Constellation

A constellation is a defined region of the sky where a set of visible stars creates a recognizable shape, often linked to a mythological figure, animal, or object. People likely first identified these patterns in prehistoric times, using them to tell stories about their beliefs, history, and the world around them. Many cultures and nations developed their own constellations; some of these persisted into the early 1900s before a global standard was adopted. Over centuries, constellations changed in size and form, with some gaining popularity and later fading, while others remained unique to a single society. Naming these star groups also made it simpler for astronomers and navigators to locate specific stars.

The word "constellation" comes from the Late Latin *cōnstellātiō*, meaning "set of stars," and entered Middle English in the 1300s. The Ancient Greek term was *ἄστρον* (astron). Historically, these terms referred to any recognizable star pattern tied to myths, creatures, or objects. Over time, European astronomers refined and standardized the constellations. In the 20th century, the International Astronomical Union (IAU) officially recognized 88 constellations, each with a Latin name. A constellation or star that never sets below the horizon from a given latitude is called circumpolar. From either pole, all stars south or north of the celestial equator are circumpolar. Depending on the definition, equatorial constellations may lie between 45° north and 45° south declination, or within the zodiac’s range of 23.5° north to 23.5° south.

Stars in a constellation may appear close together in the sky, but they are usually at very different distances from Earth. Because each star moves independently, all constellations slowly change shape over time; after tens or hundreds of thousands of years, familiar outlines become unrecognizable. Astronomers can predict past or future outlines by measuring stars’ proper motions and radial velocities. The 88 official constellations, along with those from various cultures, are imagined figures based on observable star patterns. Many official constellations draw from ancient Near Eastern and Mediterranean myths, with some stories linked to the stars’ positions—for instance, the tale of Orion being killed by Scorpius, with their constellations appearing at opposite times of year.

Constellation positions shift throughout the year because Earth’s night side faces different parts of its orbit. As Earth rotates east, the celestial sphere appears to rotate west, with stars circling counterclockwise around the northern pole star and clockwise around the southern pole star. Due to Earth’s 23.5° axial tilt, the zodiac is spread evenly across both hemispheres along the ecliptic, forming a near-perfect great circle. The northern zodiacal constellations are Pisces, Aries, Taurus, Gemini, Cancer, and Leo; the southern ones are Virgo, Libra, Scorpius, Sagittarius, Capricornus, and Aquarius. The ecliptic is overhead from 23.5° north to 23.5° south. In both hemispheres, the zodiac appears low in summer and high in winter; it descends nightly in spring and rises in autumn. The Milky Way’s galactic plane is tilted 60° from the ecliptic, lying between Taurus and Gemini (north) and Scorpius and Sagittarius (south).

Twelve (or thirteen) ancient constellations form the zodiac, straddling the ecliptic path of the Sun, Moon, and planets. The zodiac’s origins are uncertain, but its astrological divisions became prominent around 400 BC in Babylonian or Chaldean astronomy. These constellations entered Western culture through Greece and appear in works by Hesiod, Eudoxus, and Aratus. The traditional 48 constellations—the zodiac plus 36 others (now 38, after splitting Argo Navis into three)—were listed by Ptolemy in his *Almagest*. Their mythology was extensively developed, notably in Ovid’s *Metamorphoses*. Constellations of the far southern sky were added from the 1400s to the mid-1700s as European explorers reached the Southern Hemisphere.

In 1922, the IAU formally accepted the modern list of 88 constellations, and in 1928 adopted official boundaries that together cover the entire celestial sphere. Every point in a celestial coordinate system lies within one modern constellation. Some astronomical naming systems include the constellation to indicate a celestial object’s approximate location—for example, the Flamsteed designation of a star uses a number and the genitive form of the constellation’s name. Other star patterns, called asterisms, are not formal constellations but are used by observers to navigate the sky. Asterisms may consist of stars within a single constellation or share stars across multiple constellations; examples include the Teapot in Sagittarius and the Big Dipper in Ursa Major.