# Natal Chart Lab

> Free astrology calculators with published methods. Planet positions are apparent geocentric longitudes on the tropical zodiac from Astronomy Engine, checked against NASA JPL Horizons (largest difference 19.5 arcseconds in a published 50-point comparison). Historical time zones come from the IANA database; birthplaces from GeoNames. Birth details are not stored.

## Calculators
- [Birth chart calculator](https://natalchartlab.com/birth-chart-calculator): planets, Ascendant, Midheaven, Placidus or Whole Sign houses, major aspects.
- [Rising sign calculator](https://natalchartlab.com/rising-sign-calculator): Sun, Moon and rising sign, with the minutes of birth-time error the rising sign tolerates.
- [Moon sign calculator](https://natalchartlab.com/moon-sign-calculator): works without a birth time; flags days the Moon changed signs.
- [Venus sign calculator](https://natalchartlab.com/venus-sign-calculator)
- [Midheaven calculator](https://natalchartlab.com/midheaven-calculator): MC and IC sign and degree, with the minutes of birth-time error the MC tolerates.
- [North Node calculator](https://natalchartlab.com/north-node-calculator): mean North and South Node signs, degrees and house.
- [Black Moon Lilith calculator](https://natalchartlab.com/lilith-calculator): mean Lilith (lunar apogee) sign, degree and house.
- [Saturn return calculator](https://natalchartlab.com/saturn-return-calculator): every exact date of the first, second and third Saturn returns, to the minute.
- [Synastry chart](https://natalchartlab.com/synastry-chart): aspects between two charts.
- [What zodiac sign am I?](https://natalchartlab.com/what-zodiac-sign-am-i): Sun sign by date, exact for cusp birthdays; table of sign dates.
- [Is Mercury retrograde?](https://natalchartlab.com/is-mercury-retrograde): live status and dates.
- [Moon sign today](https://natalchartlab.com/moon-sign-today): the Moon's current sign and degree, phase, next sign change and this week's void-of-course times (ET and UTC).
- [Retrograde planets now](https://natalchartlab.com/retrogrades): live status of Mercury through Pluto, next dates, and a lookup for any date 1950–2050.
- Retrograde calendars by year with station times and shadow periods, for Mercury, Venus, Mars, Jupiter and Saturn, e.g. [Mercury retrograde 2027](https://natalchartlab.com/mercury-retrograde/2027), [Venus retrograde 2028](https://natalchartlab.com/venus-retrograde/2028), [Mars retrograde 2027](https://natalchartlab.com/mars-retrograde/2027), [Saturn retrograde 2027](https://natalchartlab.com/saturn-retrograde/2027).
- [Zodiac compatibility](https://natalchartlab.com/zodiac-compatibility): all 78 sign pairings with element, modality and the angle between the signs; one page per sign and per pairing (e.g. https://natalchartlab.com/zodiac-compatibility/aries-and-leo).

## Data
- [Retrograde dates 1950–2050, CSV](https://natalchartlab.com/data/retrogrades.csv) and [JSON](https://natalchartlab.com/data/retrogrades.json): station times to the minute (UTC) and shadow periods for Mercury through Pluto. CC BY 4.0. Also published on [Kaggle](https://www.kaggle.com/datasets/xucheng2022/planetary-retrogrades-and-shadows-1950-2050) and [Hugging Face](https://huggingface.co/datasets/eatfreshapple/planetary-retrogrades-1950-2050).
- [Planet positions API](https://natalchartlab.com/api/positions?t=2026-01-01T00:00Z): JSON longitudes, signs, speeds and retrograde flags for the Sun, Moon, planets, mean node and mean Lilith at any UTC instant 1800–2200. No key needed; CORS enabled.
- [OpenAPI description](https://natalchartlab.com/openapi.json) of the API and data files.
- [JPL Horizons comparison data](https://natalchartlab.com/data/horizons-validation.json): 50 longitudes compared; largest difference 19.5 arcseconds.

## Method
- [Methodology and accuracy check](https://natalchartlab.com/methodology)
- [Checking a birth chart calculator against JPL Horizons](https://natalchartlab.com/articles/checking-birth-chart-calculator-jpl-horizons): query settings, all 50 comparison results, Python example and validation limits.

Astrology is a symbolic tradition; interpretations on this site are not scientific predictions.