The zodiac

What it is

A band of sky either side of the ecliptic — the apparent path of the sun through the year — divided into twelve equal parts of thirty degrees each. The sun, moon and planets all appear within this band, because the solar system is roughly flat and we are looking at it edge-on.

The critical distinction, and the source of nearly all public confusion:

Signs are not constellations. A sign is a thirty-degree arc of the ecliptic. A constellation is a group of stars with a name and a ragged, arbitrary boundary. They share names because the signs were named after the constellations that occupied them roughly two thousand years ago. They have not lined up for a long time, and even when they did, the fit was approximate — the constellations are wildly unequal in size, and the twelve equal signs were imposed on them deliberately.

Where it came from

The twelve-fold division is Babylonian, established by around the fifth century BCE. It was an arithmetical convenience: Babylonian astronomers needed a coordinate system for recording and predicting planetary positions, and twelve equal divisions of thirty degrees fit a base-sixty number system and an approximately twelve-month year very neatly.

Before that, Babylonian celestial observation used an irregular set of reference constellations along the ecliptic — the "path of the moon" — of which there were more than twelve. Making them twelve and equal was a mathematical decision, not an observational one.

The Greeks inherited the system, Hellenistic Egypt built horoscopic astrology on it, and it passed through Arabic astronomy back into medieval Europe. It remains the coordinate system of Western astrology, and — under a different anchoring, described in tropical and sidereal — of Vedic astrology too.

How it is used

The zodiac provides the frame against which everything else is measured: where the sun, moon and planets were at a given moment, and where the horizon and midheaven fell.

Each sign carries a bundle of traditional attributions — an element (fire, earth, air, water), a mode (cardinal, fixed, mutable), a ruling planet, and a set of characteristics. In technical practice these are the raw material for judgements about placement, not descriptions of people; the compression of "sun in Leo" into "a Leo" is a modern popular usage that practitioners generally regard as a caricature.

What we can and cannot say

We can say the twelve-fold division is a human convention with a documented origin and a practical reason. Nothing in the sky is divided into twelve; Babylonian arithmetic is.

We can settle the thirteenth sign story, which resurfaces in the press every few years — most memorably when NASA published an educational page explaining precession and was widely reported as having "changed the zodiac". The claim is that the sun passes through Ophiuchus, the serpent-bearer, between Scorpius and Sagittarius, so there ought to be thirteen signs.

The sun does pass through Ophiuchus. The constellation boundaries that make this true were fixed by the International Astronomical Union in 1930 for the convenience of astronomers, and they carve the sky into 88 unequal regions. But signs are not constellations. A Western astrologer's zodiac has twelve equal thirty-degree divisions anchored to the equinox and is not claiming that the sun is in front of any particular group of stars. Adding Ophiuchus to it makes about as much sense as adding a thirteenth hour to a clock face because the sun took longer to cross the sky in summer.

This matters for the site's method rather than for astrology's credibility. It is a bad objection, and using bad objections against a practice is a way of avoiding the good ones — which exist, and are covered under the evidence.

We cannot say the sign attributions correspond to anything. The elemental and modal system is elegant, internally consistent and very old, and none of that is evidence that it describes people. That question is treated separately.

Further reading

• Francesca Rochberg, The Heavenly Writing (2004) — the standard scholarly account of Babylonian celestial science and where the zodiac came from.
• The IAU's constellation boundary definitions (1930) — for what the "thirteenth sign" story is actually about.