Tropical zodiac

The tropical zodiac divides the ecliptic into twelve 30° signs (Aries → Pisces). 0° Aries is tied to the vernal equinox — the season-based frame — so the zodiac drifts relative to fixed constellations over centuries (precession).

Sign from longitude

sign_index = floor(longitude / 30)
degree_in_sign = longitude mod 30

core.chart calculate uses tropical longitudes in body snapshots.

Sidereal alternative

Jyotish and some Western siderealists subtract an ayanamsa so signs align more closely with star regions. Same engine, different display: calculate_sidereal — sidereal-vs-tropical.

The zodiac belt is a measurement system on the ecliptic (ecliptic-longitude), not identical to IAU constellation boundaries.

References

Zodiac — Wikipedia
Axial precession — Wikipedia