Constellations serve to locate and identify objects in the night sky and many have been known since ancient times. Currently, the International Astronomical Union recognises 88 constellations distributed across the celestial sphere with formally recognised boundaries between the constellations. These constellations are often named by the pictorial representations created by the stars of each constellation, but sometimes a great deal of imagination is needed to see the image. A handful of these constellations hold special significance and are still seen in the increasingly light-polluted skies of the northern hemisphere.
The constellation Ursa Major, or "Big Bear," contains an asterism--a special grouping of stars that is not a constellation--called the Big Dipper or the Plough. This star formation consists of stars at each corner of a box attached to a long handle consisting of additional stars resembling a large spoon or plough. Following an imaginary line from the two stars forming the outside edge of the box, named Dubhe and Merak, leads to a faint star named Polaris, or the North Star, located in Ursa Minor.
Ursa Minor, or "Little Bear," contains the asterism the Little Dipper. At the end of the handle of the little dipper holds lies Polaris, or the North Star. Polaris lies near the North Celestial Pole, the point in the sky the other stars appear to rotate. It should be noted that the nightly movement of the starry sky is due to the Earth's rotation and not the stars themselves. Polaris has been used since antiquity to locate the direction of north in the night sky throughout the northern hemisphere.
While the Big Dipper can be used as a north-finding asterism, on the opposite side of Polaris lies the constellation Cassiopeia, with another north-finding asterism called the Lazy W. The Lazy W receives its name from five stars within the constellation that appear to form the points of a sloppy letter W (or M depending on the orientation). An observer can use the middle point of the W to draw an imaginary line to the vicinity of Polaris.
Perhaps one of the most recognisable constellations in the northern hemisphere is the constellation Orion, associated with the image of a hunter. Three relatively bright stars form Orion's belt and can be used to orient Orion and easily locate two of the brightest stars in the sky, Betelgeuse in Orion's shoulder and Rigel in Orion's knee. Below Orion's belt lie stars commonly associated with Orion's scabbard or sword. However, upon careful observation with the unaided eye, the middle of these stars may appear fuzzy and is actually the Great Orion Nebula, a cloud of dust where stars are being born.
Many hunters are accompanied by hunting dogs, and Orion is no different. At Orion's side lie the constellations Canis Major and Canis Minor. Canis Major, or "Big Dog," features Sirius, the brightest star in the sky after our sun. Both constellations can be found to the observer's left of Orion, with Canis Major above Canis Minor.
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