On a clear night, away from artificial light, it's possible to see over 5000 stars with the naked eye. These appear to orbit the Earth in a fixed pattern, as if they are attached to a giant sphere that makes one revolution a day.
These objects all move in predictable patterns over constant periods of time, and so people could use them to predict the seasons and to aid in navigation. People probably first formed constellations in order to make maps and calendars easier to understand.
There’s evidence that people may have formed constellations by at least 30,000 BCE, when an image that may have been the constellation Orion was carved into a mammoth tusk.
Other evidence of prehistoric star maps come from cave paintings found in the Lascaux caves in France, which date back to 15,000 BCE. These are thought to depict the Pleiades and Hyades star clusters, which are part of the Taurus constellation.
2. The zodiac ↑
The Sumerians in southern Mesopotamia, which is part of modern-day Iraq, are thought to have developed one of the first written languages in about 3000 BCE. They were also the first to record the names of the constellations sometime before 2000 BCE[5a].
The constellations that were most important to the Sumerians were those that the Sun, Moon, and planets all pass through, these are the zodiacal constellations. The Sun moves relative to these constellations, making one full revolution a year. You can find out which constellation the Sun is in by looking at the last constellation to rise before it, or the first to set after it.
Sumer became part of Babylon in about 1900 BCE. The Babylonians conserved much of the Sumerian's knowledge, including the names of their constellations and their calendar system. By 400 BCE, the Babylonians may have divided the path the Sun takes, known as the ecliptic, into 12 equal segments, one for each month.
Many of the zodiacal constellations originate from Sumer or Babylon. The rest are probably Greek in origin. The first evidence of Greek astronomy comes from Homer's epic poems, the Iliad and the Odyssey, which mention six constellations and the star Sirius, and Hesiod's poem Works and Days, which refers to a calendar system. These are thought to have been composed in about 700 BCE[5b].
Ancient Greek mathematician Thales of Miletus used his knowledge of the movement of the Sun and Moon to predict the solar eclipse of 585 BCE.
The Ancient Greeks traded with Egypt and Babylon and, in around 370 BCE, Ancient Greek astronomer Eudoxus of Cnidus learned the names of at least 48 Babylonian constellations, including those of the zodiac. His work is lost but a guide to his constellations can be found in the poem Phaenomena by Aratus, first published in 275 BCE[5c].
The Greeks also devised their own names for the constellations, naming them after animals, objects, and gods. The constellations that we have now are almost exclusively Greek and Babylonian in origin. The word 'zodiac' derives from the Greek word for animal, since animals represent eleven of the signs.
2.1 The precession of the equinoxes ↑
Ancient Greek astronomer Hipparchus showed that the Sun's path around the ecliptic is gradually changing in about 127 BCE. The constellation the Sun appears in during any particular month moves backwards once every 2000 years or so, with the exact time depending on the size of the constellation. This is known as the precession of the equinoxes.
Hipparchus discovered this by recording the location of several stars and seeing how they had moved, with respect to the path of the Sun, since Ancient Greek astronomers Timocharis and Aristyllus had measured them in the 3rd century BCE.
During Hipparchus' time, the vernal (spring) equinox - the date when incoming solar energy is equal in both hemispheres, half way between the winter and summer solstices - occurred in Aries, but by about 1 CE, it had moved to Pisces. At this rate, it will move into Aquarius in about 2600.
3. The first star catalogues ↑
Hipparchus compiled a star catalogue and devised the magnitude system by 125 BCE. The magnitude system categorises stars according to their brightness. Hipparchus gave the brightest stars a magnitude of one and the faintest stars a magnitude of six.
Dutch astronomer Jacobus Kapteyn extended the magnitude system in 1902. Kapteyn introduced the term 'absolute magnitude'. This allows people to compare stars by stating the magnitude they would have if they were all viewed from an equal distance.
Roman astronomer Ptolemy reproduced many of Hipparchus' constellations in the Almagest written in about 150 CE. All of Ptolemy's 48 constellations are still used today, with the exception of Argo Navis (the Ship Argo), which has been broken up into smaller parts, Carina (the Keel), Puppis (the Stern), and Vela (the Sail)[5d].
In 964, Iranian astronomer 'Abd al-Rahman Al-Sufi published a revised edition of Ptolemy's Almagest, the Book of Fixed Stars. Al-Sufi had determined the magnitudes for most stars and added two images of each of Ptolemy's 48 constellations. He named single stars that did not fit into a constellation after different animals or people[5e].
Sagittarius from Al-Sufi's Book of Fixed Stars. Image credit: 'Abd al-Rahman Al-Sufi/Public domain.
Al-Sufi also made the first recorded observation of Andromeda, the closest spiral galaxy to our own, describing it as a 'little cloud'.
The Almagest was updated once again in 1437, when Iranian astronomer Ulugh Beg determined the new positions of almost 1000 stars. He had previously commissioned one of the largest observatories of the time to be built in Samarkand, Uzbekistan[5f].
In 1536, German astronomer Caspar Vopel added two new constellations, Antinous and Coma Berenices. These were stars that Ptolemy had catalogued but not fitted into constellations[5g].
