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AI Prompts for a Birth Chart

Astrological calculation with LLM prompts for any AI platform for accurate natal chart interpretation.

A natal chart is a snapshot of the sky at the moment of someone's birth. Astrologers have used it for centuries to describe character, personality, and recurring life themes. Traditionally, an astrologer draws the natal chart as a circle with symbols representing the positions of the sun, moon, and planets at the moment of birth. Skilled astrologers must learn to calculate and read the chart as a first step to providing interpretations. Since the 1970s, computers have automated calculations and chart drawing but still, only skilled astrologers could read and explain a birth chart. Now, if AI chatbots are given accurate astrological data, anyone can obtain high-quality astrological interpretations without needing to learn to read a chart. Natal chart drawings remain a helpful visualization tool for skilled astrologers but they are a hindrance for AI; it's better to provide the output of astrological calculations directly to AI models as text.

A detailed birth chart contains more data than ordinary people can easily comprehend. For astrologers, a visual chart wheel is a way to organize and synthesize the firehose of data. For AI large language models (LLMs), the data from birth chart calculations is a tiny dataset. It is easy for an LLM to integrate and synthesize birth chart data into a coherent interpretation that's informed by the vast corpus of astrological texts processed during AI training. LLMs excel at synthesizing and interpreting complex data sets, making them well-suited for generating astrological interpretations.

When you give accurate astrological birth chart data to an LLM, there's a very good chance you'll experience the "aha!" moment of self-recognition and psychological insight that you'd get with a skilled professional astrologer. With an LLM, the interpretation contains a level of detail and unique relevance that can't be obtained from popular horoscopes or past computer-generated chart readings. But LLMs are useless, even harmfully misleading, if asked to provide astrological interpretations without being given data from astronomically accurate birth chart calculations.

To get astronomically accurate birth chart data to give to an LLM, you can use the software tools that professional astrologers have used in recent years: websites such as astro.com and astro-seek.com or software applications such as SolarFire, Time Passages, or Astrolog. Don't take screenshots of a chart wheel to upload to an LLM (parsing graphics is error-prone); rather, copy and paste the text output from the software tool or website. You may need some repeated copy-paste steps to get the full birth chart data into an AI chat window. If you want to simplify the workflow, use AstroPrompt, which was built to obtain accurate data from Astrolog and assemble it with suitable prompts for an LLM.

In AstroPrompt, the Personality view uses Astrolog to obtain planetary positions by sign, house, and degree, retrograde flags, the Ascendant and Midheaven, all twelve house cusps when birth time is known, and a full aspect grid with orbs. The LLM receives the accurate chart as context, a system prompt that sets the tone, and a full-report prompt enphasizing analysis and synthesis. The user never has to copy chart positions by hand, upload a chart wheel screenshot, or worry that the model fabricated the chart.

You can have a conversation with the AI about personality questions that actually matter to you: why you react the way you do, what your natural strengths are in work and relationships, what patterns tend to repeat in your life, and where your greatest potential lies. As counseling astrologers say, you do not have to "believe in astrology" for the conversation to be useful, because the process provides a level of self-reflection that is difficult to achieve without astrology as a tool.

For an introduction to the process, see calculation-first AI astrology workflow.

Before you get started

Personality is an option in the AstroPrompt navigation bar, and the fastest way to get a result is to use AstroPrompt. The tutorial below explains how to obtain the same result from any LLM yourself. To do it yourself you will need to install Astrolog, write the chart output to a file, and combine the Astrolog dataset with the prompts shown below. AstroPrompt makes the workflow easier: it runs Astrolog automatically, lets you pick from popular LLMs (such as OpenAI GPTs, Claude Opus, or Google Gemini), and streamlines the prompt assembly. For advanced users, AstroPrompt Pro lets you edit the prompts so you can customize tone, interpretation, or output structure. Try AstroPrompt now; it is FREE.

Birth chart

The Personality view computes a complete natal chart from birth date, time, and location with the planetary positions of Sun through Pluto plus the North Node, retrograde flags, the Ascendant and Midheaven, all twelve house cusps when birth time is known, and a full aspect grid with orbs.

The chart data is supplied to the LLM as context, so questions like "What are the core strengths of this person?" or a full psychological-astrology report is produced with accurate planetary positions instead of fabricated ones.

How to get an accurate LLM response

AI can't calculate planetary positions accurately or read an ephemeris reliably. To get an accurate and reliable interpretation for a birth chart, you must provide this astrological data:

  • The full set of planetary longitudes by sign, degree, and house.
  • Retrograde flags on each planet that is currently retrograde.
  • The Ascendant and Midheaven with degrees, when birth time is known.
  • A full aspect grid with orbs and aspect types.
  • House cusps for all twelve houses using the house system you prefer.
  • A flag indicating whether birth time is unknown so the model knows to skip house and angle interpretation.

Astrolog commands

The Personality view calls the Astrolog binary with a single command shaped by two toggles: whether birth time is known and whether minor objects (Chiron, asteroids, Lilith) are enabled. The canonical pattern with a known birth time and minor objects on:

astrolog -q {month} {day} {year} {utc_time} -z 0 -zl {lon} {lat} \
                 -zi {name} {location} -v -a0 -C
  • `-q` runs Astrolog in quick chart mode and accepts the date and time inline.
  • `-z 0` forces UTC, eliminating daylight-savings ambiguity. AstroPrompt converts the user's local birth time to UTC during atlas lookup.
  • `-zl {lon} {lat}` supplies the birth coordinates explicitly, in Astrolog's compact format (such as 74W00 or 42N21).
  • `-zi {name} {location}` labels the chart so the LLM sees a named subject in the data block.
  • `-v` produces the verbose planetary positions list (sign, degree, house, retrograde flag).
  • `-a0` outputs the full aspect grid with orbs.
  • `-C` is added when birth time is known so house cusps participate in aspect calculations. Without it, conjunctions to the Ascendant and Midheaven would be missing.
  • `-R` is added when minor objects are turned off, restricting output to the major planets.

