Throughout the centuries man has looked skyward with both curiosity and awe. Long before space travel became possible, history has shown us mankind’s fascination with what lay up in the sky. Whether stars, moon, or even clouds could be seen there have been stories written, myths generated and actions taken based on what was up above. These stories and myths are unique to each culture throughout the world but even today many casual astronomical observers may wonder what is a constellation? Here is a brief description and history of constellations-
A constellation is a group of celestial bodies that are connected together in some arrangement. Typically these stars form a visible figure or picture. The term can also be used less formally to mean any group of stars that are visibly related to each other, if they are considered as a fixed configuration or pattern. Some well-known constellations contain striking and familiar patterns of unusually bright stars. For example: Orion (containing a figure of a hunter), Leo (containing bright stars outlining the form of a lion), Scorpius (a scorpion), and Crux (a cross).
There is a slightly different meaning of constellation used by astronomers. Astronomers call a group of stars that can be connected to form a figure or a picture an asterism, while a constellation is an area on the sky. The International Astronomical Union (IAU) divides the sky into eighty eight official constellations each with exact boundaries, so that every direction or place in the sky belongs within one constellation. These groupings are mostly based upon the constellations of the ancient Greek tradition, passed down through the Middle Ages, and all contain the signs of the zodiac. According to this method the sun passes through all of the twelve constellations of the zodiac (plus Ophiuchus). Pagan and ancient Greek astronomers believed they each had a special significance.
Eugene Delporte drew up the constellation boundaries in 1930. He drew them along vertical and horizontal lines of right ascension and declination. However, the proposal that he based his work on was from Benjamin A. Gould works. The consequence of basing his boundaries on this work is that due to precession of the equinoxes, the borders on a modern star map are already somewhat skewed and no longer perfectly vertical or horizontal. This skew will continue to increase over the years and centuries to come.
It is interesting to note that a star pattern may be widely known but may not be recognized by the International Astronomical Union. An example is the commonly known grouping called the Big Dipper (in North America) or the Plough (in the United Kingdom). Astronomers feel that the stars in this constellation or asterism rarely have any astrophysical relationship to each other; they just happen to appear close together in the sky as viewed from Earth and typically lie many light-years apart in space.
The ancient Greek had works which dealt with the constellations which were known as books of star myths. The oldest of these works was a poem composed by Hesiod in circa 8th century BC of which only fragments survive. Most Greeks felt that these constellations were superstitious. The most complete existing works dealing with the mythical origins of the constellations are by the Hellenistic writer Eratosthenes and an early Roman writer known as Hyginus.
Another interesting note is that there are other types of constellations besides stars. For example: The “Emu in the sky”, is a ‘constellation’ defined by dark clouds rather than the stars. The head of the emu is the Coalsack. Members of the Inca civilization identified various dark areas in the Milky Way as animals, and learned to associate their appearance with the seasonal rains. These types of areas are commonly referred to by modern researchers as dark cloud constellations or dark nebulae.