If you are going to read a birth chart, the calculated planet positions need to be right. Surprisingly, a lot of free chart sites are not careful here. Astrolas is built around Swiss Ephemeris, the same calculation engine professional astrologers and academic astronomers use. This post explains what that is, why we picked it, and what it changes about your chart.
Three arc-seconds is the difference between an aspect that completes today and one that completes tomorrow. We do not round.
- โSwiss Ephemeris is the gold-standard astronomical engine
- โAccuracy within 3 arc-seconds for positions, decisive for aspects
- โWe chose it so the timing of every transit is defensible
What an ephemeris actually is
An ephemeris is a table (or modern algorithm) of planet positions over time. To compute where Mars was at 5:25 PM Lisbon time on February 5, 1985, you need either:
- A printed ephemeris (the old way - thick books with planet positions for noon GMT, plus interpolation),
- Or a numerical model that can compute the positions of every planet for any moment.
The accuracy of every astrology tool depends entirely on the accuracy of that underlying ephemeris.
Swiss Ephemeris in one paragraph
Swiss Ephemeris is a free, open-source astronomical computation library maintained by Astrodienst in Switzerland, based on NASA JPL ephemeris data (DE431 / DE441). It computes planet positions to sub-arcsecond accuracy across thousands of years. It is the same library used by professional astrology software (Solar Fire, Kepler, Sirius) and many academic and observatory tools.
Translation: it is as accurate as it gets without running NASA-grade orbital mechanics from scratch.
What that means for your chart
A planet's position changes faster than people realize:
- The Moon moves about 13ยฐ per day, or roughly 0.5ยฐ per hour. A 30-minute error in your birth time shifts your Moon by about 0.25ยฐ.
- The Ascendant moves 1ยฐ every 4 minutes on average. A 10-minute error in birth time shifts your Rising sign cusps by 2.5ยฐ.
- The house cusps rotate at the same rate as the Ascendant.
So precision matters. A cheap ephemeris that gets Mercury wrong by 0.3ยฐ will tell you the same Sun sign as the right one - but will mislabel the orb of a Mercury aspect that was important to your chart. With Swiss Ephemeris, that drift is gone.
What we use it for inside Astrolas
Every chart on Astrolas is calculated with Swiss Ephemeris for:
- Geocentric positions of Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto
- True Lunar Node (mean nodes available as an option)
- Chiron, Pallas, Juno, Vesta, Ceres
- Vertex, Part of Fortune, East Point
- All major Arabic Parts via configurable formulas
- House cusps in Placidus, Whole Sign, Koch, Equal, Porphyry, Regiomontanus, Campanus, Topocentric, Alcabitius
For synastry and transits, we run Swiss Ephemeris twice (or more) per chart pair and align the results to the requested time zone. All of it under sub-arcsecond precision.
What changes day-to-day for you
A few practical things:
- The Rodden rating of your birth time matters more than the chart engine, beyond a certain accuracy. Even Swiss Ephemeris cannot save a B-rated birth time from being off. If you have a DD or X rating (unverified or unknown time), the Sun and most planets are still trustworthy; the Moon, Rising, and houses get fuzzier.
- Astrolas's daily transit page uses Swiss Ephemeris to compute planet positions for the current minute. You will see the same numbers professional software shows.
- For compatibility and synastry, the orb of an aspect (how exact the angle is) is calculated to two decimal places of a degree. We do not round to whole degrees.
What we do not use
We do not use:
- Older / simplified ephemerides (e.g. Lahiri tabular interpolation) for natal charts.
- Sidereal calculations by default (we use the tropical zodiac unless you switch settings).
- Free third-party APIs that wrap unknown back-ends.
If you switch your settings to Vedic / sidereal, we still use Swiss Ephemeris with the Lahiri ayanamsha (configurable) applied. The underlying engine is the same.
- โEvery aspect timing - transit, progression, return - is computed to NASA-grade precision
- โEdge cases at the second-of-arc level are decided correctly, not rounded out
- โYou can defend the numbers if a working astrologer ever questions them
Where chart errors actually come from
Once the ephemeris is accurate to fractions of an arcsecond, it stops being the weak point. The errors that still show up in real charts come from the inputs.
Time zone history. Before standardized time zones were adopted, most places ran on local mean time, set by the Sun at that city's longitude. Daylight saving has been switched on and off, moved, and suspended during wars, often differently in neighboring regions. A chart that applies today's time zone rules to a 1943 birth can be off by a full hour, which moves the Ascendant by roughly 15 degrees. Good software uses a historical time zone database for this, and it matters far more than the difference between ephemeris engines.
Coordinates. A birth town and a nearby city can differ by half a degree of longitude, which shifts the Ascendant by a small but measurable amount. For most charts this is minor. For a planet sitting right on a house cusp, it can decide which house it falls in.
The recorded birth time itself. Hospital records are often rounded to the nearest 5 or 15 minutes. Parents' memories round even more. That rounding is the reason astrologers use birth time rectification, working backward from known life events to find the most likely minute.
Delta T, briefly
The Earth's rotation is not perfectly steady. Clock time (based on atomic seconds) and rotation-based time drift apart slowly, and the difference is called Delta T. For births in the last century it is around a minute or less. For historical charts, say a king born in 1500, it is a few minutes, and the uncertainty grows the further back you go.
Swiss Ephemeris handles Delta T internally, so modern charts do not need to think about it. It is one of the details that separates a proper astronomical library from a simple formula that assumes the Earth spins like a clock.
How to check any chart tool
A quick test for any chart site, including ours:
- Pick a birth with a well-documented time, such as a public figure with an AA Rodden rating.
- Compare the Moon's position to two decimal places. The Moon is the fastest-moving body, so it shows errors first.
- Compare the Ascendant. If it differs by more than a fraction of a degree, the difference is almost always the time zone handling, not the planets.
If both line up, the tool is doing its job. If only the Ascendant is off, look at the time zone and daylight saving rule it applied.
What to do next
Open your free natal chart and check the planet positions to two decimal places. Compare them to any other free chart site. If the numbers differ by more than 0.05ยฐ, that other site is using a lower-precision ephemeris.
For more on how chart accuracy affects interpretation, read the 12 houses explained and pay particular attention to angular planets - they are the most sensitive to birth-time error.