When birth time is unknown, the command swaps the time for 12:00 (noon UTC) and drops `-C` because angles cannot be trusted without an accurate birth time.

Prompts

Astrological data is combined with a user's prompt to create the context for a question that begins an AI chat. The LLM receives everything (including its responses to previous questions) as a single context block, but it is easier for us if we assemble the context from three specific prompts: a system prompt, a data description prompt, and a user prompt that asks for a report. In AstroPrompt Pro, you can edit and save each of these prompts to customize tone, interpretation, or output structure. For this tutorial, combine each of these prompts with the astrological data and then paste the entire block into the AI chat.

System Prompt

The foundational instruction that sets the AI's overall tone, style, and approach, which applies to every message in the conversation. The default produces balanced, measured analysis. You can change it to anything: terse bullet-point summaries, a specific cultural or traditional framework, a particular language, or even a lengthy style guide that reproduces your own voice for client-facing work. This is the most powerful lever for fundamentally changing the character of the AI's responses.

The system prompt is the same across every AstroPrompt view. It instructs the model to drop horoscope-style language and stay analytical:

When generating astrological forecasts, do not use rhetorical or
        persuasive devices typical of popular horoscope writing. Avoid inserting
        emotional appeals or literary devices. Avoid the following:

        • Imperatives and exhortative tone (e.g., "Embrace change," "Let go of
          the past," "Surround yourself with warmth")
        • Evocative metaphors and symbolic language (e.g., "personal
          renaissance," "magnetic energy," "mirror of the soul")
        • Overly enthusiastic, poetic, or cute phrasing
        • Vague affirmations or inspirational advice not grounded in planetary
          positions

        Instead, use a neutral, informative tone. Focus on:

        • Relevant planetary influences and their timing
        • Which life areas are affected and how
        • Expected themes based on astrological interpretation
        • Possibility framing using modal verbs (e.g., may, might, could)
          without interpretive flourish
        • Objective language suitable for readers seeking analysis rather than
          affirmation

        Structure any planner forecast by timeline or topic (e.g., career,
        relationships, health).

AstroPrompt adds a terminology directive (advanced or beginner) based on its settings.

Data Description

This prompt tells the AI what the raw astrological data actually represents. This is how the AI understands the underlying dataset. Advanced users can use this to direct the AI's attention toward specific technical elements they care about most.

After the chart data block, AstroPrompt appends a one-sentence description so the model knows exactly what it is looking at:

This data contains a natal chart showing planetary positions by sign,
        house, and degree; aspects between natal planets with orbs; house cusps;
        and planetary dignities. The chart represents the foundational
        astrological profile for this person based on their birth data.

This sentence explains the dataset's structure (positions, aspects with orbs, cusps, dignities) and its purpose (foundational profile).

Full Report Prompt

AstroPrompt provides detailed instructions when a user wants a full report. This is typically the longest prompt and has the greatest single impact on report quality and structure. The default walks the AI through a specific analytical sequence. You can replace this entirely as you wish, for example, to focus the report on career and finance, restructure it around specific techniques, or direct the LLM to use the point of view of a favored lineage or tradition.

When the user clicks "full report" rather than asking a free-form question, the LLM receives this prompt:

As a psychological astrologer, provide a comprehensive natal chart
        interpretation. Analyze the following in order: (1) The Sun, Moon, and
        Ascendant (the "Big Three") — sign, house, and major aspects — as the
        foundation of personality. (2) Mercury, Venus, and Mars — communication
        style, relationship approach, and drive/ambition. (3) Jupiter and Saturn
        — growth opportunities and life lessons. (4) Outer planets (Uranus,
        Neptune, Pluto) — generational themes and personal transformation points
        based on house placement and aspects to inner planets. (5) Notable
        aspect patterns (T-squares, grand trines, stelliums, yods) and their
        psychological implications. (6) House emphasis — which life areas are
        most activated. Conclude with a synthesis of the chart's dominant
        themes, potential strengths, and areas for growth. Use possibility
        language ("may," "might," "could") throughout.

The framing constrains the model for a structured analysis. The "possibility language" rule keeps the response suggestive rather than deterministic. The closing synthesis requirement forces the model to integrate rather than list.

Unique features of the Personality prompt

  • The chart is fully calculated by Astrolog before the LLM sees it, so the model never attempts to compute planetary positions.
  • The system prompt actively bans "popular horoscope" language, which tends to arise in natal-chart LLM output, given a preponderance of sun-sign astrology in the LLM training data
  • The full report prompt enforces a Big-Three-first ordering, which is a foundation for modern astrology.
  • House cusps are always included via `-C` when birth time is known so the Ascendant and Midheaven get full aspect treatment.
  • The "birth time unknown" path uses noon UTC and skips angles, and the data description tells the model to do the same.
  • Minor objects (Chiron, asteroids, Lilith) are opt-in via Settings, reducing the complexity in the default chart. LLMs can handle the complexity but many users prefer a simpler view.

What's Next

This was written in May 2026, describing the data and prompts used in AstroPrompt version 1.0. Astrologers are just beginning to explore the capabilities of AI as a tool for astrological insight. If you've got suggestions for a better dataset or improved prompts, I'd love to hear from you. Email me at hello@timecasters.com.