The Big Dipper
| Basic Celestial
Constellation: URSA MAJOR (The Big Bear)
Asterism: The Big Dipper (Plough, Drinking Gourd)
Ursa Major (Big Bear) is
the third largest of the 88 officially-recognized
stars form a smaller asterism within the
constellation, variously called the "Big Dipper" (America), "Plough"
(Britain), or many other names. The Big Dipper is one of the most
easily recognizable groups of stars in the sky, being
setting below the horizon) and therefore visible in northern skies
The "handle" of the dipper represents the "tail" of the bear on
ancient star maps--even though modern bears don't have long tails! A
Greek story (the Big Bear was so-called before Homer) explains that
Zeus fell in love with a mortal woman named Callisto, who was a
far-traveler and a huntress. His wife Hera became jealous, and
changed Callisto into a large bear. When Callisto failed to return
after a long journey, her son Arcas set out to find her, and in a
forest one day met a huge bear. To his horror, the bear started to
run toward him. Not perceiving it was his mother, Arcas fitted an
arrow to his bow and was about to slay the bear, when Zeus, to avert
the impending tragedy, changed Arcas into a smaller bear (he could
not undo the spell of Hera). Then Zeus grabbed both bears by the
tails, swung them around (thus stretching their tails out), and
hurled them into the sky where they would be safe and immortal.
However, Hera had the last word, moving them to the portion of the
sky that never sets, so that until the end of the world Callisto and
Arcas must endure weariness without rest.
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.
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 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
- 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
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
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?
- Is it visible in the OBU planetarium?
- 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
(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.
Star chart created with Voyager II Software for Macintosh,
published by Carina Software. This is just a taste of what Voyager
can do! For info on Voyager II software, call Carina Software at
(510) 355-1266, write them at 12919 Alcosta Blvd Suite #7, San Ramon,
CA 94583, or visit Carina Software's
home page and check out
Voyager II for yourself.
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