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In short
The tropical zodiac starts its twelve signs at the March equinox rather than at a star. Western astrology uses it, and the astronomical formulas this app relies on return tropical positions. The app never reports a chart in it: every position is converted to the sidereal zodiac by subtracting the Lahiri ayanamsa, a little over 24 degrees today.
What the term means
The tropical zodiac measures the twelve signs from the March equinox: 0° of Aries is where the Sun stands at that equinox, and the project's own checks expect the tropical Sun near 0 degrees then and near 90 at the June solstice. Because the equinox drifts against the stars, the tropical signs slide slowly past the sidereal ones. Western sun-sign columns use the tropical zodiac; Vedic astrology, and this app, use the sidereal.
Where the term comes from
None of the classical texts in this project's library uses the word. BPHS does use equinox-based positions where a rule needs them. For Ayan Bal (Ch. 27 Sl. 15-17) the ayanamsa is added to a graha's position to find its distance from the nearest equinox, and for the Sun's motional rays (Ch. 28 Sl. 3-4) the starting point is the Sun "with Ayanamsa", the Sayana Sun.
How this app uses it
- The Sun, Moon, planets and Ascendant are first computed as tropical longitudes, measured from the equinox of the birth date; the planets' published elements are moved to that equinox first.
- They are then converted. For 19 January 1985, 11:14 IST, New Delhi, the tropical Moon is in Capricorn at 6.30 degrees and the tropical Ascendant in Aries at 16.68; less the ayanamsa of 23.64 degrees, the chart shows a Sagittarius Moon and a Pisces Ascendant.
- One rule adds the ayanamsa back: the Sun's motional rays in the Ishta and Kashta Phala follow BPHS Ch. 28 Sl. 3-4 and start from the tropical Sun. Ayan Bal is not computed.
Pages that use this term
- Moon sign and nakshatra - a tropical and a sidereal sign side by side.
- Capricorn (Makara Rashi) - the Sun's tropical and sidereal Capricorn dates.
- Why apps disagree on your nakshatra - when one app is tropical, every planet shifts.
- What is a Mahadasha - why the dasha needs a sidereal Moon.
- Ayanamsa calculator - the gap between the two zodiacs on any date.
- Sun sign calculator - the sidereal Sun sign, not the Western one.
Related terms
Ayanamsa (Lahiri)
The gap between the tropical and sidereal zodiac, for any date you name - no birth details needed.
This date is sent only to compute the answer. Nothing is stored, no account is created, and no email is asked for.