A star constellations chart is useful for determining the viewer's position on Earth by comparing the locations of visible stars. Constellation charts are also useful for astronomers in plotting the locations of the stars and the Earth.
A constellation is a group of stars that looks like a picture when grouped together. Each constellation contains a specific group of stars that have been plotted by astronomers. Most constellations appear in different locations in the sky depending on the time of night and the season. Therefore, the constellations are useful as location markers if the viewer is aware of what time it is. Some stars and constellations are visible all year round in certain hemispheres. Knowing these constellations is a useful tool for navigation. For example, the star Polaris always points north. Using the relative locations of nearby constellations such as the Big Dipper helps the viewer find Polaris and determine which way is north.
Astronomers use constellation charts to map the locations and movements of stars, thereby learning more about the Earth's orbit and rotation. Most constellation charts are marked with the location, time and date that they represent. Separate charts are used for the Northern and Southern hemispheres because some stars and constellations are only visible in one of these hemispheres.