How your chart is actually calculated

Every chart goes through the same four stages. No tool invents its own numbers along the way — they all read from one shared calculation engine, so the natal chart, the synastry wheel and the retrograde calendar can never disagree with each other.

• 1 · Locate & time: turn your birth city into coordinates and your local clock time into precise astronomical time (UTC), with historical daylight-saving applied.
• 2 · Ephemeris: compute where every body actually was in the sky at that instant.
• 3 · Geometry: derive the angles, house cusps, aspects and dignities from those positions.
• 4 · Interpretation: wrap the facts in plain-language prose that is explicitly framed as a centuries-old tradition.

A chart is a snapshot of the sky at a specific instant, so getting from “3:40pm in Lyon” to a true Universal Time is the step most sites fumble silently. We resolve your birthplace to latitude and longitude, then apply the timezone that was actually in force at that date and place — including the daylight-saving rules that change by year and country. A one-hour DST error slides the whole chart and flips the Ascendant, so we treat it as a correctness problem, not a rounding one.

When you type a birthplace, the text you enter is sent (place name only — never your name, date or time) through our own proxy to the free OpenStreetMap Nominatim geocoder. If you'd rather not, you can enter coordinates directly; either way your chart is computed the same way.

Planet positions come from the sub-arcsecond astronomy (VSOP87/ELP via astronomy-engine) — a JavaScript implementation of the same VSOP87 planetary and ELP lunar theories that underpin the Swiss Ephemeris professionals use. Geocentric ecliptic longitudes, latitude, distance, retrograde stations, the lunar nodes and the lots are all computed for your exact moment, not looked up in a coarse table or approximated.

Accuracy is genuinely sub-arcsecond for the Sun, Moon and planets across the date range we support — which is far tighter than any interpretation actually depends on. The degrees are right; that was never the hard part.

We default to porphyry quadrant houses (whole-sign available on every chart) and offer the alternative on every chart. Porphyry is a quadrant system we can derive and verify directly from the ascendant and the midheaven without relying on iterative solving, and it behaves sensibly at high latitudes where some systems break down.

• Porphyry: the default — equal trisection of the quadrants between the Ascendant and MC.
• Whole-sign: offered on every chart, each sign counting as one house from the Ascendant's sign.

Charts are cast in the tropical, with a sidereal (lahiri) option clearly labelled. The tropical zodiac anchors 0° Aries to the March equinox and is what the vast majority of Western astrology — and most of our reference material — is written against.

If you work in the sidereal zodiac, you can switch to the Lahiri (ayanamsa) frame on the charts that offer it, clearly labelled, so you can compare the two without confusion about which you're looking at.

From the raw positions we derive one consistent feature set that every tool reuses: major aspects with their conventional orbs, the essential dignities (domicile, exaltation, detriment, fall and the triplicity/bound tables we cite), element and modality balance, and the angular houses. Because it's computed once and shared, a dignity that shows up in a report is the same dignity the natal chart found — no two pages of ours will quietly disagree.

This is the distinction Astrology Charts is built around. Planet positions, retrogrades, aspects and house cusps are measurable facts about the sky — you can check them against any reputable ephemeris. What a placement is said to mean is a different kind of thing entirely: layered, inconsistent, four-thousand-year-old symbolism that cultures have built on the sky.

We write the interpretation as tradition, in the language of reflection, and we never dress it as a guaranteed prediction. When a page tells you something "the chart shows", the astronomy is doing the showing; the meaning is the old story, attributed as such.

We don't ship a number we can't reproduce. The build runs a probe harness that asserts positions and derived features against the engine for fixed test dates, and the engine is verified end-to-end before the site is served. Any worked example you see in our articles is generated from a real computation, not a plausible-looking one typed by hand.

• Engine gate: the calculation engine is verified as part of every build; the site won't pass if the maths regresses.
• Probe assertions: spot-checks pin known planets, angles and lots to expected values so a refactor can't silently shift them.
• Content audit: readability and completeness checks run across the catalogue so explanations stay plain and never become filler.

Long-form pages carry a published and a last-updated date. When we find an error — a formula, a sourcing, a house-system claim — we fix it and bump the date rather than editing quietly. If you catch something wrong, tell us and we'll credit you in the note.

The whole calculation runs in your browser, so your birth date, time and place never have to reach us. That isn't a policy bolted on at the end — it's a consequence of how the engine is built. We set no cookies, run no analytics and store nothing on your device. The only thing that ever leaves is the birthplace text for the geocoder described above. Details are in the Privacy Policy.

Astrology has no controlled evidence base for making predictions, and we're not going to imply otherwise. The astronomy is precise; the interpretive layer is a tradition you may find genuinely useful for reflection and have no obligation to believe. Nothing here is medical, psychological, legal or financial advice, and a chart with an approximate birth time carries that uncertainty in every house-based reading. Where a technique is too unreliable to present honestly, we simply don't offer it.

Positions derive from the VSOP87 and ELP planetary/lunar theories (the same foundations as the Swiss Ephemeris), implemented by the open-source astronomy-engine; locations resolve through OpenStreetMap Nominatim; house and dignity conventions follow the classical and medieval texts we cite on each individual tool page. Every calculator lists its own sources so you can trace any claim back to where it came from.