The Two Zodiacs Compared
The whole difference between the two zodiacs is a rotation of the scale by about twenty-five degrees. Everything that changes between them, and everything that does not, follows from that one fact.
What changes
Every sign and degree label. A body at 15° Taurus becomes 20° 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 date 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 are reading a different sign for most bodies.
What does not change
This half is less obvious than the first, so it is set out in full.
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, and 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 depend on which zodiac is in use, because none of them are measured from the zodiac's zero.
A change of zodiac rotates the labels. It does not move the sky, and it does not move any body relative to any other.
What the difference turns on
The difference is not astronomical. The geometry is identical either way, and no observation distinguishes them: they are two answers to the question of what the signs are measuring.
- Read as the seasonal cycle, the signs are anchored by the equinox, and the tropical zero is the one that matches that reading. This is the mainstream Western position.
- Read as regions of the actual sky, the signs are anchored by the stars, and the sidereal zero is the one that matches. This is the mainstream Indian position, and it has the stronger claim to the original Babylonian scheme.
The two readings have been argued for two thousand years and the argument is not an astronomical one. What the astronomy settles is narrower and firmer: the two scales are offset by the ayanamsa, so a position from one is not a position from the other, and the difference between them is a difference of scale rather than a disagreement about the sky.
What a position leaves out
A longitude on its own does not say which zodiac produced it, and the two readings are twenty-five degrees apart, which is large enough to matter and too small to be obvious from the numbers alone. A chart computed in either zodiac looks entirely plausible on its face.
Saved work generally keeps its own setting. What travels without one is a position that has left the program: copied into notes, quoted in correspondence, or set beside another program's output. Nothing in the number itself recovers the zodiac it was measured in.
What comes next
The zodiac decides where the counting starts. The next category takes up a question that sits underneath that one and is easy to miss: from what point was the body's direction measured in the first place? That point is the origin.
Next: What an Origin Is.