Daylight-saving changes
The app uses the time-zone rules for the place and date of birth, including daylight saving.
Calculation accuracy
Precision you can check.
Across 5,183 chart comparisons, every tested longitude from the Sun through Pluto differed from Swiss Ephemeris by less than 0.014 arcseconds.
Evidence: Tropical charts
5,183
Chart comparisons
Tropical charts, dates spanning 1900–2100
<0.014″
Largest planetary longitude difference
Sun through Pluto, measured in arcseconds (″)
0
Sign or retrograde mismatches
Compared with Swiss Ephemeris
House cusps.
Six additional house systems were compared alongside Placidus, across 2,328 charts each. Every tested cusp matched Swiss Ephemeris on zodiac sign. The largest difference in cusp longitude was less than 0.04″.
| House system | Cusps compared | Largest difference |
|---|---|---|
| Equal | 27,936 | 0.022″ |
| Campanus | 27,936 | 0.022″ |
| Koch | 24,840 | 0.035″ |
| Porphyry | 27,936 | 0.022″ |
| Regiomontanus | 27,936 | 0.037″ |
| Whole sign | 27,936 | 0.000″ |
Birth times and clock changes.
When the clock repeats
When clocks go back, a time such as 1:30 am can occur twice. Ephemeris Lab presents both possibilities so the correct occurrence can be chosen.
When the clock skips
When clocks go forward, some local times never occur. The app flags these missing times.
When the birth time is unknown
With an unknown birth time, the Ascendant and houses are unavailable.
About the comparison
The figures compare tropical, apparent geocentric positions from the Sun through Pluto with recorded Swiss Ephemeris 2.08 results, using Placidus houses.
The app’s default date range is approximately 1950–2050. These comparisons also include dates outside that range.
Inputs rejected by both calculations are excluded from these figures. The Koch comparison also excludes cases where neither calculation returned houses.
These figures describe agreement on the tested positions and house systems, not an accuracy guarantee for every date, body or astrological technique. They do not measure the validity of astrological interpretation.