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What a Zodiac Is

The Zodiac dropdown offers two settings, Tropical and Sidereal. Before the difference between them can mean anything, it is worth being precise about what they are two versions of.

A ruler, laid along a circle​

A zodiac is a circle in the sky divided into twelve equal parts of 30° each, with a fixed starting point.

That is the whole definition. Three components:

  • A circle. One particular great circle on the celestial sphere — the ecliptic, which the next page introduces.
  • Twelve equal divisions. Each exactly 30°, because twelve of them have to close a 360° circle. These are the signs.
  • A starting point. Somewhere on the circle counts as 0°, and everything is measured from there.

The zodiac drawn as a ring divided into twelve equal thirty-degree segments, named in order from Aries, with the boundary at zero marked as the starting point and the direction of increasing longitude shown running anticlockwise around the ring.

A position in the zodiac is then just a distance around that circle from the starting point, quoted in two pieces: which 30° segment it fell in, and how far into that segment it got. 15° Taurus means 45° around the circle — one full sign of 30°, plus fifteen more.

It is a coordinate, not an object​

This is the point most worth holding on to, because almost every later confusion on this site traces back to losing it.

A sign is a stretch of a measuring scale. It is not a region of stars, not an object, not a thing in the sky at all. If you flew out to 15° Taurus you would find nothing there, in the same way that flying to 45° longitude on the Earth finds you no line painted on the ground.

Two consequences follow immediately, and both are dealt with properly later:

  • Because the divisions are equal by construction, they cannot match the constellations, which are unequal. That is where the signs came from.
  • Because the scale needs a starting point, and there is more than one defensible place to put it, there is more than one zodiac. That is the whole subject of this category.

Why the equality is a choice​

Twelve equal 30° divisions is not something the sky offers. Nothing up there is divided into twelve of anything, and nothing marks a boundary every 30°.

The equality was imposed, deliberately, by people who found an even ruler more useful than an uneven one. That decision was made a very long time ago and it has a history worth knowing, because knowing it disposes of a whole genre of argument about the zodiac being wrong. It is taken up four pages from here.

What the applications mean by it​

In Aquila and Compass the Zodiac setting decides only one thing: where the counting starts. Everything else about the zodiac — the circle it is laid on, the twelve equal divisions, the direction of counting — is identical in both settings.

That is why the setting is a simple pair rather than a long menu. Tropical and Sidereal are two answers to one question, and the question is: where is zero?

What comes next​

The circle first. Twelve divisions of what has been deferred long enough.

Next: The Ecliptic.