The Big Dipper

Ursa Major's Most Famous Star Configuration

The Big Dipper
Aaron McCoy / Getty Images

The Big Dipper is one of the most well-known configuration of stars in the northern celestial sky  and the first one many people learn to identify. It is not actually a constellation, but rather an asterism consisting of seven of the brightest stars of the constellation, Ursa Major (Great Bear). Three stars define the handle of the dipper, and four stars define the bowl. They represent the tail and hindquarters of Ursa Major.

The Big Dipper is well-known in many different cultures, although by different names: in England it is known as the Plough; in Europe, the Great Wagon; in the Netherlands, the Saucepan; in India it is known as the Saptarishi after the seven ancient holy sages. 

The Big Dipper is located near the north celestial pole (almost the exact location of the North Star) and is circumpolar in most of the northern hemisphere beginning at 41degrees N. latitude (the latitude of New York City), and all latitudes farther north, meaning it does not sink below the horizon at night. Its counterpart in the southern hemisphere is the Southern Cross.

Although the Big Dipper is visible all year in northern latitudes its position in the sky changes - think “spring up and fall down.” In the spring the Big Dipper rises higher in the northeast part of the sky, but in the autumn it falls lower in the northwestern sky and may even be hard to spot from the southern part of the United States before it sinks below the horizon.

To see the Big Dipper completely you need to be north of 25 degrees S. latitude.

The Big Dipper’s orientation also changes as it rotates counterclockwise around the north celestial pole from season to season. In the spring it appears high in the sky upside down, in summer it appears to be hanging by the handle, in autumn it appears close to the horizon right side up, in winter it appears to be hanging by the bowl.

BIG DIPPER AS A GUIDE

Because of its prominence The Big Dipper has played a key role in navigational history, enabling people throughout the centuries to easily locate Polaris, the North Star, and thereby plot their course. To find Polaris, you need only extend an imaginary line from the star at the bottom of the front of the bowl (furthest from the handle), Merak, to the star at the top of the front of the bowl, Dubhe, and beyond until you reach a moderately bright star about five times that distance away. That star is Polaris, the North Star, which is, itself, the end of the handle of the Little Dipper (Ursa Minor) and its brightest star. Merak and Dubhe are known as the Pointers, because they always point to Polaris.

Using the Big Dipper as a starting point can also help you locate multiple other stars and constellations in the night sky.

According to folklore the Big Dipper was instrumental in helping fugitive slaves of the pre-Civil War era from Mobile, Alabama in the southern United States find their way north to the Ohio River and freedom, as portrayed in the American folksong, “Follow the Drinking Gourd.”  The song was originally published in 1928, and then another arrangement by Lee Hays was published in 1947, with the signature line, “For the old man is awaiting for to carry you to freedom.” The “drinking gourd,” a water dipper commonly used by slaves and other rural Americans, was code name for the Big Dipper.

Although the song has been taken at face value by many, when looked at for historical accuracy there are many weaknesses.

STARS OF THE BIG DIPPER

The seven major stars in the Big Dipper are the brightest stars in Ursa Major: Alkaid, Mizar, Alioth, Megrez, Phecda, Dubhe,and Merak. Alkaid, Mizar, and Alioth form the handle; Megrez, Phecda, Dubhe, and Merak form the bowl. The brightest star in the Big Dipper is Alioth, at the top of the handle near the bowl. It is also the brightest star in Ursa Major and the thirty-first brightest star in the sky.

Five of the seven stars in the Big Dipper are believed to have originated together at the same time from a single cloud of gas and dust and they move together in space as part of a family of stars. These five stars are Mizar, Merak, Alioth, Megrez, and Phecda.

They are known as the Ursa Major Moving Group, or Collinder 285. The other two stars, Dubhe and Alkaid, move independently of the group of five and of each other.

The Big Dipper contains one of the most famous double stars in the sky. The double star, Mizar and its fainter companion, Alcor, are known together as “the horse and rider,” and are each themselves actually double stars, as revealed through a telescope. Mizar was the first double star to be discovered through a telescope, in 1650. Each has been spectroscopically shown to be a binary star, held together to its companion by gravity, and Alcor and Mizar are binary stars themselves. This all means that in the two stars that we can see in the Big Dipper side by side with our naked eye, assuming it is dark enough that we can see Alcor, there are in reality six stars present.

DISTANCES TO THE STARS

Although from Earth we see the Big Dipper as though it is on a flat plane, each of the stars is actually a different distance from earth and the asterism lies in three dimensions. The five stars in the Ursa Major Moving Group - Mizar, Merak, Alioth, Megrez, and Phecda - are all about 80 light years away, varying by “only” a few light years, with the greatest difference between Mizar at 78 light-years away and Phecda at 84 light-years away. The other two stars, however are further away: Alkaid is 101 light-years away, and Dubhe is 124 light-years away from Earth.

Because Alkaid (at the end of the handle) and Dubhe (at the outer rim of the bowl)  are each moving in their own direction, the Big Dipper will look noticeably different in 90,000 years than it does now. While that may seem like a very long time, and it is, it is because planets are very far away and revolve very slowly around the center of the galaxy, seeming not to move at all during an average human lifespan. However, the celestial skies do change, and the Big Dipper of our ancient ancestors 90,000 years ago was vastly different from the Big Dipper we see today and the one that our descendants, if they exist, will see 90,000 years from now.

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