Contents

Cover

Title Page

1 A big Universe

2 In cosmic realms

3 Third rock

4 Our cosmic backyard

5 The galactic neighbourhood

6 Far and away

7 The Universe revealed

Glossary

Need to know more?

Further reading

Index

Acknowledgements

Copyright

About the Publisher

1 A big Universe

Ever since our ancestors were first struck by the majesty of the heavens and pondered its meaning, human beings have felt a profound curiosity about realms beyond the Earth. Unimaginably distant and seemingly untouchable, the wider cosmos – from our closest planetary neighbours to the stars beyond – has inspired humans in countless ways through the centuries.

Splendour of the heavens

Although science has provided incredible insights into the Universe, our feelings of awe at viewing a star-spangled sky are the same as those experienced by our prehistoric ancestors.

Inklings of the infinite

Throughout history, everyone with the slightest degree of curiosity about the world around them has looked up at the sky and asked questions about it – seeking answers by questioning others, formulating their own theories and by arriving at their own answers through observation.

Galaxies in their thousands – a deep view of the Universe, captured by the Hubble Space Telescope.

Eternal musings

How far away are the stars and what’s the furthest thing in the Universe that our eyes can see? Does it all go on forever? Are there other planets like the Earth, inhabited with beings intelligent enough to question who they are, and how they, their world and the Universe itself came into being? If the Universe is infinite, could there be another person like me wondering exactly the same things at that very same moment in time? Will they reach the same conclusions as me? These sorts of questions have been asked by people of all ages ever since our ancestors developed the mental capacity to lay aside their basic survival instincts for a moment or two and view the world around them – and the skies above them – with a genuine curiosity and a desire to know more about the cosmos.

Cosmic connection

The cosmos includes everything – the Earth included – and while science has amassed a great deal of knowledge about the physical nature of our home planet, the intricate processes at work on its living surface, and in its oceans and atmosphere, remain only partly understood. In modern times, living in a polluted big city where the Sun itself competes for attention amid the concrete canyons, it is easy for an individual to feel utterly detached from the Earth and the rest of the Universe – spiritually, mentally and physically. In the pre-industrial age, our ancestors were much more aware of the cycles of the heavens and the Earth. However, plant any 21st century urbanite beneath a dark, star-studded night sky, away from light pollution and the trappings of ‘civilisation’, and all those hardwired visceral feelings soon return.

Our physical bodies, our clothing and jewellery, the book you’re holding right now and the chair you’re sitting on – everything around you, including the planet beneath your feet – is all made from material produced inside long-dead stars. Knowing that we are made of ‘star-stuff’ allows us to feel more intimately connected with the Universe.

Viewing the awesome cosmos.

Sunshine and starlight

Without the stars, the night skies would lose much of their splendour; without the Sun, there would be nobody around to appreciate the stars. The Sun may not be the most important star in the cosmos, but it is critical to the existence of the Earth.

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Cosmic speed limit

Light travels at the staggering speed of around 300,000km per second. The Moon is 1.3 light seconds away and sunlight is around nine minutes old. Light from the nearest stars in our galaxy takes more than four years to reach us, and light from the most distant galaxies is billions of light years old. Nothing in the Universe can travel faster than light.

Stellar energy, cosmic distances

That blindingly brilliant object that illuminates the daytime sky – our nearest star, the Sun – appears to be the single most important object in the heavens. If it emitted less heat and light than it currently does, humans would struggle to survive; our species would face the bleakest of prospects as the Earth’s oceans froze and most plant and animal life on our planet became extinct. If its heat and light were switched off, human life could not survive at all. If the Sun were a solid lump of coal, it would burn itself to a cinder within a few thousand years. After millennia of speculation, the source of the Sun’s prodigious output of energy (and that of all the stars visible in the sky) was finally explained in 1926 by the British scientist Arthur Eddington. Something far more powerful than simple chemical combustion powers the Sun. We rely on the thermonuclear processes elegantly encapsulated within Einstein’s formula E=mc² for our continued existence.

As the skies gradually darken after sunset, stars begin to appear. It may seem ironic that the starlight upon which such hopes and dreams for the future are made actually set out on its journey across the galaxy in the remote past – its light may be just over four years old or more than 3,000 years old. Rigil Kent, a star in the constellation of Centaurus, is 4.4 light years away, and the bright star Deneb in Cygnus is 3,200 light years distant. Starlight takes 2.9 million light years to reach us from the nearest big galaxy to our own, the great spiral in Andromeda.

With the exception of the Sun, the stars are too far away to perceive as globes of glowing gas; even when viewed through a powerful telescope, they appear as mere pinpoints of light. Slight variation in colour can be noticed between the stars – some appear white, others bluish, some slightly red. A star’s colour tells us a lot about its physical status – blue stars have intensely hot surfaces, while red stars have comparatively cool surfaces.

Examples of light travel time to the Earth for various objects visible with the unaided eye.

6.5 billion terranauts

We are all separated from the vacuum of space by just a few kilometres of breathable atmosphere. Earth spins on its axis once every 24 hours and revolves around the Sun once a year at a speed of more than 107,000 km/hour, accompanied by the Moon.

A crescent Earth rises over the rugged lunar surface, photographed by the crew of Apollo 17.

