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THE BIG DIPPER - Roadmap of the Northern Sky

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TO KNOW
 

To begin with, the Big Dipper is NOT a constellation. It is what is known as an "asterism". A pattern of stars found within a larger constellation or spread among several constellations. The Big Dipper is part of the constellation Ursa Major (The Big Bear). Ursa Major is one of the 88 officially recognized constellations spanning the Northern and Southern skies and is the third largest constellation. Another asterism is the "Teapot" in the constellation of Sagittarius, as well as the "Summer Triangle", spanning an area from the stars Vega, Altair and Deneb in the constellations Lyra, Aquila and Cygnus respectively. That's about 38° on it's longest length! It's huge! There are other asterisms, but that is a subject for another page.


Image Source: Stellarium
Move cursor over image to see asterism outline.

Variously called the "Big Dipper" (America), "The Plough" (Britain), or many other names, the Big Dipper is one of the most easily recognizable groups of stars in the sky. Since it's circumpolar (appearing to travel around the pole star and never setting) it is therefore visible in northern skies year-round.

Finding other constellations using the Big Dipper

If you can find the big dipper in the sky, you have a starting point for identifying many other stars. Learn to use it as a starting point for the finding other constellations.

The Pointers: The two stars forming the front edge of the Big Dipper's bowl (on the side away from the handle) point to Polaris, the north star, in the constellation Ursa Minor (the Little Bear). Polaris is a rather faint star about five times farther away than the distance between the pointers themselves.

If you continue on this line from the Pointers on past Polaris, at an equal distance opposite the big dipper, you will intersect Cassiopeia. Cassiopeia, Queen of Ethiopia, is a stretched out W-shaped constellation reclining in the starry band of the Milky Way. Cassiopeia is circumpolar, like the Big Dipper, and therefore is a familiar constellation, easily learned, visible no matter what the season of time of night from most of the United States. Cassiopeia may also be found by tracing a line from Mizar (ζUMa) (the second star of the Big Dipper's handle) through Polaris.

Trace a line from the Pointers of the Big Dipper to Polaris and past Cassiopeia, and you will come to a large, nearly perfect square of four stars (almost directly overhead in autumn) called the Great Square of Pegasus (Pegasus was a flying horse). At one corner of the Square of Pegasus is Andromeda (daughter of Cassiopeia). The constellation Andromeda contains the Andromeda galaxy, also known as M31. The Andromeda galaxy is relatively close to the Milky Way, and is a bit larger than our own galaxy.

"Arc to Arcturus." Follow the curve of the Big Dipper's handle away from the bowl to the fourth brightest star in the earth's sky, Arcturus, of the ancient constellation Boötes (pronounced "boo-oh-tees"). Boötes is a herdsman, or shepherd, and is found in cave paintings commemorating successful hunts of gazelles, zebras, and giraffes in the Sahara--this constellation was named before the Sahara became a desert. Arcturus is best seen in late summer.

Continue past Arcturus on the same curve away from the Dipper's handle. After going the same distance again as it took to reach Arcturus, you will come (if it's not below the horizon) to a bright star of the constellation Virgo called Spica (spy-ka). "Speed on to Spica!" is a handy way to remember this. Alternatively, the phrase "Spike to Spica" refers to Spica's usual location near the horizon. Spica lies nearly on the ecliptic--the path the Sun follows across the sky. Spica may have an occasional bright visitor nearby--a planet wanderer, not a permanent resident!

Return to the bowl of the Dipper. A line running through the two stars nearest the handle points almost directly to two other notable stars. Pointing down beneath the bottom of the Dipper bowl the line would take you to Regulus, the brightest star in the constellation Leo (a lion, whose mane looks like a backward question mark--Regulus is the "dot" at the bottom of the mark).

In the other direction, pointing above the open bowl, the line runs to Deneb, in the constellation Cygnus (the Swan, which looks like a cross). Deneb is the tail of the swan, which is flying south for the winter along the Milky Way. Deneb, together with two other stars (Vega and Altair) form the summer triangle, an asterism which dominates the night sky all summer long.

Look at the photo at the top of this page. Look at the second star from the end of the Dipper's handle. Can you see anything unusual about it?

Have binoculars? Test your eyesight by looking at the second star again. Look closely, and you may see two stars, which have been called the Horse and Rider. According to the Greeks, the second star is one of the Pleiades sisters, who left her six sisters over in Taurus when she married. Mizar, the brightest of the two, resolves into a double star (A & B) in a large telescope. Interestingly, from spectroscopic evidence it is known that Mizar A and Mizar B are each double stars as well, although these pairs are not resolvable by existing telescopes.




On The Hunt With Orion - Finding More Objects

Likewise, the constellation of Orion (the Hunter) [see image below] can be used as a sign post for the southerly viewed sky. While Orion is not a circumpolar constellation, it does however dominate the sky when looking toward the south in North America from winter through spring. It can be used much like the Big Dipper stars are used to find other objects of interest in that part of the night sky.


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