About TropicalAstro
How the positions, houses and timings on this site are calculated, and where the numbers come from.
What This Site Is
TropicalAstro is a set of free astrology calculators in the tropical zodiac: birth charts, relationship charts, predictive techniques, planetary hours, retrograde and combustion calendars, and astrocartography maps. None of them needs an account. This page sets out the data and the conventions behind them, so you can compare a result with another program and see why it matches or differs.
Where the Positions Come From
Every planet, point and house cusp is calculated from a high-precision ephemeris built on NASA JPL's DE431. Positions are geocentric and apparent: as seen from the center of the Earth, allowing for light travel time and aberration, and measured from the true equinox of the date.
The full-precision ephemeris on the server covers 1800 to 2399. Inside that range, planet positions are accurate to well under an arc-second. Outside it, the planets fall back to a built-in analytical model, which is less precise, and Chiron and the asteroids cannot be calculated at all.
Conventions
| Zodiac | Tropical everywhere: 0° Aries is the March equinox point. The zodiac sign pages also show sidereal (Lahiri) dates for comparison. |
|---|---|
| Lunar nodes | True node. |
| Black Moon Lilith | Mean lunar apogee. |
| Asteroids | Chiron, Ceres, Pallas, Juno and Vesta. |
| Day or night chart | Day when the Sun is above the horizon, measured from the true Ascendant–Descendant axis rather than whole-sign houses. |
| Part of Fortune | By day Ascendant + Moon − Sun; by night Ascendant + Sun − Moon. |
| Houses | Placidus by default on the birth chart, synastry and Davison charts and the celebrity charts; the birth chart and celebrity charts can switch to Whole Sign or Equal. The other calculators use Whole Sign houses. |
Aspects and Orbs
Each aspect has a base orb, scaled by the average of the two bodies' multipliers. A Sun–Moon square, for example, allows 7° × 1.25 = 8.75°, which the API rounds to 8.8°; a Saturn–Pluto square allows 7° × (0.85 + 0.7) ÷ 2, about 5.4°.
| Conjunction, opposition | 8° |
|---|---|
| Trine, square | 7° |
| Sextile | 5° |
| Quincunx | 3° |
| Semi-sextile, semi-square, sesquiquadrate | 2° |
| Sun, Moon | × 1.25 |
|---|---|
| Mercury, Venus, Mars | × 1 |
| Jupiter, Saturn | × 0.85 |
| Ceres, Pallas, Juno, Vesta | × 0.75 |
| Uranus, Neptune, Pluto | × 0.7 |
| Nodes, Chiron, Lilith | × 0.6 |
Birth Times and Time Zones
A birth time is read in the time zone of the birthplace, using the IANA time zone database with its historical daylight saving rules, and converted to Universal Time before anything is calculated. When a time falls in the hour that repeats as clocks go back, it is read as the second, standard-time occurrence; a time in the hour that clocks skip going forward is also read as standard time.
Planetary Hours
Sunrise and sunset are the moments the Sun's upper edge meets the horizon, allowing for refraction, computed by the site's ephemeris for the city's coordinates. The daylight between them is divided into twelve equal hours and the night until the next sunrise into twelve more, assigned in the Chaldean order starting from the weekday's ruler. The week and year views on each city page come from the same calculation, so they match the day's hours to the second.
Calendars
- Retrograde stations are found by stepping through each day and then narrowing in on the moment the planet's speed reaches zero. The shadow runs from the day the planet first reaches its direct-station degree to the day it passes its retrograde-station degree again.
- Combustion is a planet within 8.5° of the Sun, under the beams within 17°, and cazimi within 17′ of exact conjunction.
- A void-of-course Moon runs from the Moon's last major aspect (conjunction, sextile, square, trine or opposition) to the Sun or a planet through Pluto, until it enters the next sign.
- The calendar data covers 2025 to 2035.
Astrocartography
The map draws zodiacal lines: for every latitude from 66° south to 66° north, in half-degree steps, it finds where each planet's ecliptic longitude sits on the Ascendant, Descendant, Midheaven or IC. Zodiacal lines ignore a planet's distance above or below the ecliptic, so they can differ from lines drawn in mundo, most for the Moon and Pluto. Where two lines cross, the crossing's 1° latitude band is shown as a paran.
Checking the Numbers
Results are spot-checked against published charts. For Steve Jobs (7:15 PM, February 24, 1955, San Francisco, from his birth certificate), the site puts the Sun at 5°44′ Pisces and the Moon at 7°44′ Aries; for Prince William (9:03 PM, June 21, 1982, London), the Sun at 0°06′ Cancer and the Moon at 4°57′ Cancer. Positions are shown truncated to the arc-minute, the usual convention, so a program that rounds can show one minute more.
If a result looks wrong, write to hello@tropicalastro.com with the birth data you entered and what you expected. Corrections go into the code and the page they affect.