How accurate is a Lunonis chart?
In 60 test charts, every position we calculate was within 18.2 arcseconds of Swiss Ephemeris. That is about 0.3 of a minute of arc, far smaller than any chart wheel can show.
The results
| Position | Compared | Average difference | Largest difference |
|---|---|---|---|
| Sun | 60 | 0.5″ | 1.5″ |
| Moon | 60 | 0.9″ | 4.4″ |
| Mercury | 60 | 1.8″ | 8.9″ |
| Venus | 60 | 2.0″ | 9.8″ |
| Mars | 60 | 1.0″ | 3.8″ |
| Jupiter | 60 | 3.1″ | 7.3″ |
| Saturn | 60 | 2.6″ | 8.6″ |
| Uranus | 60 | 3.7″ | 11.0″ |
| Neptune | 60 | 10.6″ | 18.2″ |
| Pluto | 60 | 1.2″ | 2.9″ |
| Ascendant | 60 | 0.1″ | 0.3″ |
| Midheaven | 60 | 0.1″ | 0.2″ |
| Placidus house cusps | 720 | 0.1″ | 0.3″ |
How we tested
The sample is 60 dates and places from 1930 to 2028, at latitudes from 54°S to 65°N. For each one we compared the tropical ecliptic longitude of the ten bodies, the Ascendant, the Midheaven and the twelve Placidus cusps.
The reference is Swiss Ephemeris 2.10.03 through pyswisseph, in its built-in Moshier mode. Lunonis calculates with astronomy-engine 2.1.19 and its own house code. Our automated tests repeat this comparison on every code change.
Check it yourself
The data file holds every input and both sets of positions. The script recalculates the reference on your computer and prints the differences.
pip install pyswisseph
curl -O https://lunonis.com/accuracy/lunonis-vs-swiss-ephemeris.json
curl -O https://lunonis.com/accuracy/verify.py
python verify.py lunonis-vs-swiss-ephemeris.jsonWhat this does not show
- It measures astronomy only. It says nothing about whether astrological interpretations are true.
- Moshier mode isn’t Swiss Ephemeris’s highest-precision mode, which uses extra data files.
- Times here are in UTC, so this doesn’t test time zones. A wrong birth time or place changes a chart far more than these differences.
- The sample stops at 65°N. Nearer the poles, where Placidus houses stop working, we use whole-sign houses and flag it on the chart. That isn’t covered here.
More on the method: how we calculate. Or calculate your own chart, free and without an account.