Why there are twelve
The Sun takes a year to return to the same point against the background sky. Divide that circle into twelve equal parts and each one takes about a month — which is roughly a lunation, and that is almost certainly why twelve rather than ten or sixteen. The division is Babylonian, it is at least two and a half thousand years old, and it is arithmetic rather than observation: every sign is exactly thirty degrees, regardless of how much sky the constellation of the same name actually occupies.
That last point is where most confusion starts. The constellation Virgo sprawls across more than forty degrees of the ecliptic; the constellation Scorpius is clipped to about seven. The signs are all thirty. They are two different systems that were briefly aligned and have not been for two thousand years — which is a story worth reading in full.
The three properties
Each sign is defined by three things, and the way they interlock is the actual engineering of the zodiac.
- Element — fire, earth, air or water. Four elements, three signs each, spaced 120° apart. Signs sharing an element always stand at a trine to one another.
- Mode — cardinal, fixed or mutable: whether the sign starts, holds or adapts. Three modes, four signs each, spaced 90° apart.
- Ruler — the planet the sign answers to. Before the telescope there were seven visible bodies for twelve signs, so five planets ruled two signs each; Uranus, Neptune and Pluto were assigned later, and both readings are still in use.
Because element and mode are both fixed functions of position on the wheel, no two signs share both. That is not a coincidence — it is the constraint the whole scheme was built to satisfy. The elements and modes are set out here, with the grid that shows why.
Every sign, side by side
| Sign | Dates | Element | Mode | Ruler | Opposite | |
|---|---|---|---|---|---|---|
| ♈︎ | Aries | 21 March – 19 April | fire | cardinal | Mars | Libra |
| ♉︎ | Taurus | 20 April – 20 May | earth | fixed | Venus | Scorpio |
| ♊︎ | Gemini | 21 May – 20 June | air | mutable | Mercury | Sagittarius |
| ♋︎ | Cancer | 21 June – 22 July | water | cardinal | the Moon | Capricorn |
| ♌︎ | Leo | 23 July – 22 August | fire | fixed | the Sun | Aquarius |
| ♍︎ | Virgo | 23 August – 22 September | earth | mutable | Mercury | Pisces |
| ♎︎ | Libra | 23 September – 22 October | air | cardinal | Venus | Aries |
| ♏︎ | Scorpio | 23 October – 21 November | water | fixed | Mars, and Pluto in modern practice | Taurus |
| ♐︎ | Sagittarius | 22 November – 21 December | fire | mutable | Jupiter | Gemini |
| ♑︎ | Capricorn | 22 December – 19 January | earth | cardinal | Saturn | Cancer |
| ♒︎ | Aquarius | 20 January – 18 February | air | fixed | Saturn, and Uranus in modern practice | Leo |
| ♓︎ | Pisces | 19 February – 20 March | water | mutable | Jupiter, and Neptune in modern practice | Virgo |
What a sign is not
A Sun sign is one placement in a chart that has a dozen. It says where the Sun was on a particular day, and nothing at all about where the Moon was, what was rising, or what angles the planets stood at. Two people born the same week can share a Sun sign and have almost nothing else in common on paper.
That is not a criticism of Sun signs — they are a genuine and very old part of the tradition. It is an argument for not stopping there. The moon phase you were born under is a second light with its own reading; the sidereal system answers the same question from a different zero; the Chinese zodiac is a different scheme entirely. Where this site stops, and what a full birth chart would add, is set out on the method page.