The lights from the parlour and kitchen shone out
Through the blinds and the windows and bars;
And high overhead and all moving about,
There were thousands of millions of stars.
–  Robert Louis Stevenson, Escape at Bedtime

Although there are countless stars in the Universe, on a clear night you will only be able to see about 2,500 of them. Only 5,000 stars are visible to the naked eye, and only half of these are above the horizon at any time.

The stars are at many different distances from us, but are all so far away that they appear as points of light. Because of this, they seem to be set in a black sphere around the Earth. It can be quite useful to think of the stars as set in a fixed sphere, known as the 'celestial sphere', even though we know this is not the case. The Earth is at the centre of the sphere and rotates once a day. The positions of the stars relative to each other don't change, but the whole sky appears to rotate around the Earth once a day.

Random Dots and Patterns

If you present anyone with a selection of random dots, they will begin to see patterns in them, and many ancient peoples did just that, seeing pictures of animals, monsters and gods in the stars. These patterns are known as constellations, from the Latin con (together) and stella (star). As civilisations came and went, different patterns were recognised as constellations.

You should remember that stars in a constellation may be physically close together or might be in entirely different parts of the Universe, just happening to be in the same direction as seen from Earth.

The Ancient Greeks

In about 150 AD, the Greek-Egyptian astronomer Ptolemy published a book on astronomy known as the Almagest, and included in it a list of 48 constellations visible from Alexandria. Ptolemy's list was as follows:

Andromeda - Aquarius - Aquila - Ara - Argo Navis - Aries - Auriga - Boötes - Cancer - Canis Major - Canis Minor - Capricornus - Cassiopeia - Centaurus - Cepheus - Cetus - Corona Australis - Corona Borealis - Corvus - Crater - Cygnus - Delphinus - Draco - Equuleus - Eridanus - Gemini - Hercules - Hydra - Leo - Lepus - Libra - Lupus - Lyra - Ophiuchus - Orion - Pegasus - Perseus - Pisces - Piscis Austrinus - Sagitta - Sagittarius - Scorpius - Serpens - Taurus - Triangulum - Ursa Major - Ursa Minor - Virgo

All of these except Argo Navis are still in existence and are used today.

New Constellations

Over the centuries since the time of Ptolemy, new constellations have been added. There are three main reasons:

1. Splitting up Greek ones – the constellation of Argo Navis, the ship Argo from the legend of Jason and the Argonauts, was considered by French astronomer Nicolas de Lacaille (1713 - 1762) to be too big to be manageable. He decided to split it into three new constellations: Carina, 'the hull', Vela, 'the sail', and Puppis, 'the stern'. These three constellations are now used instead of Argo Navis.

2. Ptolemy, situated in Alexandria which is about 30° north of the equator, couldn't see the southernmost part of the celestial sphere – anything further south than 60° was permanently below the horizon, so the Greeks never had any constellations for this part of the sky. This actually amounts to just less than 7% of the celestial sphere. Constellations for this area were first devised by Dutch explorers in the 16th Century – Pieter Dirkszoon Keyser (1540 - 1596) and Frederick de Houtman (1571 - 1627). This process was continued by Lacaille in the 18th Century.

3. The Greeks only made constellations for the bright stars. They ignored areas of sky in between these with only faint stars. In the 17th and 18th Centuries, astronomers wanted to fill in the gaps, so they invented some nondescript constellations to feature the faint stars, with names like 'the lizard', 'the hunting dogs' and 'the fox'.

Some Constellations that Didn't Quite Make the Grade

Once the idea of inventing new constellations became established, astronomers had a field day – no self-respecting astronomer could publish a star chart without throwing in a few constellations of his own devising, so we got such creations as King Charles's Oak, Frederick's Honour, the Flamingo, the Quadrant, the Electric Generator and the Print-shop. Most of these were short-lived.

The Modern 88 Constellations

In 1922, the International Astronomical Union (IAU) produced a definitive list of 88 constellations, including most of the various constellations defined over the years:

Area, not Pattern

Up to 1930, constellations were considered to be star patterns. This meant that if a new star were discovered between two constellations, then there would have to be debate to decide which constellation it belonged to. In 1930, a change was made to the way astronomers see constellations – the IAU defined the exact positions of the boundaries between the constellations. This meant that constellations are now areas of the sky, rather than patterns of stars.

The Zodiac

One particular set of constellations deserves a special mention – these are the 13 constellations of the zodiac. Because the Earth travels around the Sun, the Sun appears to travel in a circle around the celestial globe, taking a full year to do one circuit. The path of the Sun against the background stars is known as the 'ecliptic'. Because the planets don't all go around the Sun in exactly the same plane, their apparent paths across the sky will not be exactly along the ecliptic, but they will always be close to it. If you plot the paths of the planets, you'll get a narrow band around the ecliptic, which is known as the zodiac.