Big blue marble

From his vantage point in the command module of Apollo 8 above the Moon in December 1968, astronaut Jim Lovell described planet Earth as a ‘grand oasis in the big vastness of space’. This grand oasis, a beautiful blue globe, two-thirds of whose surface is covered with water, measures 12,756km across. Our planet is the only place in the Universe known to have life. The fossil record shows that life has clung tenaciously to the Earth’s surface for billions of years, surviving the devastating effects of numerous major geological and cosmic catastrophes. Although mass extinctions of many species of animals have occurred, life – in one form or another – has carried on. Just a few dozen kilometres beneath the Earth’s solid crust is a hot mantle – a zone of molten rock which hasn’t cooled down since the Earth was born some 4.5 billion years ago. The Earth’s crust occasionally splits in a volcanic eruption, allowing this material to erupt onto the surface.

Earth – the grand oasis.

Moon musings

The same gravity that keeps you anchored to the Earth keeps the Moon in orbit. With a diameter about the same as the width of the continental USA, the Moon orbits at an average distance of 384,401km. It took the Apollo spacecraft a few days to traverse the gulf of space between the Earth and Moon, but it would take about a month to get there at the speed of a commercial jet airliner. Second in brilliance to the Sun, the Moon is 400 times smaller than the Sun, but as it is 400 times closer to the Earth, the Sun and Moon appear about the same size in our skies. As the Moon orbits the Earth every month, it goes through a sequence of phases, broadening from a narrow crescent in the evening skies to full Moon in around two weeks, and then becoming narrower again until it is a thin crescent in the morning skies.

At new Moon, the Moon is sometimes seen to move directly in front of the Sun, blocking its light and causing a partial or total eclipse. At full Moon, the Moon sometimes enters the Earth’s shadow and undergoes a partial or total lunar eclipse. Solar and lunar eclipses can be awesome sights.

Fellow wanderers

From little Mercury, whose rocky surface is scorched by the heat of the Sun, to countless deep-frozen cometary chunks from whose surface the Sun appears as a bright point, a remarkable collection of planets, asteroids and comets makes up the Solar System.

Two planetary monarchs compared – Earth, queen of the terrestrial planets, and Jupiter, king of the gas giants.

Meet the neighbours

Our immediate planetary neighbours – Mercury, Venus and Mars – are solid worlds like the Earth. Between Mars and the outer planets of the Solar System lies a zone occupied by countless chunks of rock. These remnants of the Solar System’s formation range from the size of houses to the size of Iceland and are known as asteroids. Four giant planets preside over the outer reaches of the Solar System. Jupiter, Saturn, Uranus and Neptune are all swathed in thick layers of mainly hydrogen gas. Jupiter, the largest gas giant, is so big that a thousand Earths could comfortably fit inside its vast volume. Pluto, the outermost planet, is a diminutive world, smaller in fact than our own Moon. More than four light hours from the Sun, Pluto is one of a number of icy worlds at the cold fringes of the observable Solar System. Far beyond the planets, clinging on to the Sun’s gravity to a distance almost half way to the nearest stars, lies an unseen realm of comets known as the Oort Cloud. Cometary visitors from this distant region occasionally speed through the inner Solar System; warmed by the Sun, their icy nuclei emit large amounts of gas and dust, producing celestial spectacles like the magnificent Comet Hyakutake of 1997.

The galactic suburbs

The Sun, attended by its nine major planets and their satellites, along with hundreds of thousands of asteroids and comets, orbits the centre of a vast spiral galaxy of some 300 billion stars called the Milky Way. A middle-aged resident of the galactic suburbs, the Sun is located around 26,000 light years from the galactic centre – about half way from its centre to the edge, within one of the galaxy’s spiral arms. Orbiting the Milky Way at a speed of around 220km per second, the Sun has made around 20 galactic circuits since its birth around five billion years ago.

Measuring around four light years across (if we include the Oort Cloud), the Solar System occupies a tiny part of the Milky Way galaxy, some 100,000 light years in diameter. Indeed, the Sun would not be visible to the unaided eye from a distance of much more than 50 light years. Viewed from the Alpha Centauri system some 4.5 light years away, the Sun would appear as a bright star in the constellation of Cassiopeia.

Our location within the suburbs of the Milky Way spiral galaxy.

Island Universe

On a clear night it’s possible to see hundreds of stars from a rural location. Each one is a relatively nearby member of our home galaxy, whose further reaches dissolve into a glowing band called the Milky Way.

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Beginnings

As we probe further into the depths of space, we are looking ever further back in time towards the beginning of the Universe. That beginning, thought to have taken place less than 14 billion years ago, may have been a single ‘Big Bang’ – the explosion of a primeval atom which created spacetime and matter. What caused this to occur, and what took place before the Big Bang, is still unknown.

A grand design

Shaped like a flattened disk with a central bulge, our home galaxy the Milky Way is arranged in a loosely-wound spiral, with a number of curving arms composed of stars and glowing gas clouds. Two nearby small satellite galaxies – the Small and Large Magellanic Clouds – drift some distance beyond the edge of the Milky Way. From our perspective deep within an arm of the galaxy, we see the Milky Way as a glowing band which circles the heavens. Looking towards the bright galactic region in the vicinity of the constellation of Sagittarius, we are peering directly towards the centre of the galaxy. Our view of the actual galactic hub is blocked by dense interstellar clouds of dust and gas, which show up as dark silhouettes against the brighter parts of the Milky Way.

