METHODOLOGY
How Astraly calculates your chart
Astraly calculates your birth chart with deterministic astronomy, not AI. Your local birth time is converted to Universal Time using the historical time-zone rules for your birthplace; planet positions come from the VSOP87 theory and Jean Meeus's algorithms, and houses from the Placidus system. AI only interprets the finished chart — it never invents a position.
- Ephemeris
- VSOP87 B + Meeus
- Houses
- Placidus
- Time zones
- IANA tz database
- AI in the math
- None
Step 1: Birth data and time zone
You enter your birth date, time and city. The city is matched to coordinates and to its time-zone name from the IANA database — for example America/New_York — through GeoNames-based geocoding.
Your local clock time is then converted to Universal Time (UTC) with the rules that applied on that exact date, including daylight saving time. A birth in New York at 9:00 on 1 July 1985 becomes 13:00 UTC, because summer time (UTC−4) was in force.
Under the hood
- Historical offsets and DST come from the IANA time-zone database built into the runtime, not from today's offset.
- If no time zone can be found for a place, the offset is estimated from longitude (15° per hour).
- For some regions, records before 1970 are less complete. If your birth certificate states a different local standard, use that time.
Step 2: Planetary positions
The UTC moment becomes a Julian Day, and every body is computed from published astronomical theories via the open-source astronomia library, a JavaScript implementation of Jean Meeus's Astronomical Algorithms.
Positions are geocentric and tropical (measured from the vernal equinox of the date), corrected for nutation. The same date, time and place always give the same chart.
Under the hood
- Sun: VSOP87 series B for the Earth, apparent longitude.
- Mercury to Neptune: VSOP87 B heliocentric positions, converted to geocentric.
- Moon: Meeus chapter 47 (a truncated ELP-2000/82 lunar theory).
- Pluto: Meeus chapter 37 series (valid 1885–2099), precessed from J2000 to the date.
- Lunar nodes and Black Moon Lilith: mean positions (Meeus polynomials).
- Chiron: a Keplerian orbit from Minor Planet Center elements.
- Retrograde: the planet's geocentric longitude 12 hours before and after the moment.
Step 3: Houses and angles
From the UTC moment and your longitude, Astraly computes local apparent sidereal time — the part of the sky crossing your meridian. With the true obliquity of the ecliptic and your latitude, that gives the Ascendant and Midheaven (MC).
House cusps use the Placidus system, the default in most Western astrology software. Above 66° latitude, north or south, Placidus is undefined for part of the zodiac, so Astraly switches to Equal houses: 30° steps from the Ascendant.
Under the hood
- Placidus intermediate cusps are solved iteratively to about 0.2 arcseconds.
- Each planet's house is the cusp-to-cusp arc its longitude falls in.
- The angles move about 1° for every 4 minutes of clock time, so they depend on an accurate birth time.
Step 4: Aspects
An aspect is a meaningful angle between two points. Astraly checks every pair of 14 points — Sun to Pluto, both lunar nodes, Lilith and Chiron — and keeps the single tightest aspect within orb for each pair.
The same table is used for natal charts, transits and synastry. Long-range forecasts for Jupiter to Pluto use tighter orbs: 2° for conjunctions, oppositions and squares, 1.5° for trines and 1° for sextiles.
| Aspect | Angle | Max orb |
|---|---|---|
| Conjunction | 0° | 8° |
| Opposition | 180° | 8° |
| Trine | 120° | 8° |
| Square | 90° | 7° |
| Sextile | 60° | 6° |
| Quincunx | 150° | 3° |
Step 5: What the AI does — and doesn't
The AI astrologer receives your finished chart as text: Ascendant and MC, every point with sign, degree, house and retrograde status, the 12 house cusps and the 14 tightest aspects. It reads those facts and explains them in plain language.
For any other date it must call a calculation tool — transits, a transit timeline, moon phases, synastry, long-term cycles or astrocartography — and is instructed never to recall or estimate positions from memory. Today's sky is calculated with the same engine.
Under the hood
- Does: interpret, connect themes, answer your questions, suggest what to reflect on.
- Doesn't: calculate or change positions, houses or aspects.
- The wording of a reading can vary between conversations; the chart underneath does not.
No birth time?
You can still build a chart. Without a time, Astraly calculates it for local noon on your birth date in your birthplace's time zone, which keeps the error from a missing time to at most 12 hours either way.
Over 12 hours the Sun moves about half a degree and the Moon up to about 7°, so your Moon can occasionally fall in the neighbouring sign. The Ascendant, MC and houses cannot be known without a time.
