The zodiac is a broad band in the heavens, centred on the ecliptic, in which lie all the motions of the sun, moon and major planets. Since the orbits of these are in planes not far removed from the plane of the Earth's orbit, the zodiac extends to about 8?on either side of the ecliptic. The zodiac was divided by the early astronomers into twelve equal parts, each of 30?of Right Ascension. Each part was named after a constellation in the neighborhood, with its name or sign represented pictorially or by a symbol. The twelve signs are Aries (Ram), Taurus (Bull), Gemini (Twins), Cancer (Crab), Leo (Lion), Virgo (Virgin), Libra (Scales), Scorpio (Scorpion), Sagittarius (Archer), Capricornus (Goat), Aquarius (Water-carrier), and Pisces (Fishes). These signs or symbols are often engraved on scientific instruments to serve as a scale of dates.