Clouds of interstellar dust and gas in Sagittarius obscure our view of the Milky Way’s bright core.

Around 2.9 million light years away lies the Andromeda Galaxy – the nearest big galaxy to our own, the Milky Way.

Galaxies galore

Only a century ago most astronomers thought that the Milky Way represented the entire Universe. We now know that the Milky Way is just one of billions of other galaxies – some larger, some smaller than our own. The nearest big galaxy to the Milky Way is the Andromeda Galaxy, 2.9 million light years away, a spiral similar to our own. Discernable with the unaided eye on a dark, clear night, the light from that ghostly oval smudge in Andromeda set off long before the first sparks of consciousness flickered within the minds of human beings.

Telescopic surveys have shown us the structure of the wider Universe. Galaxies are arranged in gravitationally-bound clusters and superclusters, immersed in vast clouds of gas. Incredibly, the matter that can be observed telescopically – planets, stars, interstellar gas and dust clouds – make up a small proportion of the matter in the Universe. A staggering 90 per cent is thought to take the form of ‘dark matter’, currently unable to be detected by any telescope. This mysterious stuff is known to exist because its mass produces a detectable gravitational pull on galaxies. What constitutes dark matter is a subject of much debate among astronomers. It may be an entirely new form of matter, quite unlike the stuff we are made of, or it may simply be as yet unobserved ordinary matter such as old failed stars known as ‘brown dwarfs’, or more exotic entities like black holes.

Zooming around the Universe

To get some sense of the size and scale of the cosmos, let’s zoom away from the Earth in giant steps, right out to the very edge of the Universe, pausing to survey the scene before our eyes after each step.

Our home planet, a sphere more than 12,700km across.

The little grey Moon is outshone by its big blue partner.

Earth

Most of the Earth is contained within a 10,000km cube – just 1/30th light second across. From here we can see the blue oceans, the brilliant white clouds and icy polar caps, and the browns and greens of the continents. You may see some large sprawling grey cities during the day, but at night you’ll have no trouble seeing city lights and illuminated roadways. From our high vantage point, the Earth’s atmosphere – our protection against cosmic threats, such as bombardment by meteorites and harmful ultraviolet radiation from the Sun – appears awfully thin and insubstantial.

Earth-Moon

Virtually a double planet, the Earth-Moon system is contained within a cube 1,000,000km across. Light would take just 3 seconds to get from one side to the other. This view shows the comparative size and brightness of the Earth and the dull grey Moon. The Moon has no atmosphere; no clouds ever appear in the Moon’s skies, rainfall never quenches the dry lunar soil and no rivers flow on its surface. Our only natural satellite has always been dry and lifeless, its surface subject to harsh treatment from cosmic impactors.

Solar System

Contained within a cube of space 10 billion km on each side (8 light hours across) are the orbits of the nine major planets of the Solar System.

The planets are utterly outshone by the brilliant Sun.

Oort Cloud

Viewing a cube of space 1 trillion km (38 light days) on each side, all the major planets in the Solar System are lost in the glare of the Sun, which appears as bright as the full Moon. The inner parts of the distant Oort Cloud of comets are contained within the cube.

At the edge of the Sun’s gravitational domain.

Stellar neighbourhood

A cube measuring 100 x 100 x 100 light years centred on the Sun takes in a sizeable portion of the local stellar neighbourhood, including many of the sky’s brightest stars, from Rigil Kent (4.4 light years away) out to Capella (42 light years away).

Rigil Kent, otherwise known as Alpha Centauri, is the brightest star in the constellation of Centaurus. It is like the Sun in terms of age, size, colour and luminosity, but because it is so near to us it appears as the sky’s fourth brightest star. Alpha Centauri is gravitationally bound to two other stars – a smaller orange star called Alpha Centauri B and a diminutive red dwarf known as Proxima Centauri. Of the three, Proxima is slightly the nearer; at 4.22 light years, it is the nearest star to the Sun. Could planets exist in orbit around Alpha Centauri? Since Alpha Centauri B orbits its brighter neighbour at a distance equivalent to the Sun and Uranus, Alpha Centauri wouldn’t be able to hold on to any planets further away from it as Jupiter is to the Sun, as their orbits would be gravitationally disrupted by Alpha Centauri B.

The Sun, just an ordinary star among its stellar neighbours.

The spiral structure of our galaxy can only be appreciated when viewed from above its plane.

Milky Way

Our home galaxy, the vast spiral of the Milky Way, is enclosed within a cube 100,000 light years across. From this distance we can see only the brightest stars within the galaxy – stars of the Sun’s brightness and dimmer are assumed within a mottled glow, interspersed with the dark silhouettes of opaque dust lanes and glowing clouds of dust and gas.

Our local group of galaxies.

Local galactic group

A cube 10,000,000 light years across will take in the Local Group of galaxies. Only three large spirals dominate the scene – the Milky Way, the Andromeda Galaxy and its neighbour the Triangulum Galaxy. The smaller galaxies dotted around are dwarf and irregular ones. Numerous gravitational interactions between the galaxies of our local group have taken place during the last 12 or more billion years.

