Conventions
- Zodiac: tropical (the Western convention). Sidereal or Vedic results differ by about 24° (23–24°, depending on the year).
- Positions: geocentric apparent positions of the Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.
- Houses: Whole Sign houses — the first house is the whole Rising sign. Ascendant and Midheaven are calculated for the exact moment and place.
- Aspects: conjunction, sextile, square, trine and opposition, with an orb of 8° when the Sun or Moon is involved and 6° otherwise.
- Retrograde: a planet is marked retrograde when its apparent motion along the zodiac is backwards, measured over the six hours before and after the moment.
- Display: degrees are truncated, never rounded, so a body at 29°59.9′ is never shown in the next sign; times are shown to the minute.
Engine and data
Positions come from Astronomy Engine, an open-source astronomy library (MIT licence), running in your browser. Birthplaces come from GeoNames (CC BY 4.0), which gives each place its coordinates and time zone. The clock time on your birth record is converted to universal time with the historical time zone and daylight saving rules of that place and date, as provided by your browser’s time zone database.
The dated tables on Current Sky and the Moon Calendar are precomputed with the same engine, so they always agree with the live view. The one exception is the short “Today: Sun in …” line on the homepage, which uses a lighter standard formula (Meeus) so the homepage stays fast; around the moment the Sun changes sign it can differ from the full engine by up to about 13 minutes.
Our reference checks cover dates from 1900 to 2100. For births before 1970, historical time zone and daylight saving rules come from your browser’s time zone data, which is less complete for some places; if in doubt, check the clock offset of your birth record.
Measured accuracy
We check the engine against reference values produced with the Swiss Ephemeris (version 2.10.03), a widely used professional ephemeris that is not part of MySkySoul itself.
- Every release must match reference charts within 0.02° (1.2 arcminutes) for the Sun, Moon, Ascendant and Midheaven, with identical signs, Whole Sign houses and retrograde flags. On our six reference Big Three charts the largest differences are under 1″ for the Sun, 1.5″ for the Moon and up to 15″ for the Ascendant.
- Moon sign changes are tested against the Swiss Ephemeris over a reference month (14 changes) to within 2 minutes.
- Before launch, 3,000 Big Three cases were compared: the largest Moon difference was 0.019° and there were no sign mismatches.
- In a separate check of 800 random dates between 1900 and 2100, 6,836 retrograde flags matched; planets within about a day of a station (moving less than 0.02° a day) were excluded, because their direction is ambiguous at that precision.
- An independent automated audit (AI-assisted) in September 2026 compared 967 charts with the Swiss Ephemeris and found the core positions within its tolerances. Its other findings were fixed and are covered by tests; the homepage Sun line was improved and its remaining margin is the one described above.
Birth time: exact, approximate or unknown
- Exact: everything is calculated for that moment.
- Approximate: we calculate the time you enter and also check an hour either side, then show every sign that is possible in that window — for the Rising sign in the Big Three, and for any planet in the birth chart.
- Unknown: we check the whole birth date in the birthplace’s time zone. If the Sun or Moon changed sign that day, you see both possible signs — for the Moon with the local time of the change. The Rising sign, houses and angles are not calculated, and we say so instead of guessing.
Edge cases we handle openly
- Clocks going back: if the local time happened twice, we ask which occurrence matches the record.
- Clocks going forward: if the local time never existed, we ask you to check it rather than shifting it silently.
- Near the polar circles: the Ascendant can race through signs or skip some; results for such birthplaces carry a clear caution.
- Cusp birthdays: we use the Sun’s actual position at birth, not a table of usual dates.
Privacy
Your birth date and time are processed in your browser and are not sent to our servers. The only birth detail sent is the text you type to search for a birthplace. Saving your details is optional and keeps them in your own browser. Usage statistics (Google Analytics) follow your cookie choices and never include your birth details. Read more in the About page.
What we do not do
No AI runs on this site: positions are computed by the astronomy engine, and the interpretive text is fixed, written copy. We do not offer predictions and we do not calculate sidereal (Vedic) charts. Interpretive text describes traditional themes for reflection; it is not advice.