The Zodiac

Over the course of a year the Sun appears to move all the way around the sky, tracing a circle called the ecliptic. The zodiac is that circle divided into twelve equal segments of thirty degrees each. Every sign is one of those thirty-degree slices, and a planet's sign is simply the segment it happens to occupy. The division is tied to the solar year and the seasons. The tropical zodiac begins at the spring equinox, so the signs mark the Sun's steady progress from spring through winter and back again. This is why the signs carry seasonal flavor, with the fresh start of Aries near the equinox and the depth of winter around Capricorn. Why twelve and not some other number? The circle divides cleanly into twelve, which makes for tidy equal parts. There is also a lunar reason: the Moon completes roughly twelve cycles in the time the Sun takes to go around once, so twelve was a natural way to split the year. The signs are equal segments of the ecliptic, distinct from the uneven constellations that share their names.