How we calculate your chart

ClearSkyCharts calculates where the Sun, Moon and planets were at your birth using modern astronomical models, the tropical zodiac and Placidus houses by default. Everything runs in your browser, and we publish how we test it.

Planet positions

We use Astronomy Engine, an open-source library based on the VSOP87 planetary theory and established lunar models. Positions are apparent geocentric positions: corrected for the time light takes to reach Earth, aberration and nutation, which is the convention Western astrology uses.

The North Node shown is the mean lunar node, the most common choice in Western astrology.

How we test accuracy

Before every release we compare 300 random charts, born anywhere from 1850 to 2090 and from 65°S to 65°N, against Swiss Ephemeris, the reference library most astrology software uses. Every planet, the Ascendant, the Midheaven and all twelve house cusps must agree to within one arcminute, that is one sixtieth of a degree. In our latest run the largest planet difference was under one arcminute and the largest house cusp difference was a few arcseconds. A sign is 30 degrees wide, so these differences never change a reading.

What does change readings is the birth time. The Ascendant moves about one degree every four minutes, so a time rounded to the nearest quarter hour can move it by several degrees. When your Rising sign is close to the edge, we tell you.

Zodiac

We use the tropical zodiac, where 0° Aries is the point where the Sun crosses the equator at the March equinox. This is the standard in Western astrology. Vedic astrology uses a sidereal zodiac tied to the stars, which today sits about 24 degrees away, so your sign there is often the one before.

Houses

Our default is Placidus, the most widely used house system in modern Western astrology. You can switch to whole-sign houses, an older system where each house is one full sign. Placidus can't be calculated near the poles (above about 66.5° latitude), so for births there we use whole-sign houses and say so.

Time zones and daylight saving

We convert your local birth time to Universal Time using the IANA time zone database built into your browser, which includes historical daylight saving and wartime rules. If your time fell in a daylight-saving gap or happened twice when clocks went back, we tell you and let you choose, instead of guessing.

If you don't know your birth time

We calculate for noon and don't show a Rising sign or houses, because those depend on the exact time. If the Moon or Sun changed sign during your birthday, we show both possible signs.

Birth places

Our place search covers about 70,000 towns and cities with populations over 5,000, from GeoNames (licensed CC BY 4.0). The list loads into your browser, so no outside service sees what you type. If your town isn't listed, choose the nearest one; a few miles changes the Ascendant by only a fraction of a degree.

Accepted dates

Births from 1800 to 2100. Time zone records before about 1900 are less reliable in many places, so treat early charts with care.