✦Guides · 6 min
Anyone looking at a chart is entitled to ask: is what I see accurate? This article describes honestly how we calculate, what we measured, and where the real uncertainties lie. Its aim is to make what we claim checkable, because trust rests on verifiability.
What do we calculate?
From the date, time and place of birth we calculate the ecliptic longitude of the Sun, Moon and planets, the lunar nodes, the Ascendant, the MC (Medium Coeli), the house cusps and the aspects between planets. Besides the tropical zodiac a sidereal view can be chosen, and the ayanamsa and house system used are always shown next to the chart.
The ephemeris: how do we know where the planets are?
An ephemeris is a table of the positions of celestial bodies as a function of time. Today's high-precision ephemerides (such as NASA's JPL DE series, on which the Swiss Ephemeris is also built) rest on observations and the numerical integration of the equations of motion. The Swiss Ephemeris is the de facto yardstick of astrological software, so we chose it as our reference: we compared our own calculations against it.
What did we measure?
We compared our own calculations with the values of the Swiss Ephemeris over many dates and places:
- Planets: the deviation in ecliptic longitude is less than 20 arcseconds (about 0.0056 degrees).
- Houses: the deviation in the house cusps is 0: we obtained identical results.
What this means: 20 arcseconds is the angle the Moon covers in about half a minute. The orb of an astrological aspect is several degrees, so this accuracy is in practice indistinguishable from the reference. We do not treat the measured results as absolute truth: the Swiss Ephemeris itself rests on a model, and differences also occur between ephemeris versions.
What does "true equinox" mean?
The 0th degree of the tropical zodiac is the vernal point, the Sun's position at the March equinox. This point has two versions:
- Mean equinox: takes only the effect of precession (the slow shift of the axis) into account.
- True equinox: takes into account, besides precession, nutation, the small periodic "nodding" of the axis (amplitude about 17 arcseconds, period about 18.6 years).
We measure from the true equinox of the given moment, because that corresponds to what can actually be observed in the sky. We calculate the apparent positions of the planets, which also take into account light-travel time and aberration (an apparent shift caused by the Earth's motion, at most about 20 arcseconds).
Time and place: the real uncertainty
The accuracy of the calculation itself has become a largely theoretical concern: the greatest uncertainty lies in the input data.
- Converting local time: we arrive at universal time (UTC) through historical time-zone and daylight-saving rules (for example the IANA database). For old dates, or before the introduction of standard zone time (in Hungary in the 1890s), local solar time may differ and the recorded time may be uncertain.
- The accuracy of the birth time: the Ascendant advances one degree about every 4 minutes, the MC similarly. A ten-minute error means a 2 to 3 degree deviation, and an hour about 15 degrees, which can change sign and house. The Moon moves half a degree per hour, the Sun about one degree per day, and the position of slower planets stays practically unchanged for days.
- The place: one degree of longitude corresponds to 4 minutes of local time, so the accuracy of the coordinates matters when you look up a town name.
- ΔT: we also account for the correction arising from the irregularity of the Earth's rotation (currently about 69 seconds); in the distant past this is a greater uncertainty.
What matters most?
In this order: 1. the accuracy of the birth time, 2. the choice of house system, 3. the zodiac (tropical or sidereal), and only after these 4. the accuracy of the calculation. Anyone who wants an accurate chart should first verify the birth time (birth certificate, hospital record).
What we do not claim
An accurate calculation does not mean that the interpretations are true. The calculation is astronomy; the interpretation is traditional symbolism. Astrology's predictive power has not been confirmed in controlled studies, so our site does not make predictions and gives no health, financial or relationship advice.
How can you check it?
You can calculate the same chart at astro.com or with the Swiss Ephemeris, and planetary positions in NASA's JPL Horizons system. If you find a discrepancy, let us know, since correcting errors is our goal.