How it works, and how accurate it is

Your birth details become one moment and one place. The positions of the Sun, Moon and planets for that moment are calculated with the library professional astrologers use, and checked against NASA. Eighteen chapters are then written from those positions and nothing else.

1. Cast. Your date, time and place become the exact moment, with the time zone and daylight saving that applied where you were born, on that day.
2. Check. Every position is held to within one arcminute of NASA’s own for the same moment, and the table below shows how far off each one is.
3. Write. Eighteen chapters are written in a few rounds from the finished chart. Every sentence that names a sign, house, degree or aspect is checked against it before you read it.

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The detail, for readers who know charts

Under Your Stars casts a natal chart from DE440, the planetary ephemeris NASA’s Jet Propulsion Laboratory publishes. It writes eighteen chapters from it in a few rounds, from the chart and notes worked out from it. Every position is calculated, not looked up, and every sentence that plainly names a sign, house, degree or aspect is checked against the chart before delivery.

Where the numbers come from

Every position is calculated from astronomy. None is estimated, typed in, or written by the language model.

Every number on the site and in the analysis comes from one calculation module built on DE440, the planetary ephemeris NASA’s Jet Propulsion Laboratory publishes from decades of radar, spacecraft and laser measurements. Positions are apparent geocentric longitudes on the tropical zodiac, referred to the true equinox of date: what every mainstream astrology program reports, across the supported years, 1800 to 2100. Chiron comes from JPL Horizons; the houses, the nodes and Lilith are the published geometry worked from those positions.

The tests behind the table above hold every body in those three charts to within one arcminute, and check the Ascendant against an independent sidereal-time formula.

Further tests cover an unknown birth time, a southern-hemisphere chart, the spring-forward hour that does not exist, the fall-back hour that happens twice, and pre-1970 time zone rules. We never loosen a test to make it pass: if one fails, the calculation is fixed.

Time and place

Your birth time becomes one exact moment, using the clock rules of where you were born, on that day.

The birth place is matched to a built-in database of about 34,000 towns and cities from GeoNames, each with its IANA time zone, and nothing is sent to a geocoding service. The local birth time is converted to Universal Time using the rules in force that day, including historical daylight-saving changes: a birth in 1962 gets 1962’s offsets.

If the birth time is unknown, the chart is cast for noon and the houses and the Ascendant are left out rather than guessed. The page says whether the Sun or Moon changed sign during that day, and the analysis skips the chapters on the Ascendant and the houses. What time and place change, in plain words.

House systems

Houses divide the sky into twelve areas of life. There is more than one way to cut it, and Placidus is the one most people see.

Placidus is the default. You can switch on the chart page and the chart is cast again.

Placidus

The most widely used today, and the default in most astrology software. Houses grow and shrink with your birthplace and time.

Whole Sign

The oldest way: your rising sign is your whole first house, and each sign after it is the next house. Favoured in traditional astrology.

Koch

Close to Placidus, drawn a slightly different way. Popular in German-speaking astrology.

Equal

Twelve houses of exactly 30°, counted from your rising degree.

Placidus and Koch are undefined inside the polar circles. Above about 66° of latitude the chart uses Porphyry houses instead and says so. What each house is about.

Aspects, patterns and balance

Aspects are the angles between planets. Patterns are the shapes several aspects draw together. Balance counts how many bodies stand in each element.

Aspects are found between the Sun, Moon, planets, the North Node, the Ascendant and the Midheaven. An orb is the room allowed either side of an exact angle: 8° for conjunctions and oppositions, 7° for trines and squares, 5° for sextiles and 3° for quincunxes, plus 2° when the Sun or Moon is involved.

Patterns are detected from those aspects: stelliums, grand trines, T-squares, grand crosses and yods. T-squares that share an apex are reported as one pattern.

Balance and dignities. Element and modality balance count the ten classical bodies. Traditional rulerships and exaltations give the dignities (domicile, exaltation, detriment and fall) and the chart ruler. Every aspect and pattern, explained one page at a time.

How the written analysis is grounded

The writing only ever sees your finished chart, and every fact in it is checked before you read it.

What the language model receives. It never sees a request to work out a chart. For each chapter it receives a plain-text summary of the finished chart (every placement with degree and house, every aspect with its orb, the patterns, the balance and the dignities), notes worked out from the chart and a brief for that chapter, and it writes that chapter. The summary also states what it would otherwise have to work out: how many aspects each body makes, the tightest aspects, whether an aspect is applying or separating, and the phase of the Moon. The notes do the same for the reading itself: which chapter reads which aspect in full, so that each is read once; which planet hosts which, and who rules each house; and which placements the chapter reads together.

How it is written. The eighteen chapters are written in a few rounds rather than all at once. A chapter in a later round is shown the first and last sentence of each chapter already written, so that it opens and closes differently from each of them, and the closing chapter is written last, with every other chapter in view. The language model is asked to say what a placement or an aspect means before giving the figure that proves it; every degree and orb stands in the tables at the front of your report, so you can check the reading against them.

What is checked. Every chapter is checked against the chart as it arrives: where a sentence plainly names a sign, house, degree, aspect or count, it must be the chart’s own. It is also read against the voice it is written in and against the chapters before it, and a hedge such as “perhaps”, a closing question, or a scene an earlier chapter already used sends it back. A chapter that fails is rewritten, at most twice. A sentence that still plainly contradicts the chart is taken out, and anything still flagged is kept for a person to read.

The other analyses

Weekly and monthly analyses follow the moving sky. Compatibility compares two charts. The year ahead reads the chart of your birthday, the solar return.

Weekly and monthly analyses read the transits, the current sky against the natal chart. The period is sampled at midnight UTC each day, and every transiting body is measured against the natal Sun, Moon, planets, Ascendant and Midheaven on tight orbs: 2° for conjunctions and oppositions, 1.5° for squares and trines, 1° for sextiles. Every contact, ingress, station and lunation is refined to its exact moment, so the dates are right; the Moon is in the weekly analysis and left to the lunations in the monthly one.

Compatibility compares two natal charts three ways. Interaspects are every aspect between a body in one chart and a body in the other, on orbs two degrees tighter than the natal chart, since synastry is read on closer contacts. House overlays place each person’s bodies in the other’s houses, in whichever direction a birth time allows.

The year ahead reads the solar return, the chart for the moment the Sun returns to its natal longitude in a chosen year. That moment is found by bisection on the Sun’s position to well under a second, and the chart is cast for the place you choose, to the minute. It is read against the natal chart on orbs of 3° for conjunctions and oppositions, 2.5° for squares and trines and 2° for sextiles, a degree more with the Sun or Moon.

Sources and credits

• DE440 (NASA Jet Propulsion Laboratory): the positions of the Sun, Moon and planets, from which the nodes, houses and angles are calculated; Chiron from JPL Horizons.
• NASA JPL Horizons: the check every position is held to.
• GeoNames (CC BY 4.0): the towns and cities, with their time zones.
• Yale Bright Star Catalogue: every star to the fifth magnitude in the skies drawn on the home page and each chart page.
• NASA/GSFC/Arizona State University: the Lunar Reconnaissance Orbiter’s maps, from which the Moon on the analysis’s last page is drawn, lit at the phase she was in at the minute of birth.

Where to go next

• Cast your chart: free, and it takes a minute.
• Read the sample analysis: 53 pages, from a real chart.
• New to astrology? Your birth chart in five minutes.
• Learn the signs, planets, houses and aspects, one short page at a time.
• Questions and answers, from what a chart is to how long an analysis stays on your account.