Finding your way around the sky using the Big Dipper
Ursa Major (Big Bear) is the third largest of the 88 officially-recognized constellations. Several stars form a smaller asterism within the constellation, variously called the "Big Dipper" (America), "Plough" (Britain), "Wagon" (Europe) or many other names. The Big Dipper is one of the most easily recognizable groups of stars in the sky, being circumpolar (never setting below the horizon) and therefore visible in northern skies year-round.
For skylore and images see the Big Dipper asterism page.
Observe:
Notice whether the dipper would be full of water, or whether it's upside down in the sky. Some Native Americans associated the dipper with the colors of autumn leaves, poured out in the fall by the upside down bowl.
You can learn, as did ancient sailors or western cowhands on the night watch, to tell the time of the night by the position of the big dipper. Due to the daily rotation of the earth, the dipper rotates around the north star (Polaris) every twenty four hours.
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 skymark for the following constellations. Practice tracing from the Big Dipper by finding them on your planisphere--but remember that the shapes of constellations are distorted as you move toward the outer edge of the planisphere, and lines that appear straight in the real sky will not look straight on the flat surface of the planisphere.
- The Pointers: The two stars forming the pouring 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, Little Bear. Polaris is a rather faint star about five times farther away than the distance between the pointers themselves. No matter where you are in the northern hemisphere, when you face Polaris you will be facing north. Polaris marks north more accurately than a magnetic compass. The angle between your horizon and Polaris is equal to your latitude on earth (can you prove this geometrically?).
- 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 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 Epsilon (the first 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.
- Is the Andromeda galaxy marked on your planisphere?
- Can you see it with binoculars or your unaided eyes?
- "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 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.