The Sidereal Zodiac
Sidereal is the second setting, and it puts 0° Aries at a fixed point among the stars.
The word comes from sidus, Latin for star. Where the tropical zodiac is nailed to a crossing of two circles, the sidereal zodiac is nailed to the stellar background — and it stays there.
Anchored to the stars
Precession slides the equinox westward along the ecliptic. The sidereal zodiac does not follow it.
The practical consequence is the mirror image of the previous page:
The signs stay with the stars. A star that has not moved keeps very nearly the same sidereal longitude century after century. The zodiac is measuring the sky it appears to be measuring.
The signs drift against the seasons. The Sun no longer enters sidereal Aries at the vernal equinox. It arrives about twenty-five days later, and that lag grows by a day roughly every seventy-two years. Given long enough, sidereal Aries will fall in the northern winter.
Whether that is a loss depends, again, on what you think the zodiac is for.
The case for it
The sidereal position is that the signs are regions of the actual sky, and that a zodiac which has come loose from the stars has lost the thing it was measuring.
The historical argument is strong. As Where the Signs Came From set out, the Babylonians built the twelve divisions on a band of constellations and knew where the equinox sat within them. The scheme was star-anchored from the start, so on this reading the sidereal zodiac is not an alternative to the original but the original itself, still doing what it was set up to do.
Indian astrology, Jyotisha, is predominantly sidereal, and it is the dominant tradition by number of practitioners worldwide. A minority of Western astrologers work sidereally too.
Where exactly is the star-anchored zero?
Unlike the tropical zero, this one has to be chosen.
The tropical zodiac gets its zero for free: two planes cross, and the crossing is a real feature of the geometry that anyone can find. The sidereal zodiac has no such point. There is no star sitting at the natural beginning of anything, because the stars are scattered and the twelve divisions are an imposition.
So a sidereal zodiac has to nominate a reference — a particular star at a particular longitude, or a stated offset from the equinox at a stated date — and different schools have nominated different ones. They agree to within about a degree and disagree in the detail.
That offset is called the ayanamsa, and The Ayanamsa is given over to it.
Neither zodiac matches the constellations
This is worth restating here, because this is the page where the mistake is most likely.
The sidereal zodiac is anchored to the stars. It still consists of twelve equal 30° divisions, and the constellations are still thirteen unequal patches. So sidereal Leo and the constellation Leo are not the same region of sky either — they merely stay in a constant relationship, rather than sliding apart.
Anchoring to the stars fixes the drift. It does not, and cannot, fix the mismatch built in at the beginning.
What comes next
There are now two zodiacs with two anchors, and one set of twelve names shared between them. The next page explains how the two came to share those names, which came about through a coincidence of timing.