A constellation is simply a collection of stars, imaginatively linked together to represent a person, animal or object in the night sky. In this way astronomers are better able to organize the heavens into some recognizable form, and make locating stars and other astronomical objects easier. In 1922, the International Astronomical Union (IAU) recognized 88 modern constellations, 48 of which are based upon those recorded by the Greek astronomer Ptolemy around 150 AD in his treatise called ‘Almagest’.
Many of the constellations in use today have the names of characters from Greek and Roman mythology. These ancient civilizations believed their gods dwelled in the celestial heavens, and so naturally looked for and found those deities in the pattern of the stars. As well as gods, many mortals who had done something heroic got to live in the skies too, as well as the creatures and objects associated with their legends. Out of all the recognized constellations, 42 depict animals, 29 relate to objects, while 17 are either humans or mythological characters.
The location of many different constellations in the night sky is important, as together with some of their neighbors, they often recount stories from the world of legend. Likewise, constellation families refer to groupings of constellations within the same region of the night sky, some of which also share some kind of ancient mythological relationship. Of the eight constellation families, Hercules (19) contains the most constellations, followed by La Caille (13), Bayer (11), the Zodiac (12), Ursa Major (10), Perseus (9), Heavenly Waters (9), and Orion (5).
The word Zodiac is a Greek word meaning ‘pertaining to animals’ and these Zodiac constellations are included in the overall list of 88 constellations. Contrary to popular belief, there are 13 signs of the Zodiac: : Capricornus, Aquarius, Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, and Sagittarius. The 13th Zodiac sign is called Ophiuchus. The Zodiac forms a ring of constellations in the sky and if we could see “behind” the Sun in the daytime we would notice the Sun slowly drifting in an easterly direction across a different zodiac constellation each month or so. That motion traces out a 23.5 degree wide path in the sky in which the Sun, Moon, and planets seem to orbit, called the ecliptic. This ring is called the ecliptic because it is also the line along which any solar eclipse will occur.