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The mythological connection
In Greek mythology, Ursa Major is the nymph Callisto, who was
placed in the heavens by Zeus in the form of a bear together with
her son Arcas (Ursa Minor) as “bear keeper” or “Arcutus”. The Greeks
therefore named it “Arctos”, the she-bear, or Helice, from its
turning around Polaris, the Pole star.
Ptolemy catalogued eight of the
constellation’s stars. Of these, the seven brightest constitute one
of the most characteristic figures in the northern sky and have
received various names - Septentriones, the Wagon, Plow, Big Dipper
and Charles’ Wain. For the Hindus, these seven stars represented the
seven Rishis (or sages). Two of the constellation’s stars, Dubhe and
Merak, are called pointers because the line Merak-Dubhe points to
the Pole star.
Five stars of the constellation form an
associated group with common proper motion, but Dubhe (the upper
pointer) and Alcaid (the last star of the tail) have no connection
with the others. Stars like “Sirius” in other parts of the sky have
been found to belong to the same cluster.
Asterisms of Ursa Major and Ursa Minor
Some well-known groups of stars form only part of a
constellation and are called Asterisms. The Big and Little
Dippers are examples of asterisms.
The Big Dipper forms part of the
Ursa Major and is shaped like a long handled cup, or dipper. The cup
forms the hindquarters of the bear, the handle forms the tail, and
fainter stars outline the head and legs. It consists of seven stars
that can be used to point to other parts of the sky. For example, an
imaginary line extended northward from the two stars at the front of
the cup points to the Polaris. The line points to Leo when extended
southward. The stars Mizar (at the centre of the handle) and its
companion star named Alcor have been used for hundreds of years as a
test for keen eyesight. Viewed through a telescope, Mizar appears as
two stars.
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The Big Dipper shown with five of the brighter stars
joint with a faint line. |
The Little Dipper is an asterism of
seven stars of almost the entire constellation of Ursa Minor. It
consists mostly of faint stars, visible only in a very dark sky. The
Little Dipper has been an important indicator of the north, because
Polaris, at the end of the Little Dipper’s handle, marks roughly the
position of the north celestial pole. The constellation lies at
about 15 hours right ascension and 80o north
declination.
Position of the Dippers:
On January evenings, the Little Dipper is west of the Big
Dipper. The Little Dipper’s handle points upward while that of the
Big Dipper’s, down. These positions are reversed on July evenings.
These positions also change at night, owing to the rotational
movement of the earth.
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