Most of the Universe

From our impossible perspective, viewing a cube of space 10 billion light years to each side, most of the objects in the visible Universe can be seen. Each dot represents a cluster of galaxies.

As we look further out into space, we are looking backwards in time. Light from the nearest galaxies in our local group takes a few hundred thousand to several million years to reach us, and we are seeing them as they appeared before homo sapiens sapiens walked the Earth. Galactic clusters and superclusters are arranged in filaments and sheets surrounding huge empty voids. Astronomers work out the distance of far away galaxies by measuring how much light is shifted towards the red end of the spectrum or redshifted. The greater the observed redshift, the greater a galaxy’s distance. Telescopic deep field images have revealed faint galaxies so distant that we see them as they appeared several billion years ago – before life itself developed on Earth.

Our picture of the wider Universe is based on redshift measurements.

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Take it to the next level . . .

Stellar distances 63

Galactic redshifts 64

Expanding Universe 66

The Big Bang 66

Other sources. . .

Computer software – some programs give detailed data and images about near and deep space

Go observe! Invest in a pair of 7x50 binoculars and begin exploring the skies

Weblinks. . .

Learn about the Solar System www.nineplanets.org Astronomy Picture of the Day – discover the cosmos http://antwrp.gsfc. nasa.gov/apod/astropix.html

NASA’s Imagine the Universe http://imagine.gsfc. nasa.gov

2 In cosmic realms

Humans have been watching the Universe for countless millennia. Records of celestial events go back tens of thousands of years. Our attempts to understand the Universe and our place within it have occupied the thoughts of curious folk since the dawn of civilisation. We take a look back at how our ancestors first interpreted what they saw in the skies above them and how that has evolved into the science we now use today.

Cosmic notions

Our distant ancestors certainly kept a watchful eye on celestial goings-on, for preserved in ancient cave paintings, rock carvings and other human artefacts are indisputable records of a variety of astronomical events.

Observant ancestors

Patterns of dots thought to be star maps and what appears to be an attempt to populate the heavens with figures representing constellations in the form of animals have been discovered at a number of prehistoric sites in various parts of the world. They show an amazing sophistication of thought in our distant ancestors, proving that they were immensely observant of the world around them and the skies above their heads.

Wonderful scenes like this – brilliant Venus and ruddy Mars in Scorpius, rising over the French Alps – have been witnessed by human eyes for countless millennia.

Moon markings

One of the most familiar symbols to be found is the crescent representing the Moon and markings that indicate the monthly lunar cycle of phases. One such lunar calendar of 29 markings – one for each day of the lunar month – has recently been identified in the caves at Lascaux in France, dated to around 15,000 BC. This symbolism, acknowledging the Moon’s importance, is hardly surprising; the Moon’s light will have been of immense value to any culture without the benefit of streetlights and flashlights, in terms of hunting, nocturnal survival and in primitive religions which attached significance to Moon worship and observing the lunar cycles. What many experts consider to be a depiction of the Moon showing the main features discernable with the unaided eye – the dark patches, known as the ‘maria’ – has been discovered in 5,000-year-old rock carvings in prehistoric tombs at Knowth, County Meath in Ireland.

Celestial purpose

Tens of thousands of years ago our ancestors believed the skies had a greater purpose than to be simply admired for their splendour. The heavens were considered to have a purpose; those fixed stellar points of light actually meant something. Those bright objects which appeared to move – the Moon, Sun and the five planets – were imagined to have a special significance. So the skies became incorporated into human affairs, and astrology was born. Unexpected changes in the skies – bright comets, eclipses and novae – were considered to be potent warnings or revelations to humanity.

Carved into Irish rock 50 centuries ago, this set of curves found in the Knowth prehistoric tomb in Ireland may represent the oldest map of the Moon.

Astrology

The notion that the movements of the Sun, Moon and five planets have an influence on the lives of individual people and the course of world events has its roots in ancient concepts and beliefs.

Predictive power

Being able to predict celestial events, from the movements of Venus to eclipses of the Sun and the Moon, appeared to give a great advantage to any society capable of mastering such complex astronomical and mathematical problems. Great civilizations, such as those of ancient Babylon, Egypt and China, attached great importance to observing, recording and predicting heavenly phenomena. Astrologer-priests kept a constant vigil on the skies, ostensibly for society’s well-being and to keep their rulers informed of any celestial portents that might affect the status quo. Astrology was considered such a precious asset that its use without the ruler’s permission was often punishable by execution. Sometimes, either through lack of attention to the skies, sloppy mathematics or just plain bad luck, the astrologers got things wrong – and paid a severe penalty as a result. The ancient Chinese Book of History reports that two court astrologers were executed for having failed to announce a total lunar eclipse in 2136 BC.

Comets were once regarded as celestial omens.

Portents of doom

Of course, not everything in the heavens can be predicted. Even these days, bright novae or supernovae (stars which temporarily brighten far beyond their ordinary brightness), bright comets, fireballs (brilliant meteors) or aurorae (displays of the northern/southern lights) often take astronomers by complete surprise. Although such sights can be awesome to behold, we know how these phenomena are caused and realise that, for the most part, they pose no threat to the Earth or its inhabitants. Only a few hundred years ago, things were very different, and a bright comet might have been regarded as an omen of impending change on the Earth – of natural catastrophe, famine, pestilence, war or a change of ruler.

