Constellation

Lore & Background

The origins of the zodiac remain historically uncertain; its astrological divisions became prominent c. Constellations appear in Western culture via Greece and are mentioned in the works of Hesiod, Eudoxus, and Aratus. The traditional 48 constellations, consisting of the zodiac and 36 more (now 38, following the division of Argo Navis into three constellations), are listed by Ptolemy, a Greco-Roman astronomer from Alexandria, Egypt, in his Almagest. The formation of constellations was the subject of extensive mythology, most notably in the Metamorphoses of the Latin poet Ovid. 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. 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's name. Other star patterns or groups called asterisms are not constellations under the formal definition, but are also 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.

Reader's Guide

Constellations serve as a fundamental framework for organizing the night sky, enabling astronomers and navigators to identify stars and celestial objects more easily. This system replaced earlier, more variable constellations that differed across cultures and changed over time. The zodiac, consisting of twelve or thirteen ancient constellations straddling the ecliptic, remains significant in astrology and astronomy, with its origins historically uncertain but prominent by c. Constellations also provide a basis for naming conventions, such as the Flamsteed designation of stars. Additionally, asterisms—star patterns not formally recognized as constellations—aid observers in navigating the night sky. Over long timescales, constellations slowly change due to the independent motion of stars, becoming unrecognizable after tens to hundreds of thousands of years, though astronomers can predict past or future outlines using astrometry and spectroscopy.

Did You Know?

• The word 'constellation' comes from the Late Latin term cōnstellātiō, meaning 'set of stars,' and came into use in Middle English during the 14th century.
• A constellation or star that never sets below the horizon when viewed from a particular latitude on Earth is termed circumpolar.
• The traditional 48 constellations listed by Ptolemy in the Almagest now number 38 after the division of Argo Navis into three constellations.