Tropical vs Sidereal Zodiac

The tropical zodiac starts 0° Aries at the point of the March equinox, so its signs are tied to the seasons. The sidereal zodiac starts from a reference point fixed against the stars. Because the equinox point drifts slowly westward against the stars, the two starting points no longer coincide: today they differ by roughly 24 degrees, and the gap widens slowly over the centuries.

Neither zodiac changes where a planet is. Both describe the same longitude along the ecliptic; they disagree only about where counting begins, and therefore about which sign name the longitude receives.

What the ayanamsa is

The ayanamsa is the offset between the two starting points on a given date. A sidereal longitude is the tropical longitude minus the ayanamsa. Because the offset grows with time, the ayanamsa depends on the date of the calculation, which is why a sidereal chart for a birth in one decade uses a slightly different offset from a chart for a birth in another.

There is more than one convention for fixing the reference point. Lahiri is the most common in Indian astrology; others, such as Fagan-Bradley or Raman, choose a different star or epoch and produce offsets that differ from one another by amounts large enough to move a body across a sign boundary. A sidereal chart is therefore not fully specified until the ayanamsa is named.

The same moment in both zodiacs

The table takes the demonstration chart as NatalChart.AI calculates it and subtracts an approximate Lahiri ayanamsa for 1991, about 23°44′. The sidereal column is illustrative only: the product does not compute it, and a sidereal tool with a different ayanamsa or a more exact value for the date will differ by some minutes of arc.

Demonstration birth: 1991-09-06 at 18:30 local time, Kyiv, Ukraine (50.4501° N, 30.5234° E), Europe/Kyiv at UTC+3, so 1991-09-06 15:30 UTC. Tropical zodiac, Placidus houses, Swiss Ephemeris 2.10.03. Full result and settings on How to verify your birth chart.

Every body moves back by the same amount, so most positions land about 24° earlier along the zodiac. Whether the sign name changes depends on the degree within the sign: Sun, Moon, Venus, Ascendant cross into the previous sign, while Mercury and Jupiter stay in the same sign with a lower degree. The Placidus cusps shift by the same offset, so each planet keeps its house; only the sign written on the cusp changes.

Comparing NatalChart.AI with another tool

1. Check the zodiac setting of the other tool first. If it is sidereal, expect every longitude to sit roughly 24° earlier and many sign names to move back by one.
2. If the zodiac is the same, match the birth time, the historical timezone offset and the coordinates. A wrong offset moves the Ascendant and the house cusps, not the zodiac.
3. Match the house system. NatalChart.AI uses Placidus; see Placidus vs Whole Sign.
4. Compare the planets before the houses and aspects, as described on how to verify your chart.

A difference of about 24° that applies to every body at once is the zodiac convention, not a calculation error. A difference that affects only the Ascendant and the cusps points at the time or place instead.

What NatalChart.AI does

Every chart is calculated with Swiss Ephemeris in the tropical zodiac with Placidus houses. There is no sidereal option and no zodiac switch, and the ayanamsa is not applied anywhere in the product: not in the natal chart, not in transits, not in compatibility or the year ahead. The readings and the explanations refer to tropical positions throughout.

Which zodiac is right?

This is a question of convention and tradition rather than of measurement. Western astrology has used the tropical zodiac for most of its history; Indian astrology uses sidereal zodiacs. Each is internally consistent, and each interprets its signs within its own tradition. What matters for a comparison is that both charts are read in the zodiac they were calculated in.

Questions

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