How Astra calculates a birth chart
Astra product guide · Updated
Astra calculates tropical, geocentric planetary longitudes with Astronomy Engine 2.1.19. It resolves a local birth date and time to a UTC instant, calculates the selected house system, and derives aspects from angular separations. Astronomical positions and symbolic interpretations are separate: agreement between calculators does not validate personality claims or predictions.
Inputs and time resolution
The calculation uses a calendar date, local clock time, latitude, longitude and an IANA time zone such as Europe/Madrid. City search uses a bundled GeoNames directory; manual coordinates and the selected time zone are not independently verified. Astra uses Temporal to resolve daylight-saving changes. A repeated local time requires choosing the earlier or later occurrence; a nonexistent local time is rejected.
Positions and supported points
For the Sun, Moon and planets from Mercury through Pluto, Astra converts Astronomy Engine's aberration-corrected geocentric vectors to ecliptic longitude. The tropical zodiac divides longitude into twelve equal signs. The North Node is a separate mean-node calculation, not the true node. Chiron, asteroids, declinations and minor aspects are not included.
Retrograde motion is determined from the direction of longitude change around the calculated instant. Astra does not display a separate stationary state. Element and modality totals count the ten planetary bodies and exclude the North Node.
Houses and aspects
Astra implements whole sign, equal and Placidus houses. Placidus cusps are calculated by Astra's own numerical solver, not supplied by Astronomy Engine. Houses and angles require a known time. Compare the calculated systems and polar limit.
The natal aspect list contains conjunctions, sextiles, squares, trines and oppositions. One selected orb applies equally to all of these aspects; there are no planet-specific allowances. The displayed list is ordered from the tightest orb. Differences from another calculator may reflect these settings rather than different astronomical positions.
Reproducible reference
Reproduce this example: 1990-11-02, 04:30 (Europe/Madrid), Barcelona, 41.38879°, 2.15899°. The public Sofia sample is demonstration data, not a customer record. Its calculated instant is . The tables round positions to two decimal places; calculations use unrounded values.
Engine recorded in the calculated chart: Astronomy Engine 2.1.19 · tropical, geocentric · mean North Node.
| Quantity | Value |
|---|---|
| Sun longitude | 219.452069° |
| Moon longitude | 28.737215° |
| Ascendant longitude | 184.582877° |
| House system | Placidus |
| Natal aspect orb | 6° |
Extra decimal places make comparisons reproducible; they are not an accuracy guarantee. Astra's automated numerical checks compare selected cases with independent Swiss Ephemeris fixtures. These checks cover a limited set of dates and locations, not every possible input. Stored charts retain the engine label and settings with which they were calculated.
Sources and interpretation limits
Astronomy Engine supplies the astronomical position library. GeoNames supplies the city data. Astra adds its house calculations, aspect rules and symbolic text. Neither source establishes that astrology predicts outcomes.
Continue with your chart
Explore the calculated sample without an account, or sign up and confirm your email to save your own chart. Treat interpretations as reflection, with the limits of your inputs in mind.