Astrology may look glamorous, but it is as insubstantial as the paper its predictions are written on.

Pseudoscience

Ancient astrology was practised by making astronomical observations (in an age long before the telescope was invented), so the two subjects of astrology and astronomy were once closely intertwined. Modern forms of astrology, which claim to foretell the destinies of individuals, have often come under close scientific scrutiny, yet have been consistently found to have no provable predictive powers. While many find modern astrology to be entertaining, it has very little basis in science, apart from using certain scientific terms and nomenclature.

Ancient cosmology

Although humans around the world have viewed the same range of celestial phenomena through the ages, our interpretation of them – how they were thought to have been created and what they were believed to have meant – has been immensely varied.

A copy of observations of Venus made in Babylon in 1700 BC is recorded on this clay tablet made at Nineveh in the 7th century BC.

Mesopotamian skies

The science of astronomy can trace its roots back thousands of years to Mesopotamia, a land bordered by the rivers Tigris and Euphrates in modern Iraq.

Fertile minds

Mesopotamia actually means ‘land between two rivers’. The region is also known as ‘the fertile crescent’, its fertility being perhaps the source of the biblical Garden of Eden. Here, from around 4000 BC, the first complex human civilisations grew: first Sumeria, then the kingdoms of Babylon and Assyria.

Omens in the heavens were deemed of tremendous importance to the rulers of ancient Mesopotamia – all events visible in the skies were believed to take place for a reason, and it was the job of the astronomer-priests to note such events, predict them where possible and to interpret their meaning. Astronomical observations were deemed so important that they were preserved on clay tablets imprinted with cuneiform script – a form of writing made with narrow wooden scribes with tapered ends.

A flourishing sky lore also developed in Sumeria, in which ancient myths and legends were projected into the heavens, creating some of the constellations with which we are familiar today. Zodiacal constellations such as Sagittarius, Scorpius, Capricornus, Leo, Gemini and Taurus – areas of the sky through which the Sun, Moon and five planets were observed to travel – were devised by the Sumerians around 5,000 years ago. In addition to having a spiritual significance, the constellations had a practical use – their visibility throughout the year (notably the times of the year when certain constellations were first seen rising or setting) was used to mark agricultural seasons.

Lunar calendar

A calendar based on the Moon’s cycles was used. Each month began with the first sighting of the narrow crescent Moon at sunset, and 12 lunar months made one year. Since 12 lunar months is 11 days short of a solar year (365 days), the Sumerian calendar was synchronized with the solar year by adding a leap month every three or four years.

Babylonian astronomy

The Babylonians adopted and added to ancient Sumerian sky lore, their calendar and scientific knowledge. Hundreds of Babylonian clay tablets noting scientific and astronomical subjects have been discovered. The Babylonians acknowledged the ‘morning star’ and the ‘evening star’ to be a single object –the planet Venus. Lunar eclipses were capable of being predicted with reasonable accuracy.

Babylonian world map, early 5th century BC on a clay tablet, shows a flat, round world with Babylonia at the centre. Eighteen of the animal constellations are named on the tablet.

Ancient Egypt

Ancient Egyptians paid little attention to observing and recording the movements of the Sun, Moon and planets. Instead, special significance was attached to certain bright stars and constellations.

The ceiling of the Temple of Hathor at Denderah (above) depicts a magnificent series of ancient Egyptian constellations (illustrated, top).

Flood warning

By chance, the annual flooding of the River Nile – so vital to irrigating the crops that grew near the river – happened to coincide with the first sighting of Sirius, the sky’s brightest star, as it rose in the east before dawn. It was deemed that the start of each new year would occur with the first new Moon following the reappearance of Sirius. When the system was adopted around 3,000 years ago, the rising of Sirius (known to the Egyptians as Sopdet) just before sunrise took place in early July; it now it takes place around three weeks later because the Earth’s axis points to a slightly different place today than it did in those far off times.

Gods in the sky

A catalogue of the heavens made in 1100 BC lists just five major constellations. In addition, 36 smaller star groups enabled the time to be calculated during the night – helpful tables allowing these calculations to be made have been found inscribed on a number of coffin lids. Osiris, the god of death, rebirth and the afterlife, was represented by the bright, familiar constellation of Orion, and the swathe of the Milky Way represented the sky goddess Nut, who gave birth to the Sun god Ra each day. Circumpolar stars – those stars near the north celestial pole which never set from Egyptian latitudes – were deities known as the ‘Imperishable Ones’.

Pyramid scheme

A collection of large stone slabs recently discovered in Nabta in Egypt (in the Sahara Desert) represent the world’s earliest known astronomically-aligned construction. Dated between 6,000 to 6,500 years old, it is thought that the megaliths were erected by the direct ancestors of the builders of the pyramids, in what was once a fertile area. Climate change caused the residents of Nabta to move eastwards into the Nile Valley, where the great pyramids at Giza were built some time between 2700 to 2500 BC. Astronomy was used to precisely position the pyramids, since their sides are aligned almost exactly north-south and east-west. The fact that they are more accurately aligned east-west suggests that the main positional sighting was made by observing the rising and setting points of a star due east and west; the star Acrab (Beta Scorpii) best fits for the era of the pyramids’ construction. It is likely that the positions of the three great pyramids were meant to reflect the belt stars of the constellation of Orion.

