How ZenAstro calculates your chart
Every chart on ZenAstro is calculated by our own code from published astronomical series. This page says which methods are used and how close the results are to a professional reference.
Zodiac and ayanamsa
Charts use the sidereal zodiac with the Lahiri ayanamsa, the standard for Vedic astrology. The ayanamsa is calculated with a straight-line model of precession. Against a reference it adds at most about 0.005° to a position.
Houses and ascendant
Houses are whole-sign: the sign that rises is the first house, and each following sign is the next house. The ascendant (Lagna) comes from the local sidereal time, the tilt of the Earth’s axis and the latitude of the birth place. Because the ascendant changes sign roughly every two hours, the exact birth time matters most.
Planetary positions
- Sun, Mercury, Venus, Mars, Jupiter, Saturn: the VSOP87 series, with corrections for light travel time, aberration and nutation.
- Moon: Brown’s lunar theory.
- Rahu and Ketu: the mean lunar node. Ketu is always opposite Rahu.
- Uranus, Neptune, Pluto: fitted to NASA JPL’s DE440 ephemeris (public domain) for the years 1850 to 2150. They are shown for reference and are not used in the classical interpretation.
Accuracy
We compare our results with the Swiss Ephemeris, a professional reference, on 200 random charts between 1900 and 2040. The largest difference in longitude for each body was:
| Body | Largest difference |
|---|---|
| Ascendant | 0.010° (0.6 arcminutes) |
| Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn | 0.006° to 0.014° |
| Rahu and Ketu | 0.004° |
| Uranus, Neptune, Pluto | 0.014° to 0.015° |
None of the 1,800 planet placements fell in a different sign from the reference.
Dasha
The Vimshottari Dasha follows the 120-year cycle. The starting lord comes from the Moon’s nakshatra at birth, and the remaining part of that first period comes from how far the Moon has travelled through it. Mahadashas are divided into Antardashas in the same proportions.
Panchang
Tithi, nakshatra, yoga and karana are worked out from the longitudes of the Sun and the Moon at the chosen moment. Vara is the weekday of the local calendar date.
Interpretation
Yogas, planetary strengths, house lords and dasha readings come from fixed rules (classical yogas, house significations and lordships) applied to your chart by the program. They are the same for everyone with the same placements. You can read the classical texts in Learn Vedic Astrology.
Limits
- The Kundli form accepts birth years from 1900 to 2100.
- Local time is converted to universal time with the historical time zone rules for the birth place and date. A wrong time or place changes the ascendant and can change house placements.
- ZenAstro is for education and the study of traditional Jyotish. It does not give medical, legal or financial advice.