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docs: note that calculate_planets applies ayanamsa via naive subtraction #9

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Expand Up @@ -121,6 +121,40 @@ planets.forEach((p) => {
});
```

#### Note on ayanamsa application

If you're comparing `calculate_planets(jd, mode)` output to KP / Vedic stacks
like VedicRishi (`astrologyapi.com`), it's helpful to know how the ayanamsa
ends up in the returned longitudes:

**Ayanamsa appears to be applied as naive subtraction** — i.e. the returned
sidereal longitudes match `swe_calc(jd, planet, TROPICAL) − swe_get_ayanamsa(jd)`
in our testing — rather than via Swiss-Ephemeris's `SEFLG_SIDEREAL` flag,
which rotates ecliptic coordinates in the ayanamsa reference frame. The two
agree to first order but differ by ~3–6″ per planet. Naive subtraction matches
the convention most KP / legacy Vedic software uses, so it is often the
desired behaviour.

If you specifically want the `SEFLG_SIDEREAL` path, you can call
`calc_ut(jd, planet, SEFLG_SWIEPH | SEFLG_SPEED | SEFLG_SIDEREAL)` directly,
but the sidereal mode must be set first. In our testing, calling
`calculate_planets(jd, mode)` once appears to set the global sidereal mode as
a side effect, so a subsequent `calc_ut(..., SEFLG_SIDEREAL)` returns
positions in the expected ayanamsa frame:

```typescript
import { p_julday, calculate_planets, calc_ut, Constants } from "@fusionstrings/panchangam";

const jd = p_julday(2024, 1, 1, 12.0, 1);
calculate_planets(jd, 1); // prime sidereal mode (mode 1 = Lahiri)

const flags = Constants.SEFLG_SWIEPH | Constants.SEFLG_SPEED | Constants.SEFLG_SIDEREAL;
const sun = calc_ut(jd, Constants.SE_SUN, flags); // sidereal Lahiri Sun
```

These are observations from external testing rather than statements about the
library's internals; the maintainer is the authoritative source.

### Muhurat Calculation

Determine auspicious and inauspicious time windows.
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