The Big Dipper is an asterism, a recognizable pattern of seven bright stars, that lies entirely within the larger constellation Ursa Major (the Great Bear). Three stars form the handle and four form the bowl. Known across cultures as the Plough and Charles’s Wain, the Big Dipper is one of the most familiar sights in the Northern Hemisphere night sky. Its distinct shape has made it a guidepost for navigators and a focus of folklore for thousands of years.
Seasonal and Hourly Movement
The visibility of the Big Dipper changes due to Earth’s rotation and its annual orbit around the Sun. On any given night, the asterism appears to rotate counter-clockwise around the North Star, Polaris, completing a full circle every 23 hours and 56 minutes. This daily motion shifts the Big Dipper’s orientation about 15 degrees every hour.
The Big Dipper’s position at the same time each evening also changes throughout the year because of Earth’s revolution. The pattern shifts approximately 30 degrees each month. In the Northern Hemisphere, the Big Dipper shines highest in the evening sky during spring and early summer.
During autumn and winter evenings, the Big Dipper sweeps closer to the northern horizon. While the asterism is always “up” for many northern observers, its height and angle vary significantly from month to month. The difference in position between a summer and winter evening, when viewed at the same time, is about 180 degrees around Polaris.
How Latitude Affects Visibility
The visibility of the Big Dipper every night depends entirely on geographic latitude. For observers in the mid-to-high Northern Latitudes, the Big Dipper is circumpolar, meaning it never sets below the horizon. This occurs because of its proximity to the North Celestial Pole.
To see the entire Big Dipper remain above the horizon, an observer needs to be situated north of roughly 41 degrees North latitude. At these latitudes, the asterism is always revolving around Polaris, making it a constant fixture in the night sky. Closer to the equator, the Big Dipper’s circumpolar status changes.
South of about 35 degrees North latitude, the Big Dipper will dip below the horizon for a period of time each night. For example, observers near 32 degrees North will find the asterism hidden for a few hours daily. Below approximately 25 degrees North, the Big Dipper becomes non-circumpolar, meaning there are several weeks out of the year when it is not visible at all.
Using the Big Dipper to Navigate the Sky
Once located, the Big Dipper is a tool for finding other celestial objects. Its most famous use is locating Polaris, the North Star, which marks the direction of true north. This is done by identifying the two stars at the outer edge of the Dipper’s bowl, known as the “Pointer Stars.”
By drawing an imaginary straight line through these Pointer Stars and extending it outward, the first moderately bright star encountered is Polaris. This line points almost directly to the North Star, regardless of the Dipper’s orientation. Polaris is the star at the end of the handle of the Little Dipper, which helps confirm the identification.
The Big Dipper’s curved handle can also be used to locate other bright stars. If you follow the arc of the handle away from the bowl, you will arrive at the reddish-orange star Arcturus in the constellation Boötes. Continuing that curved line onward leads to the bluish-white star Spica in the constellation Virgo.