Choosing a Zodiac

The setting has two values and the difference between them is a rotation of the scale by about twenty-five degrees. Here is what that does to output you are looking at.

What changes

Every sign and degree label. A body at 15° Taurus becomes 21° Aries. Because the shift is nearly a full sign, most bodies drop back into the previous sign, and a chart can look almost unrecognisable at a glance.

Ingress dates. The Sun enters tropical Aries at the equinox, around 20 March, and sidereal Aries about twenty-five days later. Every ingress event in a study moves by roughly that much.

Anything that depends on which sign a body is in. Sign rulerships, dignities, and any technique that treats a sign boundary as significant will be reading a different sign for most bodies.

What does not change

This is the surprise, and it is worth being explicit because it is not obvious.

Aspects do not move. Every longitude in the chart shifts by the same twenty-five degrees, so the angle between any two bodies is exactly what it was. A trine is still a trine, to the arcsecond. A conjunction is still a conjunction.

The Ascendant and Midheaven do not move relative to the planets. They are positions on the same ecliptic and shift with everything else. Their sign labels change; their relationship to the chart does not.

House cusps do not move relative to the planets, for the same reason. A body in the seventh house stays in the seventh house.

Nothing physical moves at all. No calculation of a body's actual place in the sky is affected. Rising times, eclipse circumstances, declinations, distances, retrograde stations — none of them know or care which zodiac you selected, because none of them are measured from the zodiac's zero.

Switching zodiac rotates the labels. It does not move the sky, and it does not move any body relative to any other.

So which should you choose?

The applications take no position, and neither does this site. The choice is not astronomical — the geometry is identical either way — and it comes down to what you think the zodiac is measuring.

• If the signs mean the seasonal cycle, the equinox is the correct anchor and Tropical is the right setting. This is the mainstream Western position.
• If the signs mean regions of the actual sky, the stars are the correct anchor and Sidereal is the right setting. This is the mainstream Indian position, and has the stronger claim to the original Babylonian scheme.

What is not defensible is switching between them casually, or comparing a position from one against a position from the other. That is a twenty-five degree error dressed as a disagreement.

Recording the choice

One piece of practical advice, and it applies whichever you pick.

A position is meaningless without its zodiac. Twenty-four degrees is far too large to shrug off and far too small to be obvious from the numbers alone — a chart computed in the wrong zodiac looks entirely plausible and is nearly a sign wrong throughout.

The zodiac is stored with every study and every chart in the applications, so saved work keeps its own answer. The risk is in what leaves the application: positions copied into notes, quoted in correspondence, or compared against another program's output. Those carry no setting with them unless you write it down.

If you work in both — some people do, deliberately — label everything.

What comes next

The zodiac decided where the counting starts. The next category asks a question that sits underneath that one and is easy to miss entirely: from what point was the body's direction measured in the first place?

Next: What an Origin Is.