrology here is for reflection and entertainment; nothing on this page is advice — not medical, not legal, not financial.
Sources.
- Astronomy Engine — Open-source ephemeris: VSOP87 for the planets, E. W. Brown's lunar theory via the 1954 Improved Lunar Ephemeris for the Moon, a bundled numerical integration for Pluto, NOVAS-derived precession and nutation. The library that produced every figure on this page, bundled identically into the browser chart and the static build. See how Zodiux computes your chart.
- Worked charts, 1 May 1994 — Chicago 07:12 CDT and Lisbon 21:40 local, computed through the site's own ephemeris module. Reproducible from any independent ephemeris using the same UTC instants, 12:12 and 19:40. Note that Portugal ran on Central European Time from 1992 to 1996, so the offset that year is UTC+2, not the UTC+1 a present-day assumption would give.
- The shipped chart code — The notation table, the floored degree, the eight-aspect list, the orb values, the nine o'clock Ascendant and the absence of house cusps were all read directly out of the birth chart's own source rather than described from memory. See what we do not claim.
- IANA time zone database — Applied through the runtime's own historical zone rules to convert wall-clock birth time to UTC. See birth time and time zones for where its rules are weakest.
- City dataset — About 36,000 cities with latitude, longitude and IANA zone, bundled with the site rather than looked up remotely — which is what lets the free chart run without sending anything anywhere. See data sources.
Written and reviewed by the Zodiux editorial team · Last reviewed · Editorial standards and corrections · Astrology is for reflection and entertainment, not advice.
Run the chart, then read this page again
Definitions do about half the job. The other half is looking at eleven real numbers with your own date, time and city attached — free, computed in your browser, and never sent to us.
Do I need my exact birth time to get a birth chart?
No, but you get a smaller chart without it. Sun, Mercury, Venus, Mars and everything slower are effectively fixed across a whole day. The Moon is not — it covers 12° to 15° a day, so a four-hour error can change its sign, as it does in the worked example above. The rising sign is impossible without a time, because it advances about 1° every four minutes. With the field blank, Zodiux computes the chart for local noon and prints "add birth time" where the rising sign would go rather than printing a guess.
Is a birth chart the same as a natal chart?
Yes. The two names describe the same object: the positions of the Sun, Moon and planets, plus the ascending degree, for one birth moment and one location. "Natal" is the older term and is more common in technical writing; "birth chart" is the same thing in plainer English.
What do the symbols on a birth chart mean?
They are shorthand for three things and nothing more. ☉ ☽ ☿ ♀ ♂ ♃ ♄ ♅ ♆ ♇ are the ten bodies, in that order. ♈ ♉ ♊ ♋ ♌ ♍ ♎ ♏ ♐ ♑ ♒ ♓ are the twelve signs, also in order. ☌ ✶ □ △ ☍ are the five aspects — conjunction, sextile, square, trine, opposition. A number with a degree sign is how far into the sign a body sits, and ℞ means retrograde. The table above pairs each mark with what it does.
Why does my chart differ from one I got on another site?
Usually for one of four reasons, none of them the underlying astronomy. The other site may use the sidereal zodiac rather than the tropical one, which currently shifts every sign by roughly 24°. It may apply a different house system, which moves planets between houses without moving them at all. It may round the degree where Zodiux floors it, which is a one-degree presentation difference on about half of all placements. Or the two sites disagree about the historical time-zone offset for your birthplace, which is the most common cause and matters most for births before 1970.
Does a birth chart include the houses?
In the general technique, yes — houses are a standard part of a chart. In the free Zodiux chart, no. It reports the Sun, Moon, rising sign and every planet by sign and degree, and draws no house cusps, because doing so would force a silent choice between Placidus, Whole Sign and Koch that changes the result. The paid report does show houses, using whole-sign houses counted from the rising sign and naming that system on the page, and leaves them out entirely when no birth time was supplied.
Is my birth data sent anywhere when I run a chart?
Not for the free chart. It is computed in your browser using a bundled ephemeris and a bundled city dataset; your birth details are not transmitted and are not written into the page address. The one link that carries them is the one you create yourself with the "Copy link to this chart" button. A paid report is the deliberate exception, because it is rendered on the server, and the Reports page states that before payment.