The Tropical Zodiac
Tropical is the first of the two settings, and it puts 0° Aries at the vernal equinox.
That is the whole definition, and the rest of this page is what follows from it.
Anchored to the seasons
Because the vernal equinox is where the Sun crosses the celestial equator heading north, tying the zodiac's zero to it ties the whole scale to the Sun's position in the year.
The four cardinal points of the zodiac land exactly on the four turning points of the seasonal cycle, permanently:
| Zodiac position | Seasonal event | Northern hemisphere |
|---|---|---|
| 0° Aries | Vernal equinox | Spring begins |
| 0° Cancer | June solstice | Summer begins |
| 0° Libra | Autumnal equinox | Autumn begins |
| 0° Capricorn | December solstice | Winter begins |
By construction, and forever. The Sun reaches 0° Aries at the vernal equinox because that is what 0° Aries is defined to be, so the match in the table is exact and does not wear off with time.
The four positions in that figure are the four rows of the table. The tropical zodiac is, in effect, a 360° scale laid over the Earth's orbit with its zero nailed to one of the crossings.
The name records this: tropical comes from the Greek for "turning" — the same root as the Tropics of Cancer and Capricorn, the latitudes where the Sun turns back at each solstice.
What it gives up
The equinox slides westward at a degree every seventy-two years, and the tropical zodiac slides with it. So the price is exact and known:
The signs drift against the stars. Over the two thousand years since the convention was fixed, tropical 0° Aries has moved nearly a full sign's width away from the stars of Aries. It is currently in Pisces and heading for Aquarius.
Positions expire. A star that has not moved acquires a new tropical longitude every year, at about 50 arcseconds a year. For the fixed stars this matters over a working lifetime.
Whether that is a loss depends on what you take the zodiac to be measuring. That is the argument between the two zodiacs, and it is not an astronomical one.
The case for it
The tropical position is that the signs were always about the seasonal cycle, and that the constellation names were labels for that cycle rather than claims about stars.
On this reading, Aries means the surge at the start of spring, Cancer means the solstice and the turn of the year, Libra means the balance of the autumn crossing. The names were borrowed from whatever patterns happened to be nearby when the scheme was set down, and the borrowing was never the point.
If that is what the signs are, then the equinox is the only correct anchor rather than merely a convenient one, and precession changes nothing that matters. A zodiac that stayed with the stars would be the one that had drifted, because it would have come loose from the cycle it was built to describe.
Western astrology is predominantly tropical, and this is why.
One thing that follows for the southern hemisphere
The seasonal table above is written for the northern hemisphere, and the tropical zodiac is defined from the northward crossing, which is a northern-hemisphere spring.
South of the equator the seasons are reversed while the zodiac is not. The Sun still reaches 0° Aries around 20 March; in Sydney that is the start of autumn. The astronomical definition is unaffected — the equinox is a point on a circle, not a season — but any interpretation resting on Aries means spring does not carry over unchanged to the southern hemisphere.
The astronomy fixes the position and takes no view on what the position means.
What comes next
The next page takes the other anchor, the stars, which is also the older of the two: the sidereal zodiac.
Next: The Sidereal Zodiac.