In colloquial usage, a constellation is a group of celestial bodies, usually stars, which appear to form a pattern 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, Eugène Delporte aided the IAU in dividing the celestial sphere into 88 official constellations. Typically, these modern constellations 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.