A belief that cosmic events and planetary positions influenced human affairs is remarkably absent throughout most of ancient Egyptian history. Astrology did not figure until the Ptolemaic period, from around the 3rd century BC, when the culture was being influenced by nearby civilisations. Perhaps the best star map depicting astrological constellations can be found on the ceiling of the Temple of Hathor (see left, page 30), constructed in the first century BC.

The three large pyramids at Giza appear to mimic the position of the three belt stars of Orion.

Megaliths and medicine wheels

An awareness of the Universe, and a need to remain tuned in to its cycles prompted ancient cultures in Europe and America to construct earthworks and stone buildings which were aligned with celestial events.

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Little is known about the builders of these sites, but it is clear that they attached great importance to observing the Universe and perhaps being able to predict basic celestial events.

Hanging stones

Megalithic monuments dating back 5,000 years can be found across parts of western Europe and the British Isles. Weathered by the elements and damaged by people over the millennia, these battered grey stone circles and other prehistoric monuments make an eerie sight, but their former grandeur can be imagined. Perhaps the most famous of these ancient sites, Stonehenge, on the windswept Salisbury Plain in southern England, dates back to at least 2950 BC. Of unknown religious and ritual significance, Stonehenge and other megalithic constructions appear to have been used to make astronomical sightings of the Sun and Moon and to provide a means to calculate future solar and lunar solar events.

Medicine wheels

Many examples of the ‘medicine wheel’ – a feature commonly constructed from stones laid on the ground which radiate from a raised central cairn to large circular stone rings – can be found across the United States and Canada. Smaller stone circles can often be found nearby these sites. Medicine wheels were constructed and used by native Americans from 4,000 years to a few hundred years ago. Astronomical alignments certainly exist in them, not only connected with the Sun and Moon but connected with the rising and setting points of numerous bright stars, which may have had special significance in native American religion and cosmology.

Ancient Stonehenge keeps a cold and silent vigil on England’s Salisbury Plain.

Ancient China

Far removed from the cradle of western civilisation, China developed its own form of astronomy, creating its own constellations and ideas about the workings of the Universe.

Skies under scrutiny

Over an almost continuous period spanning the 16th century BC to the end of the 19th century AD, court astronomers were appointed to observe and record changes in the heavens. This legacy of almost 3,500 years’ worth of astronomy, in which sunspots, aurorae, comets, lunar and solar eclipses and planetary conjunctions were noted, has provided us with a rich source of reference material.

Ancient Chinese astronomers created a catalogue of stars visible with the unaided eye, divided the skies into constellations known as ‘palaces’ and referred to the brightest star in each palace as its ‘emperor star’, surrounded by less brilliant ‘princes’. In the 4th century BC, the astronomer Shih-Shen catalogued 809 stars and recorded 122 individual constellations.

Instruments to aid naked eye observations were used extensively in ancient China, as they were in the west. In the 1st century, Lo-hsia-Hung constructed an armillary sphere – a device representing the celestial sphere, upon which were marked 365.25 divisions (for the days of the year), and rings for the celestial equator and the meridian. Lo-hsia-Hung’s charming analogy for the Universe likened the Earth to the yolk within an eggshell, stating ‘the Earth moves constantly but people do not know it; they are as persons in a closed boat; when it proceeds they do not perceive it’. In the 15th century an observatory was built on the southeastern corner of the city wall in ancient Beijing, which was equipped with a number of accurately calibrated sighting devices made out of bronze.

Cosmic firecracker

One of the most interesting records dates from 1054 AD and describes the sudden appearance of a ‘guest star’ near the star we know as Zeta Tauri. This bright star, initially brilliant enough to be seen during the daytime and visible to the unaided eye for more than a year, was caused by a supernova – the catastrophic explosion of a massive star. Its remnants are visible today as the Crab Nebula.

Star signs

Astrology played an important role in ancient China. No fewer than 5,000 astrologers resided in 5th century Beijing! Twenty-eight constellations formed the ancient Chinese zodiac, through which the Sun, Moon and planets progressed. Each of the five planets was designated its own element – Mercury, water; Venus, metal; Mars, fire; Jupiter, wood; Saturn, earth. A person’s fate was supposedly determined by the relative position of the five planets, the Moon, Sun and any comets that happened to be in the sky at the time of that person’s birth.

Ancient Chinese astrological classification of comets – known as ‘broom stars’. Each shape was said to foretell a different event.

Ancient Greece

Many concepts about the Universe with which we are familiar today first arose in ancient Greece between 700 BC and 300 AD. Greek philosophers laid the foundations of modern astronomy.

Eudoxus envisaged the Earth at the centre of a series of crystal spheres upon which were fastened the Sun, Moon, individual planets and stars.

Cosmic questioning

The Greeks benefited from the knowledge of the Universe that had been acquired by ancient Mesopotamian astronomers, and much of its ancient sky lore was also adopted and developed into fantastic celestial myths and legends. Quite unlike ancient Mesopotamia, conditions within the Greek civilisation allowed scientific enquiry to flourish. Ancient Greek philosophers were the first to determine the size of the Earth, the distance of the Moon and deduce the cause of solar and lunar eclipses. Greek philosophers enquired into the very nature of the cosmos, as attempts were made to explain objects and phenomena from the very small to the very big, from the basic atomic essence of matter to the structure of the Universe.

