What is in a birth chart
A birth chart, or natal chart, is a map of the sky at the moment you were born, seen from where you were born. It has three layers. The planets are the actors: the Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto, plus Chiron, the lunar nodes and Black Moon Lilith. The signs describe how each planet acts, from fiery Aries to watery Pisces. The houses say where in life it acts: the 10th house is career, the 7th partners, the 4th home.
Around the outside of the wheel runs the zodiac. The horizontal axis is your horizon at birth: the Ascendant, or rising sign, on the left and the Descendant on the right. The vertical axis runs from the Midheaven at the top to the IC at the bottom. Planets above the horizon were in the sky when you were born; those below it were under the Earth.
Aspects
Aspects are angles between planets. A conjunction (0°) blends two planets. A trine (120°) and a sextile (60°) let them work together easily. A square (90°) and an opposition (180°) create tension that pushes for growth. The calculator lists the closest aspects first, with their orb: how many degrees each one is from exact. Standard orbs are used, a little wider for the Sun and Moon and narrower for points like the nodes.
| Aspect | Angle | Usual reading |
|---|---|---|
| Conjunction | 0° | The two planets act as one |
| Sextile | 60° | Opportunity and easy cooperation |
| Square | 90° | Friction that demands action |
| Trine | 120° | Natural talent and flow |
| Opposition | 180° | Balance, projection and awareness |
Why the birth time matters
The planets move slowly, so most of them are in the same sign all day. The Moon is the exception: it moves about 13° a day and changes sign every two to three days. The angles and houses move fastest of all, sweeping the whole zodiac every 24 hours: the rising sign changes about every two hours. If you don't know your time, tick I don't know the birth time. The chart then uses noon, leaves out the houses and angles, and tells you if the Moon changed sign that day. The rising sign calculator can show every possible rising sign for your birth date.
How the chart is calculated
Your local birth time is converted to Universal Time using your birthplace's historical time-zone rules, including daylight saving in that year. Planet positions come from Astronomy Engine, an open-source library tested against NASA JPL Horizons, and Chiron from a JPL table. Houses use Placidus by default, calculated from your exact sidereal time and latitude. The accuracy and sources page lists every method and test.
From the chart to the map
Every planet in your chart also has lines on Earth: places where it was rising, setting or overhead at your birth. Open your astrocartography chart to see where they fall, or relocate your chart to see how its houses change in a city you know.