Constellations are a group of stars that form patterns in the sky. Constellations played a significant role in navigation for explorers of the Earth; these navigators created extensive star charts to help them find their way around the planet. The word “constellation” comes from the Late Latin term cōnstellātiō, which can be translated as “set of stars”; it came into use in Middle English during the 14th century.
Ancient humans spent a lot of time observing star patterns in the sky. They identified clusters of stars as gods, goddesses, heroes, animals, and objects. They also created stories to go along with these star patterns, which became the basis for many of the myths passed through centuries by the Greeks, Romans, Polynesians, Indigenous Americans, and members of various African tribes and Asian cultures. Most of the constellation names we know came from the ancient Middle Eastern, Greek, and Roman culture. In some cases, the constellations may have had ceremonial or religious significance. In other cases, the star groupings helped to mark the passage of time between planting and harvesting.
In 1930 the International Astronomical Union (IAU) officially listed 88 modern and ancient constellations in the Northern and Southern Hemispheres of the sky. In 1928 adopted official constellation boundaries that together cover the entire celestial sphere. It is roughly based on the traditional Greek constellations listed by Ptolemy in his Almagest in the 2nd century and Aratus’ work Phenomena, with early modern modifications and additions by Petrus Plancius (1592, 1597/98 and 1613), Johannes Hevelius (1690) and Nicolas Louis de Lacaille (1763), who named fourteen constellations and renamed a fifteenth one.
36 modern constellations predominantly lie in the northern sky, while 52 are found in the southern celestial hemisphere. Most constellations (more than 40) represent animals. Many were named after humans or figures from mythology, while some depict inanimate objects.
Constellations are typically grouped around asterisms, which are patterns formed by bright stars that appear close to each other in the night sky. These asterisms are often the most conspicuous parts of constellations, which is why the term constellation is still colloquially (and incorrectly) used synonymously with asterism. The constellations themselves are much larger than asterisms and occupy considerably larger areas. For example, the Big Dipper, Little Dipper and Southern Cross are not constellations. They are asterisms formed by the brightest stars of the constellations Ursa Major, Ursa Minor, and Crux.
Zodiac Constellations are constellations that lie along the plane of the ecliptic. The ecliptic, or the apparent path of the Sun, is defined by the circular path of the Sun across the sky, as seen from Earth. In other words, the Sun appears to pass through these constellations over the course of a year.
There are 12 constellations in the zodiac family. They are: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius and Pisces.
The northern zodiac constellations – Pisces, Aries, Taurus, Gemini, Cancer and Leo – are located in the eastern celestial hemisphere, while the southern – Virgo, Libra, Scorpius, Sagittarius, Capricornus and Aquarius – are found in the west.
Some prominent constellations include:
Ursa Major constellation lies in the northern sky. Its name means “the great bear,” or “the larger bear,” in Latin. The smaller bear is represented by Ursa Minor. Ursa Major is the largest northern constellation and third largest constellation in the sky. Its brightest stars form the Big Dipper asterism, one of the most recognizable shapes in the sky, also known as the Plough. Ursa Major is well-known in most world cultures and associated with a number of myths. It was one of the constellations catalogued by the Greek astronomer Ptolemy in the 2nd century. In Greek mythology, it is associated with Callisto, a nymph who was turned into a bear by Zeus’ jealous wife Hera.
Cassiopeia is one of the most easily recognizable constellations in the northern night sky. Nicknamed the W constellation, Cassiopeia is easily recognizable for the prominent W asterism formed by its five brightest stars. The constellation is named after the vain queen Cassiopeia in Greek Mythology, wife of the King Cepheus of Aethiopia. Cassiopeia can be found high in the northeastern sky on October evenings, not far from Polaris, the North Star.
Andromeda constellation is located in the northern sky, between Cassiopeia’s W asterism and the Great Square of Pegasus. The constellation was named after the mythical princess Andromeda, the daughter of Queen Cassiopeia and wife of the Greek hero Perseus. It is also known as the Chained Maiden, Persea (wife of Perseus), or Cepheis (daughter of Cepheus).
Pegasus is one of the most prominent constellations in the northern sky. It was listed by the astronomer Ptolemy during the 2nd century and was named after a winged horse in Greek mythology. The brightest star in the constellation is Epsilon Pegasi, which forms the creature’s nose. Pegasus belonged to Poseidon, the god of the sea, earthquakes, and storms. In a battle between Perseus and Medusa, Perseus decapitated her and the winged horse “sprang” from her blood. In the Northern Hemisphere, the Pegasus constellation can be found high in the sky from the end of summer through autumn.
Orion is one of the brightest and best known constellations in the night sky and lies on the celestial equator. It is named after Orion the hunter in Greek mythology. In mythology, Orion was a supernaturally gifted hunter, and the son of Poseidon. He proclaimed himself as the greatest hunter in the world. This angered Zeus’s wife Hera, who had a scorpion kill him. Out of compassion, Zeus put Orion into the sky. Located on the celestial equator and made up of bright young blue giants or supergiants, it is one of the most prominent and recognizable constellations in the sky and can be seen throughout the world. Orion’s Belt includes the three most prominent stars in the constellation: Alnilam, Mintaka, and Alnitak. Orion is clearly visible in the night sky from November to February.
Each Latin constellation name has two forms: the nominative, for use, when talking about the constellation itself, and the genitive, or possessive, used in star names. For instance: Hamal, the brightest star in the constellation Aries (nominative form), is also called Alpha Arietis (genitive form), meaning literally “the alpha of Aries”.
The 88 officially recognized constellations are visible at different times throughout the year. Each season has distinctive star patterns because the visibility of stars in the sky change as the Earth orbits the Sun. The Northern and Southern Hemisphere skies are very different from each other, and there are some patterns in each that cannot be viewed between hemispheres. In general, most people can see about 40-50 constellations over a year.
Most people can see more than half of them throughout the year, though it can depend on where they live. The best way to learn them all is to observe throughout the year and study the individual stars in each constellation. To identify the constellations, most observers use star charts found online and in astronomy books. Others use planetarium software such as Stellarium or an astronomy app.