What a birth chart actually is
A birth chart is a diagram of the sky at one moment, seen from one place. It records where the Sun, the Moon and the planets stood against the zodiac, which of them were above the horizon, and which degree was rising in the east. Nothing about it is symbolic at the point of calculation; the interpretation comes afterwards and is built on top of the astronomy.
That is why the three inputs matter so much. The date fixes the planets. The time fixes the horizon, and with it the rising sign and the houses. The place fixes which part of the sky counted as up. Change any one and you have a different chart.
What the calculator gives you
Every body is placed to the arc minute, which is one sixtieth of a degree. The Sun, Moon and eight planets come from a numerical model of the solar system; Chiron, the four main asteroids and Black Moon Lilith are interpolated from a sampled ephemeris because no browser-side model covers them. The lunar nodes are the true nodes, not the mean, and the Part of Fortune uses the day formula or the night formula according to whether the Sun was above the horizon.
House cusps are given in the system you choose, and the default is Placidus. The wheel is drawn with the ascendant on the left, the traditional orientation, and the aspect lines inside it are only the major aspects unless you open the minor list.
| Shown | How it is derived |
|---|---|
| Planets and luminaries | Numerical solar-system model, checked against the Swiss Ephemeris |
| Chiron, Ceres, Pallas, Juno, Vesta, Lilith | Sampled ephemeris, cubic interpolation |
| Ascendant, Midheaven, Vertex | Spherical trigonometry from sidereal time and latitude |
| House cusps | Placidus by default; Whole Sign, Equal, Porphyry, Regiomontanus available |
| Aspects | Angular separation against the standard set, with orbs widened for the Sun and Moon |
| Dominants | Weighted scoring across bodies, rising sign and Midheaven, with the weights shown |
How to read the result, in order
Start with the three placements at the top: Sun, Moon and rising. Together they describe what you are driving at, what settles you, and how you arrive in a room, and most of what people recognise about themselves in a chart lives there.
Then look at the aspect table sorted by orb. The tightest aspects, those under two degrees, are the ones doing the most work. A wide aspect near the edge of its orb is background; an exact one is a headline. The table lists orbs precisely so you can make that distinction rather than treating every listed aspect as equal.
The house table tells you where each body sits, which is the part of a chart that makes a placement personal rather than generational. Pluto in Scorpio belongs to everyone born across a dozen years; Pluto in your second house does not.
Leave the dominants for last. They are a scoring convention rather than an observation, and they are most useful for noticing what the chart leans on when the individual placements pull in different directions.
Why the birth time matters so much
The Earth turns once a day, so the degree rising in the east moves through the entire zodiac every twenty-four hours, at roughly one degree every four minutes. A birth time that is off by an hour moves the ascendant fifteen degrees, and often into the next sign. The Midheaven and every house cusp move with it.
The Moon also moves quickly enough for the time to matter, about half a degree an hour. On the days when it changes sign, which is roughly one day in three, the hour decides which sign you get.
The planets are unaffected. If you do not know your birth time, tick the box that says so and the calculator will give you everything that depends on the date alone, hide everything that depends on the hour, and warn you when the Moon is close to a boundary. That is the honest answer, and it is a good deal more useful than a chart that quietly assumes noon.
Time zones, daylight saving and the awkward years
Birth times are recorded in local clock time, and converting them to the universal time the astronomy needs is where most free calculators go wrong. Daylight saving rules have changed repeatedly, wartime double summer time existed in some countries, and half-hour and quarter-hour zones are common. A chart calculated with the wrong offset is a chart for a different moment.
This calculator uses the full historical time-zone database that ships with your browser, so the conversion comes from the record for that place and that year rather than from a guess. When a birth time falls on the hour the clocks went forward, so that the wall-clock time never actually happened, or on the hour they went back, so that it happened twice, the result says which reading was used.
Which house system to choose
The planets are the same in every system; only the house boundaries move. Placidus is the default because it is the most widely used in modern Western practice, and most published interpretations assume it. Whole Sign is the oldest system and has come back into wide use; it is also the right choice if you want to compare with a Vedic chart or with most Hellenistic material.
The practical difference is not small. In a typical chart several bodies land in a different house depending on the system, and the house calculator on this site shows the same chart in all five side by side so you can see exactly which of your placements are affected. If a body sits within a degree or two of a cusp, treat its house placement as uncertain whichever system you pick.
Placidus has no solution above roughly sixty-six degrees of latitude, because some house circles never rise. When that happens the calculator falls back to Porphyry and tells you, rather than printing cusps that do not exist.
Tropical and sidereal
The default zodiac is tropical, which begins at the spring equinox and is what nearly all Western astrology uses. The sidereal zodiac begins against the fixed stars instead, and the two have drifted about twenty-four degrees apart since they coincided some seventeen centuries ago. Switching to sidereal moves most placements back roughly one sign.
Neither is more correct; they measure from different starting points. If you are comparing with a Vedic chart, use sidereal with Whole Sign houses, or use the dedicated Vedic calculator, which applies both and adds the nakshatras.
How accurate it is
Before any version of this calculator is published, the whole engine is compared against the Swiss Ephemeris at random dates and latitudes spanning two centuries. In the most recent check the planets agreed to well under a third of an arc minute, the angles and house cusps to under an arc second. The methodology page lists the figures. An arc second is one part in thirty-six hundred of a degree, and nothing in astrology turns on differences that small.
The places where a chart can genuinely be wrong are the inputs: a birth time rounded to the hour, a birthplace given as the nearest city rather than the actual town, a date written in the wrong order. Those matter far more than anything in the arithmetic, and the calculator surfaces the ones it can detect.
Common questions
Do I need my exact birth time?
For the planets, no: they barely move in an hour. For the ascendant, the Midheaven and every house cusp, yes, because those sweep the whole zodiac in a day. Tick the unknown-time box and the calculator gives you everything the date settles and hides the rest rather than guessing.
Which house system should I pick?
Placidus if you have no reason to prefer another, since most published interpretations assume it. Whole Sign if you are comparing with Vedic or Hellenistic material. The house calculator shows the same chart in five systems so you can see which of your placements actually move.
Why does my chart here differ from another site?
Usually the house system or the time-zone conversion. A different system moves bodies between houses without changing a single degree, and a mishandled daylight saving rule moves the whole chart by an hour. The degrees themselves should agree to within an arc minute with any competent tool.
Is my birth data stored anywhere?
No. The chart is calculated by JavaScript running on your device, so the date, time and place are never transmitted. If you save a chart it is written to your browser's local storage on that device only.
What is the Part of Fortune doing in my chart?
It is an Arabic part, derived by arithmetic from the ascendant, the Sun and the Moon rather than observed in the sky. The formula differs for day and night births, and this calculator picks the correct one from the Sun's position and tells you which it used.
Can a birth chart predict events?
No, and nothing here attempts it. A natal chart describes disposition rather than outcome. Transits and returns describe timing of a kind, but they describe conditions and pressures, not events.