The Big Dipper and the North Star (Polaris) are two of the most recognizable patterns in the Northern Hemisphere. The Big Dipper, known for its distinctive ladle shape, is often the first celestial group stargazers learn to identify. Polaris serves as an unwavering beacon for navigation. Because of their close visual relationship, many people mistakenly believe the North Star is part of the Big Dipper. However, they are separate entities, and the Big Dipper is simply used as a guide to locate Polaris.
The Big Dipper and Polaris Are Separate
The Big Dipper is not a constellation itself, but rather an asterism, which is a highly recognizable pattern of stars. This asterism is part of the much larger constellation Ursa Major, or the Great Bear, and is composed of seven bright stars. Polaris, the North Star, is a single star located in a completely different constellation known as Ursa Minor, or the Little Bear. Polaris is situated at the very end of the handle of the Little Dipper asterism. The Big Dipper and Polaris are distinct celestial bodies that simply appear close enough in the sky for one to serve as a pointer to the other. Polaris is easily visible to the naked eye with an apparent magnitude of around 2.0, though it is not the brightest star in the night sky.
How to Use the Big Dipper as a Guide
The Big Dipper is a useful tool for locating Polaris, serving as a celestial signpost that is visible year-round for most northern observers. To find the North Star, first identify the two stars that form the outer edge of the Dipper’s bowl, known as the “Pointer Stars.” These stars are named Merak and Dubhe. Merak is the star at the bottom of the bowl, and Dubhe is the star farthest from the handle.
Draw an imaginary straight line starting from Merak, passing through Dubhe, and continuing outward into the dark sky. The distance between Merak and Dubhe serves as the unit of measure. Following this line approximately five times that distance will lead directly to Polaris. This technique is reliable regardless of the Big Dipper’s orientation in the sky, as the asterism rotates around Polaris throughout the night.
Identifying Polaris, the True North Star
Once located using the Big Dipper’s pointers, Polaris reveals its identifying characteristics. It is positioned almost directly above the Earth’s north rotational axis, a point known as the north celestial pole.
This alignment means that as the Earth rotates, all other stars appear to move in circles across the sky, but Polaris remains nearly motionless. It traces out only a very small circle around the celestial pole over the course of a night. This stable position is why Polaris has been historically important for navigation, as it always indicates the direction of true north in the Northern Hemisphere. Navigators can determine their approximate latitude by measuring the angle of Polaris above the horizon. Polaris is not a single star but a multiple-star system.