Scientific Methodology

Swiss Ephemeris Accuracy and Stars.expert Astrology Calculation Methodology

Planetary positions are computed to ±0.001 arcsecond precision over 30 000 years of coverage, from 13000 BC to 17000 AD. This page documents the four house systems we support, how we resolve historical timezones, and where each number in your chart comes from — so you can check any single step against the public Astrodienst tables yourself, without taking our word for it.

Swiss Ephemeris

World standard for astronomical calculation

±0.001"

Precision to thousandth of arcsecond

IANA TZ

Official IANA timezone datesbase

Swiss Ephemeris: The Heart of Our Calculations

Swiss Ephemeris is the world reference library for high-precision astronomical position calculations. Developed by Astrodienst (Switzerland), it is used by professional software, astronomical observatories and research institutions worldwide.

Official sources:

Swiss Ephemeris (astro.com)NASA JPL HorizonsIAU (International Astronomical Union)

Key Features

  • ±0.001 arcsecond precision for all planets
  • Time coverage from 13,201 BC to 17,191 AD
  • Calculation of all planets, Moon, Sun, lunar nodes and major asteroids
  • Support for all astrological house systems

By using Swiss Ephemeris, we ensure our calculations align with the highest standards of modern computational astronomy.

NASA JPL DE431: Official Data

Our ephemeris is based on the Development Ephemeris DE431 from NASA's Jet Propulsion Laboratory.

Temporal Coverage

From 13,201 BC to 17,191 AD with sub-arcsecond precision

Celestial Bodies

Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto

Visit NASA JPL Ephemeris

Precision ±0.001 Arcseconds

Our calculation precision is ±0.001 arcseconds. To put this in perspective, one arcsecond is 1/3600 of a degree.

±0.001"

One arcsecond (1") = 1/3600°. We are precise to the thousandth.

This is the same precision used by professional astronomical observatories for satellite tracking and space navigation. You can trust that planetary positions in your birth chart are as accurate as scientifically possible.

Astrological House Systems

We support all major astrological house systems, allowing you to choose the one you prefer or that matches your astrological tradition.

Placidus

The most common system in the West. Based on the time division for a point to pass from Ascendant to Midheaven.

Koch

Popular in Europe, especially Germany. Similar to Placidus but with a different calculation method.

Equal Houses

Each house occupies exactly 30°. Simple and used since antiquity.

Whole Sign

Each zodiac sign corresponds to one house. The oldest system, used in Hellenistic astrology.

Timezones and Daylight Saving

Birth time accuracy is crucial for calculating the Ascendant and houses. We use the IANA Time Zone datesbase, the international standard recognized by ICANN.

Why IANA Matters

  • Includes all historical timezone variations from 1800 to present
  • Correctly handles daylight saving time changes for every location
  • Constantly updatesd to reflect legislative changes

Frequently Asked Questions

What guarantees swiss ephemeris accuracy on Stars.expert?
Planetary positions are computed through the Swiss Ephemeris library at Astrodienst. The Julian-to-Gregorian transition applies the 1582 calendar reform.
How does the astrology calculation methodology handle house systems?
Placidus is the default because the modern western tradition Chris Brennan and Demetra George refer to in their published work still treats it as the baseline for natal interpretation. Stars.expert lets you swap in Koch, Regiomontanus, Campanus, Porphyry, Whole Sign or Equal House at any time. For births above 66 degrees latitude Placidus becomes mathematically unstable, and the methodology page warns explicitly and recommends Whole Sign as a fallback. The house system choice is logged in every chart export.
Which time-zone database underpins the swiss ephemeris accuracy claim?
Stars.expert resolves every birth time against the IANA TZ database, which captures historical zones, pre-1970 anomalies and Daylight Saving Time rules accurately for every country. Without this layer, pre-1970 charts inherit the local-mean-time error every shareware astrology calculator still propagates — a few minutes of offset that, at the equator, can move the ascendant by a full degree.
How can I verify the calculation methodology independently, without trusting Stars.expert?
Open JPL HORIZONS in a separate browser tab and request the heliocentric or geocentric coordinates for the same date and time. The numbers will agree with Stars.expert to within ±0.001 arcsecond. Astrodienst's printed ephemeris tables provide a second independent check; Solar Fire, Kepler and Janus, which all import the Swiss Ephemeris library, provide a third.

Ready to Explore the Stars?

Now that you know our methodology, discover what the planets reveal about you.

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