What a Zodiac Is
A 360° circle cut into twelve equal parts: a ruler laid along one circle in the sky.
The Ecliptic
The Sun's annual path around the celestial sphere — the great circle the zodiac is laid along, and the narrow band everything a chart plots keeps to.
The Vernal Equinox
The crossing point that serves as a zero needing no landmark, why that makes it the strongest possible reference, and why it does not stay still.
Where the Signs Came From
The Babylonians named the constellations along the Sun's path, then deliberately replaced them with twelve equal divisions of 30° — which is why the signs have never been the constellations.
Precession Moves the Starting Point
The slow wobble of the Earth's axis, and the consequence that splits the zodiac: the equinox slides westward along the ecliptic by a degree every seventy-two years.
The Tropical Zodiac
Zero anchored to the vernal equinox, which locks the twelve signs to the seasonal cycle and lets them drift against the stars.
The Sidereal Zodiac
Zero anchored to the stars, which keeps the twelve signs with the stellar background and lets them drift against the seasons.
The Same Names on Two Zodiacs
How one Babylonian naming scheme came to serve two different zodiacs — and why the transfer, made in the third century, cost nothing visible at the time.
The Ayanamsa
The measured gap between the two zeros, currently a little over 25° and growing — and why anything sidereal has to nominate a value for it.
The Two Zodiacs Compared
What differs between the two scales and what does not — every sign label moves, and every angle between bodies stays exactly where it was.