Constellations have been observ soon in Antiquity. Many of them have curious names of animals or heros, from Greak or Roman origine: Taurus, Aries, Canis Major, Pegasus, Hercules, Cassiopeia, Andromeda...

In fact, they are not correspond to the pictures form. Ursa Major (also call the Big Bear) resemble more at an old crock with a long twist handle. Otherwise, Chineses peoples call it the Big Crock.

It is realy important and easy to find it, because its seven stars are realy bright. From it, you can find the Polar star : it is situate in the prolongation of two stars opposite at the handle. It is the travellers star : it is pointing out the North direction, and nothing was more important for olden days sailors who did not have any compass.

In reality, forming stars of constellations are not realy group in space, but seen from the Earth, they give us the illusion of.

It be 88 constellations. They are not all visible from the same place, neiher at the same hour, or seasons.

The Earth turn on itself in a day (rotation) and pass to an orbit around the Sun (revolution) in a year. But, how can we mesure a rotation movement? Astronomers calculate lenght of a day in two ways: according to the Sun and according to the stars. Those relatives measures to the stars is call siderals.

When the Earth have done a complete tour on itself, a solar day have been done. When the Earth have been done a complete tour on itself according to the stars, it also have done a distance of 1/365 on the Sun orbit. Earth have to turn a little bit more for that astronomer on Earth to see the Sun in the exact same place in sky. In fact, it have to turn again during 1/365 of day, who represent 3 minutes and 56 seconds. Then, solar days hold out 3 minutes and 56 seconds more then a sideral day.

In consequence, constellations rising almost 4 minutes sooner each day. After a month, they rising themselve approximately 30 days X 4 minutes by days = 120 minutes = 2 hours soon.

If we have been more in North, we always see the same constellations of the North hemisphere and never them of the South hemisphere. If we have been at the equator, it will be possible to see all constellations of the sky. We are at latitude locate at midway from the equator and North pole; for us, some constellations are always visibles (circumpolars) and some others are only visible at some hour and time of the year.

Circumpolars constellations are always visible sometime they are locate in top of Celestal North pole and sometime above. Those costellations are more easy to observ when they are on Celestal North pole, also more higher in sky.