The constellations are totally imaginary things that poets, farmers and astronomers have made up over the past 6,000 years (and probably even more!).
The real purpose for the constellations is to help us tell which stars are which. Constellations are names for groups of stars that appear to form shapes in the sky. A constellation is what astronomers call an 'asterism': a group of celestial bodies (usually stars) that appear to form a pattern in the sky or appear visibly related to each other.
Constellations are named patterns of stars. All societies created them. Constellations are normally the product of human perception rather than astronomical realities. Constellations move so slowly that, in our lifetime, they will always be found in about the same place. All modern constellation names are Latin proper names or words, and some stars are named using the genitive, or sometimes the ablative of the constellation in which they are found. Many names we use nowadays came from the ancient Greeks. Due to their northern location they only gave names to those regions visible from their countries.
The regions around the celestial South Pole got some of their names when the astronomer Johan Bayer made his notes about the south regions of the sky. These are formed by using the usual rules of Latin grammar. Some examples include: Aries → Arietis; Taurus → Tauri; Gemini → Geminorum; Virgo → Virginis; Libra → Librae; Pisces → Piscium; Lepus → Leporis. The stars in a constellation are often vastly distant from each other, but they appear close to each other from the perspective of Earth. The 48 ancient constellations single out only the bright patterns. From around 1600 to 1800, post-Copernican astronomers invented hosts of "modern" constellations from the faint stars that lie between the classical figures. Later astronomers broke the ship Argo into three parts, yielding 50 ancient constellations.
In the early twentieth century, the International Astronomical Union (IAU) adopted 38 of the modern constellations and drew rectangular borders around all 88. The constellations play an important role in modern astronomy. They bring order to the sky by dividing it into smaller segments, providing a base for naming celestial objects. The brightest constellation is Crux. They used to assign their gods and heroes to certain figures in the sky.
The constellations of the zodiac we know nowadays reach back to the days of the Babylonians and Chaldeans, 2000 to 3000 years ago. Back then the sun passes through twelve constellations we still assign to the zodiac.