How We Came to Know the Cosmos: Space & Time

Discover How We Came to Know the Cosmos

Chapter 1. Constellations

1.1 The first constellations

On a clear night, away from artificial light, you can 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.

You may also see some of the seven objects that appear to move independently of this sphere: the Moon, Mercury, Venus, Mars, Jupiter, Saturn, and the Sun.

The Sun and Moon and the fixed sphere of stars 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. The first constellations were probably formed to make maps and calendars easier to understand.

People might have formed constellations by at least 30,000 BCE when a possible image of Orion was carved into a mammoth tusk.[1] Other evidence of prehistoric star maps comes 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]

1.2 The zodiac

The first written languages were developed in Africa and the Middle East by about 3000 BCE. Calendars and maps were made by the ancient Egyptians and the Sumerians in southern Mesopotamia, which is part of modern-day Iraq. These show that the ancient Egyptians had divided the year into 365 days of 12 months[3] and later divided the day into 24 hours.[4]

The Sumerians were the first to record the names of the constellations by about 2000 BCE.[5] The constellations that were most important to them were those that the Sun, Moon, and planets all pass through, the zodiacal constellations.

The Sun moves relative to the zodiacal constellations, making one full revolution a year, and the Sumerians could see which constellation the Sun was in by looking at the first constellation to set after it.

Sumer became part of Babylon over the next hundred years, but the Babylonians kept the Sumerian’s calendar system and the names of their constellations.

Many of the zodiacal constellations that we use now originate from Sumer or Babylon. The rest are probably from ancient Greece. The first evidence of ancient 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.[5]

Ancient Greek astronomy was advanced enough that the mathematician Thales of Miletus was able to use 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 the ancient Greek astronomer Eudoxus of Cnidus learned the names of at least 48 Babylonian constellations, including those in the zodiac.[5] By then, the Babylonians had divided the path the Sun takes through the zodiacal constellations - the ecliptic - into 12 equal segments, one for each month.[6]

The ancient Greeks also devised their own names for the constellations, naming them after animals, objects, and gods. The word ‘zodiac’ derives from the Greek word for 'animal' since 11 of the signs are represented by animals.

1.2.1 The precession of the equinoxes

The ancient Greek astronomer Hipparchus showed that the Sun’s path through the ecliptic is gradually changing in about 127 BCE. The constellation that the Sun travels through during any particular month moves backwards once every 2000 years or so. 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 they were recorded by the ancient Greek astronomers Timocharis and Aristyllus in the 3rd century BCE.[7]

During Hipparchus’ time, the vernal (spring) equinox - the date when incoming solar energy is equal in both hemispheres, halfway between the winter and summer solstices (discussed in Chapter 2) - occurred in Aries, but by about 1 CE, it had moved into Pisces. At this rate, it will move into Aquarius in about 2600.

1.3 The first star catalogues

Hipparchus compiled a star catalogue and devised the magnitude system. 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.

The Dutch astronomer Jacobus Kapteyn extended the magnitude system in 1902. Kapteyn introduced the term ‘absolute magnitude’, which allows people to compare stars by stating the magnitude they would have if they were all viewed from the same distance.

The 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, except for Argo Navis (the Ship Argo), which has been broken up into smaller parts, Carina (the Keel), Puppis (the Stern), and Vela (the Sail).[5]

In 964, the Iranian astronomer ’Abd al-Rahman Al-Sufi published a revised edition of Ptolemy’s Almagest, the Book of Fixed Stars. Al-Sufi 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 people and animals.[5] Al-Sufi also made the first recorded observation of Andromeda, the closest spiral galaxy to our own, describing it as a ‘little cloud’.[8]

A photograph showing pages from Al-Sufi’s ‘Book of Fixed Stars’.

Figure 1.1
Image credit

Sagittarius from Al-Sufi’s Book of Fixed Stars, first printed in 964.

Depiction of constellations that are visible in the southern sky, from 1603.

Figure 1.2
Image credit

The ‘Southern Birds’ from Johann Bayer’s Uranometria, first printed in 1603: Phoenix (a mythical phoenix), Grus (the crane), Tucana (the toucan), Indus (the Indian, a term that may have referred to a person from Asia or the Americas), Pavo (the peacock), and Hydra (the water snake).

The Almagest was updated once again in 1437 when the 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.[5]

In 1536, the German astronomer Caspar Vopel added two new constellations: Antinous and Coma Berenices. These were stars that Ptolemy had catalogued but not fitted into constellations.[5]

In 1540, the 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. The German cartographer Johann Bayer modified this system in 1603. It was adopted by Kapteyn’s magnitude system and has been used ever since.[9]

1.4 Stars in the southern hemisphere

Europeans began to map the southern skies in 1595 when the 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 12 new constellations, most of which were named after animals in the Greek tradition. These included Apus (the bird of paradise), Tucana (the toucan), Chamaeleon (the chameleon), and Hydra (the water snake).[5]

Houtman originally described Tucana as “the Indian magpie, named Lang in the Indies”, which was actually a hornbill. The Flemish astronomer Petrus Plancius named the constellation ‘Toucan’ on his globe of 1598, which Bayer then used in his star catalogue.[5]

Plancius added 11 more constellations, including Crux (the Southern Cross) and Columba (the dove), but only four are still used.[5]

The English astronomer Edmond Halley created a catalogue of the southern stars in 1679 from observations at St Helena in the South Atlantic Ocean.

The German astronomer Johannes Hevelius and the Polish astronomer Elisabetha Hevelius printed a celestial atlas in 1690, which contained 10 new constellations, seven of which are still in use. These include Scutum (the shield), Sextans (the sextant), and Vulpecula (the fox).[5]

The French astronomer Nicolas Louis de Lacaille devised 14 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 reflected the spirit of the time. These include Microscopium (the Microscope), Telescopium (the Telescope), Pictor (the Painter’s Easel), and Norma (the Carpenter’s Square).[5]

Lacaille was one of the first people 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.

The French astronomer Pierre Charles Le Monnier added a few more southern constellations by 1792 including Turdus Solitarius, a type of bird. The American polymath Richard Hinckley Allen claimed it represented Rodrigues Solitaire a bird similar to a dodo. The British natural philosopher Thomas Young renamed it the Mockingbird in 1806, and the British writer Alexander Jamieson renamed it Noctua, the owl, in 1822. The constellation is no longer in use.[5]

Depiction of constellations that are visible in the southern sky, from 1825.

Figure 1.3
Image credit

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).

1.5 Modern constellations

By the mid-19th century, celestial atlases were designed for the general public. The illustrations were simplified and the number of constellations reduced.

The International Astronomical Union (IAU) formed in 1919. The IAU made a list of 88 official constellations by 1928. The Belgian astronomer Eugène Joseph Delporte improved this system in 1930 and is still in use today.

IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg), Licensed under an Attribution 4.0 International (CC BY 4.0) license,

Figure 1.4
Image credit

Hydra the water snake and surrounding constellations as depicted by the IAU today.

1.6 References

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