A night sky is seen full of stars. These stars could be more than some trillion miles away from us. Yet some seem to form a group in the sky. This group or the cluster of stars is called constellation. The constellation system comes from the ancient Greeks. Each constellation is given a name in connection to how they look. There are about 88 recognized constellations in northern and southern hemisphere. The international astronomical union (IAU) is responsible for naming celestial objects and their features. Ursa Major and Orion are some of the known ones. They are seasonal which means they are best seen in different months.
In astronomy, every celestial body other than the Earth is defined in a celestial space. The constellations are defined areas of the celestial sphere. There is an imaginary equator in an imaginary celestial sphere. This imaginary equator also called celestial equator is a circle in the same plane as earth’s equator. Celestial objects near the celestial equator are visible worldwide. Not all constellations are visible from any point on earth. The celestial equator passes through constellations like Pisces, Taurus, Orion, Aquarius, Leo and many more.
The stars within a constellation do not have any relation in astronomical terms. They could actually be so far from each other. There are also exceptions to this. For instance stars of the constellation Ursa Major are close to one another and they travel with similar velocities. This constellation is visible throughout the year in the north. The seven stars it corresponds to are called Big dipper in the US and Saptarishi in India. The Big Dipper is actually called asterism and not a constellation. Asterism is a group of stars that form patterns but are actually a part of one or more constellations. Big Dipper is an asterism of the constellation Ursa Major.