In 1540, Italian philosopher Alessandro Piccolomini created the first celestial atlas that depicted constellations without accompanying pictures. Piccolomini was also the first to use letters to represent the brightness of each star. German cartographer Johann Bayer modified this system in 1603. It was adopted by Kapteyn's magnitude system, and has been used ever since.
4. The southern skies ↑
Europeans began to map the southern skies in 1595, when Dutch navigators Pieter Dirkszoon Keyser and Frederick de Houtman filled in the gaps around the South Pole while they explored the East Indian oceans.
Keyser and Houtman added twelve new constellations. These included Triangulum Australe (the Southern Triangle), and Indus (the Indian, a term that may have referred to a native of Asia or the Americas), and ten others that were named after animals in the Greek tradition.
These were: Phoenix (a mythical phoenix), Apus (the bird of paradise), Pavo (the peacock), Tucana (the toucan), Grus (the crane), Musca (the fly), Chamaeleon (the chameleon), Volans (the flying fish), Dorado (the dolphinfish), and Hydra (the water snake)[5h].
Houtman originally described Tucana as "the Indian magpie, named Lang in the Indies", which was in fact a Hornbill. However, Flemish astronomer Petrus Plancius named the constellation 'Toucan' on his globe of 1598, which Bayer then used in his star catalogue of 1603[5i].
The 'Southern Birds' from Johann Bayer's Uranometria of 1603: Phoenix (a mythical phoenix), Grus (the crane), Tucana (the toucan), Indus (the Indian, a term that may have referred to a native of Asia or the Americas), Pavo (the peacock), and Hydra (the water snake). Image credit: Johann Bayer/Public domain.
Plancius had added another constellation, Crux (the Southern Cross), in 1589 and two more, Columba (the dove) and Polophylax (the pole-watcher), in 1592, although Polophylax is no longer in use. In 1612, Plancius used modern charts to add eight more constellations, but they were not popular and only two are still in use[5j].
English astronomer Edmond Halley created a catalogue of the southern stars in 1679, from observations at St Helena, in the South Atlantic Ocean.
German astronomer Johannes Hevelius and Polish astronomer Elisabetha Hevelius printed their celestial atlas in 1690. This contained ten new constellations, seven of which are still in use. These are: Canes Venatici (the hunting dogs), Lacerta (the lizard), Leo Minor (the little lion), Lynx (the lynx), Scutum (the shield), Sextans (the sextant), and Vulpecula (the fox)[5k].
French astronomer Nicolas Louis de Lacaille observed fourteen new constellations from his observatory on Table Mountain in South Africa in 1750. Unlike Keyser and Houtman, who had kept to the Greek tradition of naming constellations after animals and people, Lacaille named most of his constellations after scientific and artistic instruments, which he believed to reflect the spirit of the time.
These were: Antlia (the Air Pump), Reticulum (the Net), Fornax Chemica (the chemical furnace), Microscopium (the Microscope), Telescopium (the Telescope), Horologium (the Clock), Pictor (the Painter's Easel), Sculptor (the Sculptor's Workshop), Caelum (the Sculptor's Chisel), Pyxis (the Compass Box), Circinus (the Compasses), Norma (the Carpenter's Square), Octans (the Octant), and finally, Mensa (this is Latin for table and refers to Table Mountain)[5l].
Lacaille was one of the first to break Ptolemy's Argo Navis constellation into the three parts we use today. He also eliminated the constellation Robur Carolinum (the Charles Oak), which was first introduced by Halley to honour King Charles II.
Constellations depicted in 1825: Noctua (the owl), Corvus (the crow), Crater (the cup), Sextans Uraniæ (the astronomical sextant), Hydra (the water snake), Felis (the cat), Lupus (the wolf), Centaurus (the centaur), Antlia Pneumatica (the air pump), Argo Navis (the ship), and Pyxis Nautica (the mariner's compass). Image credit: Sidney Hall/Richard Rouse Bloxam/Adam Cuerden/Public domain.
French astronomer Pierre Charles Le Monnier added a few more southern constellations between 1776 and 1792, including Turdus Solitarius, a bird resembling a thrush. Le Monnier claimed to have introduced this constellation in memory of French astronomer Alexandre Guy Pingré's voyage to Rodrigues, in the Indian Ocean, in order to observe the transit of Venus[5m].
It was debatable which bird it was supposed to represent, with American polymath Richard Hinckley Allen claiming it represented Rodrigues Solitaire a bird similar to a dodo. British natural philosopher Thomas Young renamed it the Mockingbird in 1806, and British writer Alexander Jamieson renamed it Noctua, the owl, in 1822. The constellation is no longer in use[5n].
5. Modern constellations ↑
By the mid-19th century, celestial atlases were designed for the general public. The illustrations were simplified and the amount of constellations reduced.
The International Astronomical Union (IAU) formed in 1919. The IAU made a list of 88 official constellations and approved each boundary by 1928. Belgian astronomer Eugène Joseph Delporte improved this system in 1930, and is still in use today.
Hydra, the water snake, as depicted in by the IAU today. Image credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)/CC-A.