Constellation

A constellation is a group of stars that forms an imaginary outline or pattern on the celestial sphere, typically representing an animal, mythological person or creature, a god, or an inanimate object. The origins of the earliest constellations likely go back to prehistory. People used them to relate stories of their beliefs, experiences, creation, or mythology. Different cultures and countries adopted their own constellations, some of which lasted into the early 20th century before today's constellations were internationally recognized. Adoption of constellations has changed significantly over time. Many have changed in size or shape. Some became popular, only to drop into obscurity. Others were limited to single cultures or nations. The 48 traditional Western constellations are Greek. They are given in Aratus' work Phenomena and Ptolemy's Almagest, though their origin probably predates these works by several centuries. Constellations in the far southern sky were added from the 15th century until the mid-18th century when European explorers began traveling to the Southern Hemisphere. Twelve ancient constellations belong to the zodiac (straddling the ecliptic, which the Sun, Moon, and planets all traverse). The origins of the zodiac remain historically uncertain; its astrological divisions became prominent c. 400 BC in Babylonian or Chaldean astronomy,. In 1922, the International Astronomical Union (IAU) formally accepted the modern list of 88 constellations, and in 1928 adopted official constellation boundaries that together cover the entire celestial sphere. Any given point in a celestial coordinate system lies in one of the modern constellations. Some astronomical naming systems include the constellation where a given celestial object is found to convey its approximate location in the sky. The Flamsteed designation of a star, for example, consists of a number and the genitive form of the constellation name. Other star patterns or groups called asterisms are not constellations per se but are used by observers to navigate the night sky. Asterisms may be several stars within a constellation, or they may share stars with more than one constellation. Examples of asterisms include the Pleiades and Hyades within the constellation Taurus and the False Cross split between the southern constellations Carina and Vela, or Venus' Mirror in the constellation of Orion. The word 'constellation' comes from the Late Latin term cōnstellātiō, which can be translated as 'set of stars'; it came into use in English during the 14th century. The Ancient Greek word for constellation is ἄστρον. These terms generally referred to a recognisable pattern of stars whose appearance is associated with mythological characters or creatures, earthbound animals, or objects. A more modern astronomical sense of the term 'constellation' denotes one of the officially recognized 88 named constellations used today. Colloquial usage does not draw a sharp distinction between 'constellations' and smaller 'asterisms' (pattern of stars), yet the modern accepted astronomical constellations employ such a distinction. E.g., the Pleiades and the Hyades are both asterisms, and each lies within the boundaries of the constellation of Taurus. Another example is the northern asterism popularly known as the Big Dipper (US) or the Plough (UK), composed of the seven brightest stars within the area of the IAU-defined constellation of Ursa Major. The southern False Cross asterism includes portions of the constellations Carina and Vela and the Summer Triangle is composed of the brightest stars in the constellations Lyra, Aquila and Cygnus. A constellation (or star), viewed from a particular latitude on Earth, that never sets below the horizon is termed circumpolar. From the North Pole or South Pole, all constellations south or north of the celestial equator are circumpolar. Depending on the definition, equatorial constellations may include those that lie between declinations 45° north and 45° south, or those that pass through the declination range of the ecliptic or zodiac ranging between 23½° north, the celestial equator, and 23½° south.