What a natal chart calculation contains
A natal chart preserves the apparent geocentric positions of the Sun, Moon, planets, nodes, angles, and house cusps for a specific instant and location. The calculation is astronomical geometry; the interpretation is a separate symbolic layer.
How to read the result in the right order
Begin with the Sun, Moon, and Ascendant. Then inspect their rulers, angular planets, repeated elements or modalities, house concentration, and the tightest aspects. A chart becomes coherent through repetition and relationship, not by collecting hundreds of isolated placement descriptions.
Unknown birth time
Planetary signs remain useful for much of the day, although the Moon may cross a sign boundary. Houses, Ascendant, Midheaven, Vertex, and angle-dependent techniques require time. Astrarium labels this uncertainty instead of manufacturing precision. Read the full guide to birth time unknown.
Calculation method
The browser uses a pinned, MIT-licensed TypeScript ephemeris engine and an offline coordinate-to-time-zone resolver. Longitudes west of Greenwich are negative. Placidus and Koch automatically fall back to Whole Sign above latitudes where their geometry is undefined, and the interface reports the fallback.
Frequently asked questions
Is the complete birth chart really free?
Yes. Planet positions, houses, angles, aspects, dignities, multiple house systems, tropical or selected sidereal zodiacs, printing, and calculation JSON are available without a subscription.
Why does birth time matter?
Birth time determines the local angles and house cusps. A small time difference can move the Ascendant materially. When time is unknown, Astrarium uses local noon for planetary positions and withholds confident angle and house interpretation.
What happens during a daylight-saving clock change?
The calculator resolves historical time-zone rules. If a local time occurred twice or did not exist, the result is flagged and, for an overlap, you can compare both occurrences.
Which house system should I use?
Placidus is the default because many modern Western astrologers expect it. Whole Sign is a clear traditional alternative. The calculator includes twelve systems so the choice is visible rather than hidden.