The Big Dipper is one of the most recognizable star patterns in the night sky, serving as a fundamental guide for sky-watchers in the Northern Hemisphere. This famous pattern is not a constellation itself but an asterism—a prominent group of stars that forms a recognizable shape. The seven stars of the Big Dipper are the brightest members of the much larger constellation Ursa Major, often called the Great Bear. Its distinctive shape and bright appearance have made it an indispensable tool for celestial navigation for centuries.
When and Where to Look
The Big Dipper is known as a circumpolar asterism in most of the Northern Hemisphere, meaning that for many observers north of about 41 degrees latitude, it never fully sets below the horizon. Instead, it appears to endlessly circle the North Star, Polaris, throughout the night and across the year. While it is visible every night from these latitudes, its position and orientation change dramatically.
A helpful mnemonic is “spring up and fall down,” which describes the asterism’s height in the sky during the evening hours. On spring and summer evenings, the Big Dipper rides high overhead, making it easy to spot. During autumn evenings, the asterism sweeps low across the northern horizon.
Recognizing the Distinct Shape
The Big Dipper is composed of seven bright stars that form the distinctive ladle or saucepan shape. Four of these stars create the quadrilateral shape of the “bowl,” while the remaining three form the curved “handle.” The stars are exceptionally bright, with six of the seven shining at the second magnitude, making them prominent even in skies with moderate light pollution.
The stars in the bowl are named Dubhe, Merak, Phecda, and Megrez, and the three stars of the handle are Alioth, Mizar, and Alkaid. A fun detail to look for is the double star system Mizar and Alcor, located in the middle of the handle.
Navigating with the Big Dipper
The most practical use of the Big Dipper is its role as a celestial signpost for locating Polaris, the North Star. The key to this technique lies with the two stars at the outer edge of the bowl, Dubhe and Merak, which are collectively known as the “Pointer Stars.”
To find true north, draw an imaginary straight line beginning at Merak, passing through Dubhe, and continuing outward from the bowl. Extend this imaginary line a distance roughly five times the separation between the two Pointer Stars. The first moderately bright star encountered along this extended line is Polaris.
This star is positioned almost directly above the Earth’s North Pole, meaning that facing Polaris indicates the direction of north. Polaris is also the final star at the end of the handle of the fainter Little Dipper asterism, part of the constellation Ursa Minor.