← Encyclopedia of Astrology · lesson 7/235
Tropical zodiac
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What it is
The tropical zodiac is the way of dividing the zodiac that most Western astrology uses. It starts the circle at the March equinox — the moment the Sun crosses the celestial equator heading north — and calls that point 0° Aries. From there the ecliptic is cut into twelve equal signs of 30°.
Because it is tied to the equinoxes and solstices, the tropical zodiac is a zodiac of the seasons:
- 0° Aries — the March equinox, spring in the northern hemisphere;
- 0° Cancer — the June solstice, the Sun at its farthest north;
- 0° Libra — the September equinox;
- 0° Capricorn — the December solstice, the Sun at its farthest south.
The name comes from the two tropics, of Cancer and of Capricorn — from the Greek tropos, "a turning" — because at the solstices the Sun seems to stop and turn back.
Why it drifts from the stars
The Earth's axis wobbles slowly like a spinning top, tracing a full circle in about 26,000 years. As it does, the equinox point slides backwards against the background stars by about 1° every 72 years. This is the precession of the equinoxes.
The tropical zodiac moves with the equinox, so over the centuries its signs have slipped away from the constellations whose names they carry. When the signs were laid down some two thousand years ago, the two lined up fairly well; today the tropical signs lie about 24° ahead of the star groups. The Sun at "0° Aries" in March now stands among the stars of Pisces.
This is not an error that tropical astrologers overlooked. In tropical astrology a sign is defined by its place in the cycle of the year, not by the stars behind it. Aries is the start of the solar year and of increasing daylight in the north; Libra is the balance point of day and night; Cancer is the peak; Capricorn the turning back towards light. Precession does not change any of that.
Tropical and sidereal
The other main approach, the sidereal zodiac, keeps the signs anchored to the fixed stars instead. Both use the same twelve names and equal 30° signs; they simply start in different places, about 24° apart today. A planet at 10° Taurus tropical is at roughly 16° Aries sidereal.
Astrologers argue about which is right, and the arguments go back a long way. Many Western astrologers hold that the tropical zodiac fits the meanings the signs have carried since antiquity; siderealists and the Indian tradition hold that the stars are the true frame. Others regard them as two coherent systems that answer different questions. A reading made in one cannot simply be mixed with the other.
A note on the southern hemisphere
The seasonal story is told from the northern hemisphere: south of the equator, Aries begins in autumn. Most astrologers in the south still use the same zodiac unchanged, holding that the equinox and solstice points matter for the Earth as a whole; a few have proposed reversing it. This is an open question rather than a settled one.
In the tradition
Ptolemy explicitly took the signs' beginnings from the equinoxes and solstices, and Hellenistic and medieval Western astrology followed him. Western astrology has been, overwhelmingly, tropical ever since.
In a reading here
Charts here are cast in the tropical zodiac, and a reading's astrological information says which zodiac its chart used.