Tropical Zodiac
The tropical zodiac divides the ecliptic into twelve equal 30° sections, starting at the vernal point — the Sun's position at the March equinox. It's anchored to the seasons, not to the constellations.
The zero point of the tropical zodiac is the vernal point: 0° Aries is defined as the Sun's position at the spring equinox. From there, twelve exact 30° sections are counted off — Aries, Taurus, Gemini and so on. Western astrology has calculated tropically since antiquity; the zodiac sign is thereby tied to the rhythm of the seasons.
It's important to distinguish this from the constellations: the actual constellations in the sky are different sizes, and because of the precession of Earth's axis, the vernal point and the constellations have drifted about 24° apart over roughly 2,000 years. Someone born "under the sign of Aries" today usually has the Sun astronomically in the constellation Pisces. Vedic astrology instead uses the sidereal zodiac, aligned to the fixed stars — both systems are internally consistent, but they are not the same thing.
The often-repeated objection that astrology "ignores the shift of the stars" misunderstands the definition, then: the tropical zodiac never measured the constellations — it has always tracked the points of the seasons.
How we use this term
Leitstern uses the tropical zodiac and calculates positions within it using Astronomy Engine.
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