Birth Chart
A birth chart (natal chart) is a map of the planets' positions at the moment of birth, tied to a specific place. It is the foundation of all astrology: everything else, from transits and solar returns to synastry, is built around it.
What you need to cast one
Three things: the date, exact time and place of birth.
The time determines the Ascendant and the houses, which make a full turn every day, shifting about one degree every four minutes. An error of two hours gives a different Ascendant and a different house grid.
The place is needed to convert the time to sidereal time and for the latitude, which determines how the houses are divided.
If the time is unknown, a noon chart is cast: the planets will be almost right (except the Moon, which moves 12–13 degrees a day), but the houses will not.
What a chart is made of
Planets in signs: how each function expresses itself. Ten planets.
Planets in houses: in which area of life. Twelve houses.
Aspects: the angles between planets. The major ones are the conjunction (0°), sextile (60°), square (90°), trine (120°) and opposition (180°). Harmonious aspects are considered easy; tense ones, conflict-prone but growth-inducing.
The angles — the Ascendant, MC, IC and Descendant. Astrologers also often look separately at the Moon sign and Black Moon Lilith — we have calculators for all three.
All told, that is about a hundred elements to be pulled together into a coherent description. It is at this stage that astrology stops being calculation and becomes interpretation.
Why We Don't Have a Calculator
Casting a chart requires ephemerides, tables of planetary positions. Free libraries for this exist, and technically the calculation is quite feasible.
The problem lies elsewhere: a chart without interpretation is useless, and automated interpretation inevitably boils down to a list of "Sun in Aries means…" statements, a set of paragraphs with no connection to each other. Text like that creates the appearance of individual analysis without being one.
We chose not to do this and instead to explain how a chart is structured, so you can make sense of a ready-made reading or one of the many free chart generators online.
An Honest Look at the Method
Natal astrology is the oldest and most developed part of the system: it is more than two thousand years old, and its logic is internally consistent.
It does not pass tests. The best-known test is Shawn Carlson's experiment published in Nature in 1985: astrologers could not match birth charts to psychological profiles any better than random guessing. Later studies have reached the same result.
That doesn't stop the chart from being a useful tool for self-reflection, but as a conversation tool, not as a source of information about the future.