The data source: Swiss Ephemeris
Every position on this site comes from the Swiss Ephemeris, a planetary calculation library maintained by Astrodienst in Zurich and derived from the NASA Jet Propulsion Laboratory DE431 ephemeris. It is the same engine that sits underneath most professional astrology software, and it is accurate to a fraction of an arcsecond for the modern era.
That level of precision matters more than it sounds. A tenth of a degree is invisible in most readings, but it decides whether an aspect falls inside orb, whether a planet sits in the twelfth house or the first, and whether a transit is exact today or next week. We do not round positions or use simplified orbital approximations to save computation.
From your birth details to a chart
When you submit a form, four things happen. First, your birth place is resolved to latitude and longitude. Second, your local birth time is converted to Universal Time using the correct historical timezone offset for that date and location, which includes daylight saving rules that have changed many times over the last century. Third, the Julian day number for that exact moment is computed. Fourth, the ephemeris returns true geocentric positions for the Sun, Moon, planets, lunar nodes, and Lilith, along with the ascendant, midheaven, and house cusps for your coordinates.
Aspects are then measured between every pair of bodies and filtered by orb. We use conventional orbs: roughly 8 degrees for conjunctions and oppositions involving the Sun or Moon, tightening for minor bodies and smaller aspects. Retrograde status comes from the sign of each body daily motion, not from a lookup table.
House systems and the tropical zodiac
Western charts on this site use the tropical zodiac, which is anchored to the equinoxes rather than the constellations, and the Placidus house system, which divides the space between the ascendant and midheaven by time rather than by equal degrees. Placidus is the default in most modern Western practice, though it behaves poorly at extreme latitudes where houses become very distorted. If you were born above roughly 66 degrees north or south, treat the house cusps with caution.
Vedic charts use the sidereal zodiac with the Lahiri ayanamsa and whole sign houses, which is standard Jyotish practice. Because the two zodiacs currently differ by about 24 degrees, most people find their Vedic Sun sits one sign earlier than their Western Sun. Neither system is wrong. They measure different things.
What our derived tools actually compute
Secondary progressions use the day for a year key, so your chart for thirty days after birth is read as your thirtieth year. Solar returns are cast for the precise moment the Sun returns to its exact natal degree, which is rarely your calendar birthday and depends on the location you set. Composite charts take the midpoints of each planetary pair between two charts. Astrocartography projects the angles onto the globe to draw the lines where each planet would have been rising, setting, or culminating.
Our astrology calendar is not typed in by hand. Eclipse times, retrograde stations, sign ingresses, and lunation moments are scanned directly out of the ephemeris and refined by bisection until they converge, then published with the resulting UTC timestamps. When a date on this site says a planet stations retrograde at a particular hour, that number was computed, not copied.
Where the limits are
Accurate astronomy does not make astrology a science, and we will not pretend otherwise. What we guarantee is that the positions and geometry are correct. The interpretations built on top of them come from astrological tradition, and reasonable practitioners disagree about plenty of it, including which house system to use, how wide an orb should be, and whether asteroids belong in a chart at all. Where a question is genuinely contested, we say so rather than inventing certainty.
Two practical limits are worth naming. Birth times recorded on certificates are often rounded to the nearest five or fifteen minutes, so treat a chart built on a rounded time as approximate at the cusps. And historical timezone data before the 1970s is imperfect for some regions, particularly where local mean time was still in use.
Privacy of your birth data
Your birth details are used to compute a chart and are not stored. This site has no accounts, no login, and no user database. Nothing you enter is retained after the calculation is returned to your browser.