Under the hood
- The app marks charts without a time as approximate.
- The astrocartography map is not drawn until you add an exact time.
- Long-range forecasts leave out house entries and the angles.
- Add your time later and the chart is recalculated.
How accurate is it?
Accuracy of the calculated longitudes for births between 1900 and today. One arcminute (1′) is 1/60 of a degree — far smaller than any difference an interpretation relies on.
| Point | Method | Accuracy |
|---|---|---|
| Sun | VSOP87 B (Earth), apparent | < 0.1′ |
| Mercury – Neptune | VSOP87 B | < 1′ |
| Moon | Meeus ch. 47 (ELP-2000/82) | ≈ 1′ |
| Pluto | Meeus ch. 37 (1885–2099) | < 1′ |
| Nodes, Lilith | Mean positions (Meeus) | Mean point by definition |
| Chiron | Keplerian orbit (MPC elements) | ≈ 1–2° |
| Ascendant, MC, houses | Sidereal time + Placidus | Limited by your birth time: 4 min ≈ 1° |
Figures are the largest differences we measured between Astraly's output and a full apparent-place reduction of the same theories, plus the theories' own error. Chiron uses a simplified orbit and is the least precise point.
Transits, cycles, maps and compatibility
Everything built on top of your chart uses the same engine and the same rules — calculated first, interpreted second.
Daily transits
Today's Sun to Pluto are compared with your ten planets, Ascendant and MC, day by day — usually 90 days ahead. The Moon is left out because it forms a new aspect every couple of hours.
Long-range cycles
Jupiter to Pluto are sampled every two days for up to five years, giving house entries and exact dates of each pass, including retrograde loops.
Astrocartography
For each of ten bodies, Astraly draws where on Earth it was rising, setting, culminating or anti-culminating at your birth. It needs an exact birth time.
Compatibility
Aspects between two charts are scored by planet pair, aspect type and tightness into 0–100 per life area, with 50 as neutral. It's a structured summary, not a statistically validated predictor.
What astrology is — and isn't — for
The calculation is astronomy; the meaning is astrology. Astraly treats your chart as a language for reflection and timing, not fate, and it is not a substitute for medical, psychological, legal or financial advice.
Your birth data is used to calculate and keep your chart, and the free calculator needs no account. Details are in our privacy policy.
Frequently asked questions
Does Astraly use AI to calculate my chart?
No. Astraly's chart is calculated with deterministic astronomical formulas — VSOP87 and Meeus — so the same birth data always produces the same positions. AI is used only afterwards, to explain the finished chart in plain language and answer questions. It is instructed never to estimate positions from memory and to call the calculator instead.
Which ephemeris does Astraly use?
Astraly uses the VSOP87 planetary theory (series B) for the Sun and Mercury through Neptune, Meeus's lunar theory (chapter 47) for the Moon, and Meeus's chapter 37 series for Pluto, through the open-source astronomia library. Planet positions are accurate to under one arcminute; the Moon to about one arcminute.
Which house system does Astraly use?
Astraly uses Placidus houses, the default in most Western astrology software. Above 66° latitude, north or south, Placidus cannot be calculated for part of the zodiac, so Astraly automatically switches to Equal houses, placing a cusp every 30° from the Ascendant. Ascendant and Midheaven are calculated the same way in both cases.
How does Astraly handle time zones and daylight saving time?
Astraly looks up your birthplace's IANA time zone, such as Europe/Rome, and converts your local birth time to UTC with the offset and daylight-saving rules in force on that date — not today's. If a place has no known time zone, the offset is estimated from its longitude.
How accurate is Astraly's natal chart?
Accuracy is about one arcminute or better for the Sun, Moon and planets between 1900 and today, and about 1–2° for Chiron. The Ascendant, MC and houses are only as accurate as your birth time: every four minutes of error moves the Midheaven by roughly one degree.
What if I don't know my birth time?
Astraly calculates the chart for local noon on your birth date. Planet signs are almost always right — the Sun moves only about half a degree in 12 hours — but the Moon can shift up to about 7°, and the Ascendant, MC and houses stay unknown. The app marks such charts as approximate.
Will my chart change if I calculate it again?
No. The calculation is deterministic: the same date, time and place always give the same positions, houses and aspects, on the web and in the app. What can differ is the wording of an AI reading, because each conversation is written fresh from the same unchanged chart.
See it on your own chart
Calculate your birth chart free, without an account — or create one to save it and ask the AI astrologer what it means.