Earth-centred Universe

In the 4th century BC, in accordance with Pythagoras’ deduction that the circle and the sphere were perfect figures, Eudoxus devised a complete picture of the Universe, placing our planet at the centre of a nest of 27 concentric transparent celestial spheres to which were attached the Sun, Moon, planets and stars. Each of these Earth-centred spheres rotated around an axis shared with the Earth’s axis. Using 55 spheres, the system was ‘improved’ by Aristotle a century later. It was considered that the Sun, Moon and planets were perfect spherical objects, and circular motions were deemed to be the only paths that could possibly be followed by celestial objects. This notion held sway throughout the entire era of ancient Greek astronomy and persisted through to the renaissance.

In the 3rd century BC, Eratosthenes applied trigonometry to determine that the Earth is a sphere measuring around 40,000km in circumference, a remarkably accurate feat achieved by observing the length of the shadow cast by the Sun at noon from two widely separated places (Aswan and Alexandria) whose separation was known.

A contemporary of Eratosthenes, Aristarchus used geometry to calculate the sizes and distances of the Moon and the Sun. By observing the Moon’s half-phases and the angles made by the Earth, Sun and Moon, Aristarchus concluded that the Sun was 19 times further away than the Moon (the Sun’s actual distance is 400 times that of the Moon). He observed the Moon passing through the Earth’s shadow during lunar eclipses and concluded that the Moon was half the Earth’s size.

Hipparchus, working in the 2nd century BC, made accurate measurements of the orbit, distance and size of the Moon, and determined the distance of the Sun. Using an accurately constructed naked eye measuring device called an astrolabe, he observed and recorded the co-ordinates of around 850 stars to compile a star catalogue. Another major star catalogue was compiled in the 2nd century AD by Ptolemy, who included it in an encyclopedia of ancient Babylonian and Greek knowledge.

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Very little is known of Ptolemy’s life. He made astronomical observations from Alexandria in Egypt during the years 127–41 AD. The first of Ptolemy’s observations can be dated to 26 March 127, while his last known observation was made on 2 February 141.

Solar hub

Ptolemy’s Earth-centred view of the Universe held sway in the west for around 1,500 years. Credit for the idea that the Sun, not the Earth, lies at the centre of the Solar System is often given to Nicolaus Copernicus (1473–1543).

Copernican revolution

Heliocentric (Sun-centred) theories can actually be traced back to ancient Greece, to philosophers Pythagoras, Philolaus and Aristarchus. However, in 1514 Copernicus was bold enough to produce a small handwritten pamphlet (but canny enough not to put his name to it) which challenged the very fundamentals of Ptolemy’s geocentric (Earth-centred) view of the Universe. Copernicus stated that the stars are at an immense distance, compared to the distance from the Earth to the Sun. He was convinced that the Sun, not the Earth lay near the centre of the Universe, and that the apparent daily rotation of the heavens is caused by the Earth’s rotation. Copernicus went on to explain that the apparent annual circuit of the Sun around the ecliptic is caused by the Earth revolving around the Sun, and the apparent retrograde motion of the planets is caused by the motion of the Earth along an orbit inside that of the outer planets.

His explanation of the phenomenon of retrograde motion, and dispensing with the need to introduce epicyclic planetary motions, is perhaps the most insightful and original of Copernicus’ theories. Copernicus later laid out his potentially heretical heliocentric views of the Universe in his book On the Revolutions of the Heavenly Bodies, one of the first copies of which was given to him as he lay dying in 1543.

Placing the Sun at the centre of the Solar System, Copernicus’ heliocentric theory saw the Earth as one of six orbiting planets.

Copper-nosed enquirer

Tycho Brahe (1546–1601) is regarded as the last and greatest astronomer of the pre-telescopic era. A hot-headed Danish nobleman, Tycho lost part of his nose in a sword duel and later replaced it with a copper prosthesis. In an attempt to refute Copernicus’ view of the Universe, Tycho began making precise measurements of the stars and the movements of the planets with the aid of quadrants and cross-staffs from his observatory at Uraniborg on the island of Hven. Tycho’s careful naked-eye observations went on to provide plenty of evidence disproving the old established notions of the Earth-centred Universe.

Tycho Brahe, a great pretelescopic astronomer. He used naked eye devices to make incredibly accurate observations of planetary motions.

Understanding the Solar System

In the early 17th century, a potent combination of theory and observation expanded human notions about the Universe. The telescope opened everyone’s eyes to the cosmos.

Kepler, planetary law-giver

Despite finding evidence to support the idea that the Earth was a planet in orbit around the Sun, Tycho Brahe remained highly skeptical of the idea. He clung to the old notion of an Earth-centred Universe whose motions would eventually be explained as soon as the right mathematical model was found. Tycho’s assistant, Johannes Kepler (1571–1630), had none of his teacher’s confidence in the geocentric theory, and used Tycho’s extensive observations to place the heliocentric theory on a firm scientific footing. Profoundly religious, Kepler was convinced that God had created the Universe in accordance with mathematical rules, and that a knowledge of these rules was within human comprehension.