The ancient Babylonians divided the zodiac into 12 constellations, which are known as the 'signs of the zodiac'. At any time, the Sun will be between us and one of the signs; it is said to be 'in the sign'. Similarly, the planets move from sign to sign as they orbit the Sun.

The Babylonians believed that the fate of humans was governed by the positions of the Sun, the Moon and planets in the zodiac. Despite the lack of any evidence to support it, this belief appears to be still prevalent in modern society, and horoscopes based on the zodiacal signs are published in the popular culture newspapers and magazines.

When the IAU defined the boundaries of the constellations, this produced an oddity: the Sun now passes through 13 constellations on its journey around the celestial sphere. The 13th 'sign of the zodiac' is Ophiuchus the Serpent Bearer, but this sign has not been incorporated into popular culture.

Due to the precession of the Earth⁵, the dates for the Sun to be in the different zodiacal signs have changed over the two thousand years since they were worked out. For example, the Sun is in the sign Taurus from 15 May to 21 June, whereas the incorrect dates of 21 April to 20 May are given in the popular magazines. Experiments show that either set of dates can be used for fortune telling with equal accuracy.

Asterisms

Some striking patterns of stars are not in line with the modern constellations – such patterns are known as asterisms. They include:

• Patterns that form part of a constellation. The best example is the Plough, or Big Dipper, which consists of seven stars, but is only part of the constellation Ursa Major, the Great Bear. Another example is the Sickle, which is part of the constellation Leo, the Lion.

• Patterns that are formed from stars in two (or more) different constellations. The best example is the Summer Triangle, which is formed by three stars from the constellations Lyra, Cygnus and Aquila. Another example is the Great Square of Pegasus, which contains three stars from Pegasus and one star from Andromeda. Yet another example is the Winter Triangle formed from stars in Orion, Canis Major and Canis Minor.

The Naming of Stars

Many stars have their own proper names, such as Sirius, the brightest star in the night sky, in the constellation of Canis Major. Rather than thinking up proper names for every one of the 5,000 visible stars, a naming system was devised by Johann Bayer (1572 - 1625) which is roughly based on how bright the star is and what constellation it is in.

This system relies on the letters of the Greek alphabet, alpha, beta, gamma, delta and so on:

• Alpha, the first letter of the Greek alphabet, represents the brightest star in the constellation.

• Beta, the second letter of the Greek alphabet, represents the second brightest star in the constellation.

• The systems proceeds through all the letters of the Greek alphabet.

The Greek letter is combined with the genitive (possessive) form of the constellation name. For example, Centaurus is the constellation of the centaur. Its genitive form is Centauri. So the brightest star in Centaurus is called alpha Centauri. This may also be written using the actual Greek letter and the constellation abbreviation: α Cen.

These 'Greek letter names' are technically known as 'Bayer designations'. For many of the southern constellations which were unknown to the ancient Greeks, the Bayer designations are the only names the stars possess. For northern skies, most of the bright stars have a proper name and a Bayer designation. Alpha Lyrae, for example, the brightest star in Lyra, is called Vega. There are a few well-known stars which have a common name but no Greek letter, for example Maia (one of the Pleiades) has the designation 20 Tauri.

Bayer wasn't completely consistent in devising his designations for the stars: his lists of star brightnesses were not very accurate, so sometimes the exact brightness does not match with the designation. Alpha Orionis (Betelgeuse), for example, is actually not quite as bright as beta Orionis (Rigel). In other cases, he didn't use brightness at all, assigning the designations based on the position of the star in the constellation: the seven stars of the Plough (Big Dipper) are assigned in order along the line of the pattern. But in general the system works well, and we are saved from having to remember the names of 5,000 different stars.

What Use Are Constellations?

Professional astronomers these days study stars, planets, nebulas, galaxies and anything else they can find in the sky, but they don't study constellations. Any kind of interesting object can be quickly located by its celestial co-ordinates of declination and right ascension, so they have no need of the constellations.

Amateur astronomers, on the other hand, use the constellations to find their way around the sky to locate the interesting objects. For example, if you read that there is a new comet visible in Perseus as there was in 2007, you'll know just where to look in the sky, although the right ascension and declination would probably be meaningless to you.

So a familiarity with the constellations and how they are positioned relative to each other will help you to locate the things in the sky that are worth looking at. While you're at it, you can bear in mind that you are also looking at representations of myths from the dawn of humanity.