The Big and Little Dippers are nicknames used for two constellations in the northern hemisphere, Ursa Major and Ursa Minor, the bears. To call the The Big Dipper a constellation is wrong, it is actually an asterism, a pattern that is not a constellation in its own right but is part of one. Finding one, you can find the other. The Big Dipper is also known as the Plough mainly in the United Kingdom.
The constellation that seemingly separates the two constellations is Draco, the Dragon with its Alpha designated star highlighted. Ursa Minor along with Cepheus, Cassiopeia and Draco are resident in the northern skies the whole year which makes it important for navigators to use. If you can find the constellation, you can plot your course if GPS is not available :).
The Big Dipper only really refers to a certain portion of the Ursa Major constellation, that section that looks like a saucepan. Once heard it referred to as a one-legged spider, eek.... The two stars, Merak and Dubhe in alignment point to Polaris, the current North Star. Current, only because in the future as was in the past, the star Thuban will reclaim the title of being the Pole Star which is why I've chosen to highlight.
The Little Dipper is the whole of Ursa Minor rather than just part of the constellation. The constellation, visible throughout the year is the northern most constellation. Its southern equivalent is Octans. The nearest equivalent to the Northern Pole Star is Sigma Octantis
The above picture was drawn using Stellarium, an awesome free star mapping + more application.
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