The twelve-sign zodiac was fixed in Babylonian astronomy in the first millennium BCE, dividing the older constellation-based sky into equal segments for calculation, and it passed through Hellenistic astrology into the Western tradition and, in a sidereal form, into India. The signs are the framework onto which planets, houses and aspects are read.
The atlas gives each sign its element, mode, ruler, and traditional associations of body, character and affairs. The sidereal signs of Indian astrology, the rashis, share the names but are measured differently and are treated under Vedic Astrology; the constellations proper are a separate category.