Galileo’s telescopic observation of the Pleiades. Only a handful of stars in this cluster can be seen with the unaided eye.

Telescopic revelations

Having heard about a newly invented optical instrument that made distant objects appear larger, Galileo Galilei, a professor of mathematics at Padua University, ground and polished lenses to make small telescopes of his own by the end of 1609. Although a number of people had used telescopes in the months before Galileo placed his own eye to the eyepiece, Galileo is acknowledged to have been the first to turn the telescope to the skies to make scientific observations.

Anyone recalling their first telescopic views of the Moon and planets might conjure up some idea of how Galileo must have felt when he made his first observations through the telescope in 1609–10. Galileo’s telescopes consisted of two lenses mounted at either end of a tube – a plano-convex objective lens of about 30mm diameter at the front end to collect and focus light, and a smaller planoconvex eye lens at the other end to focus and magnify the image so that it can be viewed. Such an instrument is the most basic form of refracting telescope, yet despite it performing little better than today’s toy telescopes, Galileo gained amazing insights into the nature of the Universe. He discovered four small satellites orbiting Jupiter, which are still referred to as the Galilean Moons. Venus displayed phases, proving that it was a globe in orbit around the Sun. Sunspots proved that our central star is by no means perfect, and the Moon’s surface was revealed as a world with dark plains, craters and high mountains. Delving into deep space, Galileo observed that the glowing band of the Milky Way was made up of multitudes of faint stars.

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In the early 17th century Kepler formulated his famous laws of planetary motion, which stated that the planets orbit the Sun along elliptical paths and that they move faster when nearest to the Sun.

An observation of the Sun made by Galileo in July 1613 shows detail within sunspot groups.

The orbs around us

During the 17th century, as telescopes improved, an increasing number of scientists and amateur astronomers were keen to learn more about the Universe by scrutinising celestial realms previously hidden from human view.

The Moon and its craters, from an observation made by Galileo in November 1609.

Reality bites

People began to question the workings of the Universe, and they now had optical tools with which to study the heavens more closely. Yet, despite the evidence, there were many who did not accept the reality of what the newly invented telescope revealed about the Universe. Some had seen the power of the telescope with their own eyes but believed that it was trickery or the work of the devil. Galileo and others who supported the Copernican view that the Earth was a planet in orbit around the Sun were denounced as heretics. Even Galileo succumbed to pressure when he was asked by the Inquisition to refute the idea.

Our lunar companion

The Moon is so big and bright that plenty of detail can be seen on its surface through even the most basic optical equipment – a fact that has delighted lunar observers from Galileo to the present day. It is not surprising that many early telescopic observers chose to study the Moon, to draw its features and to map its surface. The Moon’s landscape really did resemble parts of the Earth – Galileo had likened it to parts of Bohemia – so did it have an atmosphere and could it support life? Large dark patches visible with the unaided eye were discovered to be relatively flat grey plains. These areas became known as ‘maria’ (Latin: seas), but it was plain to see through the telescope eyepiece that they did not represent bodies of water.

The mid-17th century saw the publication of a number of detailed lunar maps. One by Johannes Hewelke (Hevelius) was published in his Selenographia, complete with names for lunar features based upon geographical landmarks, like Sicily, Mount Etna and the Mediterranean Sea. From his private observatory, Hevelius also made accurate measurements of star positions and produced the Uranographica, the most advanced star atlas of its time; the names for his seven new constellations are still used by astronomers today.

At around the same time, Giovanni Riccioli published an accurate lunar map which incorporated nomenclature that is still current, including his names for the Moon’s seas, such as Mare Tranquillitatis (the Sea of Tranquillity) and many of the larger craters like Copernicus (Mount Etna on Hevelius’ map) – familiar names to modern watchers of the Moon.

Giovanni Riccioli’s Moon map of 1651.

Christiaan Huygens’ explanation of Saturn’s rings and their changing appearance.

Planetary perceptions

Fresh telescopic revelations answered some age-old questions about our immediate planetary neighbourhood but raised many more new ones.

Aerial adventures

In the Netherlands, Christiaan Huygens constructed lengthy refracting telescopes whose lenses were suspended on frames in the air and moved by means of ropes and pulleys. Such unwieldy devices – some of them measuring more than 8om (260ft) from objective lens to eyepiece – were required to overcome a defect inherent in simple optics known as chromatic aberration. Caused by the inability of a single objective lens to focus all the colours within white light to a single point, chromatic aberration causes bright objects to appear surrounded by coloured fringes. The longer the objective lens, the less evident the degree of aberration. Huygens also improved eyepiece design by introducing the twolensed Huygenian ocular – a design still provided today with many budget telescopes. Using these ungainly aerial telescopes he discovered Jupiter’s equatorial bulge – the consequence of Jupiter’s rapid spin and its gaseous composition. Bright polar caps were discovered on Mars, in addition to a dark V-shaped feature known as Syrtis Major. Almost half a century after Galileo had discovered the Solar System’s first planetary satellites, Huygens discovered Saturn’s largest satellite, Titan, in March 1655. A conundrum which had baffled astronomers since Galileo – mysterious appendages that seemed to cling to Saturn’s side and vary in size over the years – was finally solved by Huygens, who explained that Saturn had a flat ring system that nowhere touched the planet.

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