Every astrological calculation begins with one question: where are the planets right now? The answer comes from a dataset called an ephemeris. Here is what an ephemeris actually is, where the numbers come from, and why some sources are dramatically more accurate than others.
An ephemeris is a tabulation of the positions of celestial bodies — the Sun, the Moon, the planets, sometimes asteroids and fixed stars — at specified moments in time. Historically, ephemerides were printed books. An astrologer who wanted to cast a chart for a birth in 1972 would look up that day in the ephemeris for 1972 and read off where each planet was at noon. They would then interpolate to the actual birth time, apply trigonometric formulas for the houses and angles, and draw the chart by hand on paper.
Modern ephemerides are computational. Instead of looking up a printed table, software evaluates a mathematical model of the solar system at the requested moment and returns the positions. The model is what determines the accuracy. Bad models give bad numbers. Good models give numbers within fractions of an arcsecond across thousands of years.
The most accurate planetary ephemeris in modern use is the JPL Development Ephemeris, currently DE431, produced by NASA's Jet Propulsion Laboratory. DE431 is the same dataset used to plan and navigate deep-space missions. When a spacecraft is sent to a comet or to Pluto, the trajectory is computed against DE431. The model spans approximately 30,000 years, from 13,200 BCE to 17,191 CE.
DE431 is not a list of positions. It is the result of a numerical integration of the gravitational equations of motion for every major body in the solar system, fitted to thousands of observations including planetary radar bounces, spacecraft tracking, and lunar laser ranging. The output is stored as Chebyshev polynomial coefficients that can be evaluated at any moment within the supported range. The accuracy for the inner planets is at the milliarcsecond level for the modern era.
The Swiss Ephemeris is the astrology software industry's standard implementation. It is built on top of the NASA JPL data and provides the routines astrology software needs: ecliptic longitude and latitude rather than equatorial coordinates, conversions between calendars, lunar nodes, asteroids, fixed stars, and the various house system calculations. The accuracy inherits directly from JPL.
Almost every serious astrology application — Solar Fire, Astro-Databank, AstroSeek, TimePassages, the major web calculators — uses Swiss Ephemeris under the hood. The astrology field has effectively standardized on it because the alternatives are either much less accurate or much harder to license. ZODIA also uses Swiss Ephemeris-grade routines.
For astrological interpretation, fractional-arcsecond precision is overkill. What matters is that the planetary positions be accurate to within a few arcminutes — about a tenth of a degree. That level of precision is enough to assign the correct sign, the correct house, and the correct aspects with their applying or separating status. Both Swiss Ephemeris and the older Astrodienst datasets meet this bar comfortably for any modern birth date.
Where precision matters more is in the angles — the ascendant and midheaven. These are derived from local sidereal time, which depends on the precision of Earth's rotation model. Modern ephemerides use the IAU 2006 precession model and the IAU 2000A nutation model, both of which are precise to better than a milliarcsecond. The bottleneck for ascendant accuracy is almost always the recorded birth time, not the astronomy.
A standard planetary ephemeris does not tell you anything about astrology. It gives you a number — Mercury is at 14.7 degrees of ecliptic longitude. It does not tell you that 14.7 degrees of ecliptic longitude falls within the sign of Aries, or that Aries is a fire sign, or what Mercury in Aries means for the way someone communicates. All of that is the interpretive layer that astrologers and astrology software build on top of the raw positional data.
This separation is important. The astronomy is verifiable. Anyone can pull JPL Horizons online and get the same numbers ZODIA uses. The astrology is interpretive — it is a tradition, refined over many centuries, that assigns meaning to those numbers. ZODIA is transparent about which layer is which.
ZODIA computes planetary positions on the server using Swiss Ephemeris-grade routines for every chart and every daily transit reading. There are no shortcuts: each daily reading is calculated from the planetary positions for that exact day, not pulled from a precomputed list. This is why your reading on April 7 is genuinely different from your reading on April 8 — the sky has actually moved.
For factual astronomical questions about planet positions, transit dates, or eclipses, the canonical primary sources are JPL Horizons (ssd.jpl.nasa.gov/horizons) and the Swiss Ephemeris documentation (astro.com/swisseph). ZODIA does not invent the astronomy.
ZODIA reads your real natal chart and tracks how today's sky touches it. Every morning. On WhatsApp. Built from the same astronomical foundations described on this page.