In modern astronomy, a constellation is an internationally defined area of the celestial sphere. Historically, the term was also used to refer to a perceived pattern formed by prominent stars within apparent proximity to one another, and this practice is still common today.
In colloquial usage, a constellation is a group of celestial bodies, usually stars, which appear to form a pattern or picture in the sky. Astronomers today still utilize the term, though the current system focuses primarily on constellations as grid-like segments of the celestial sphere rather than as patterns. A star-pattern that is not officially classed as a constellation is referred to as an asterism. One famous example is the asterism known as the Big Dipper, a term unused by the International Astronomical Union (IAU) as the stars are considered part of the larger constellation of Ursa Major.
In 1922, Henry Norris Russell aided the IAU in dividing the celestial sphere into 88 official constellations. Where possible, these modern constellations usually share the names of their Graeco-Roman predecessors, such as Orion, Leo and Scorpius. While such celestial formations were originally linked to a mythical event, creature or person, the categorization of the night sky into recognizable patterns was important in early land and naval navigation prior to the invention of the compass during the Age of Discovery. With the technical advancement of astronomy, it became important to move from a pattern-based system of constellations to one based on area-mapping, which led to several historic formations becoming obsolete. Out of the modern constellations, 37 lie predominantly in the northern sky, and the other 51 predominantly in the southern. Since much of the southern sky was not visible to classical astronomers based around the Mediterranean Sea, many of the constellations of the far south were named only in the last 300 years, and have no ancient counterparts.
In 1930, the boundaries between the 88 official constellations were devised by Eugène Delporte along vertical and horizontal lines of right ascension and declination. However, the data he used originated back to epoch B1875.0, which was when Benjamin A. Gould first made the proposal to designate boundaries for the celestial sphere, a suggestion upon which Delporte would base his work. The consequence of this early date is that due to the precession of the equinoxes, the borders on a modern star map, such as epoch J2000, are already somewhat skewed and no longer perfectly vertical or horizontal. This effect will increase over the years and centuries to come.
