The International (UAI) Astronomic union divides the sky in 88 official constellations with precise confinements, of way that every direction exactly belongs to a constellation. These are mainly based on the constellations of the tradition of ancient Greece, handed down through the Middle Ages.

The 88 constellations are divided, according to a historical criterion and of importance, in three groups:

• the 12 constellations of the Zodiac, that are along the eclittica, and that they are crossed therefore from the Sun in its apparent motion on the celestial time during the year;
• the other 36 constellations listed by Tolomeo in his Almagesto, become today 38 with the subdivision of one of them (the Ship Argo) in three separate constellations;
• the remaining 38 constellations, define in modern (beginning from 1600 around) epoch in the empty spaces among the constellations of Tolomeo and in the southern hemisphere. These new constellations are generally composed by stars a little bright (those bright were, obviously, included in the constellations of the Zodiac and Tolomeo), and they can be difficult to be observed from the cities

Besides they also are divided according to their position in the sky:

• 18 northern constellations;
• 34 equatorial constellations;
• 36 southern constellations.