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William Collins

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This eBook edition published by William Collins in 2015

Text © Brian Cox and Andrew Cohen 2014

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Cover photographs © Shutterstock (ape); © NASA (arm)

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Source ISBN: 9780008125080

eBook Edition © May 2015 ISBN: 9780008129798

Version: 2018-09-28

Praise for Professor Brian Cox:

‘Engaging, ambitious and creative.’ Guardian

‘He bridges the gap between our childish sense of wonder and a rather more professional grasp of the scale of things.’

Independent

‘If you didn’t utter a wow watching the TV, you will while reading the book.’

The Times

‘In this book of the acclaimed BBC2 TV series, Professor Cox shows us the cosmos as we have never seen it before – a place full of the most bizarre and powerful natural phenomena.’

Sunday Express

‘Cox’s romantic, lyrical approach to astrophysics all adds up to an experience that feels less like homework and more like having a story told to you. A really good story, too.’

Guardian

‘Will entertain and delight … what a priceless gift that would be.’

Independent on Sunday

Dedication
From Brian

To George Albert Eagle:

It’s your future, little boy.

From Andrew

To my soulmate Anna, my beautiful children Benjamin, Martha and Theo, my wonderful mum Barbara, my brothers Paul and Howard and all of the ‘small creatures’ whom I am lucky enough to have with me in the vastness.

Contents

Cover

Title Page

Copyright

Praise for Professor Brian Cox

Dedication

Where are We?

Oakbank Avenue, Chadderton, Oldham, Greater Manchester, England, United Kingdom, Europe, Earth, Milky Way, Observable Universe …?

Off Centre

Changing Perspective

Outwards to the Milky Way

Searching for Patterns in Starlight

Beyond the Milky Way

The Great Debate

The Political Ramifications of Reality, or ‘How to Avoid Getting Locked Up’

The Happiest Thought of My Life

A Day Without Yesterday

Are We Alone?

Science Fact or Fiction?

The First Aliens

Listen Very Carefully

The Golden Voyage

Alien Worlds

The Recipe for Life

Origins

A Brief History of Life on Earth

A Briefest Moment in Time

So, are We Alone?

Who are We?

Spaceman

Apeman

Lucy in the Sky

From the North Star to the Stars

Climate Change in the Rift Valley and Human Evolution

‘An Unprecedented Duel with Nature’

Farming: The Bedrock of Civilisation

The Kazak Adventure: Part 1

Intermission: Beyond Memory

The Kazak Adventure: Part 2

Why are We Here?

A Neat Piece of Logic

New Dawn Fades

The Rules of the Game

Nature’s Fingerprint

A Brief History of the Snowflake

How the Leopard Got Its Spots

A Universe Made for Us?

A Day Without Yesterday?

What is Our Future?

Making the Darkness Visible

Sudden Impact

Seeing the Future

Science Vs. Magic

The Wonder of It All

Dreamers: Part 1

Dreamers: Part 2

The End

Plate Section Credits

Picture Section

Footnotes

Index

Acknowledgements

About the Authors

About the Publisher

WHAT A PIECE OF WORK IS A

MAN, HOW NOBLE IN REASON,

HOW INFINITE IN FACULTIES,

IN FORM AND MOVING HOW

EXPRESS AND ADMIRABLE, IN

ACTION HOW LIKE AN ANGEL, IN

APPREHENSION HOW LIKE A GOD!

THE BEAUTY OF THE WORLD,

THE PARAGON OF ANIMALS –

AND YET, TO ME, WHAT IS THIS

QUINTESSENCE OF DUST? MAN

DELIGHTS NOT ME – NOR WOMAN

NEITHER, THOUGH BY YOUR

SMILING YOU SEEM TO SAY SO.

HAMLET

What is a human being? Objectively, nothing of consequence. Particles of dust in an infinite arena, present for an instant in eternity. Clumps of atoms in a universe with more galaxies than people. And yet a human being is necessary for the question itself to exist, and the presence of a question in the universe – any question – is the most wonderful thing. Questions require minds, and minds bring meaning. What is meaning? I don’t know, except that the universe and every pointless speck inside it means something to me. I am astonished by the existence of a single atom, and find my civilisation to be an outrageous imprint on reality. I don’t understand it. Nobody does, but it makes me smile.

This book asks questions about our origins, our destiny, and our place in the universe. We have no right to expect answers; we have no right to even ask. But ask and wonder we do. Human Universe is first and foremost a love letter to humanity; a celebration of our outrageous fortune in existing at all. I have chosen to write my letter in the language of science, because there is no better demonstration of our magnificent ascent from dust to paragon of animals than the exponentiation of knowledge generated by science. Two million years ago we were apemen. Now we are spacemen. That has happened, as far as we know, nowhere else. That is worth celebrating.

WHERE ARE WE?

We shall not cease from exploration,

And the end of all our exploring

Will be to arrive where we started

And know the place for the first time.

T. S. Eliot

OAKBANK AVENUE, CHADDERTON, OLDHAM, GREATER MANCHESTER, ENGLAND, UNITED KINGDOM, EUROPE, EARTH, MILKY WAY, OBSERVABLE UNIVERSE …?

For me, it was an early 1960s brick-built bungalow on Oakbank Avenue. If the wind was blowing from the east you could smell vinegar coming from Sarson’s Brewery – although these were rare days in Oldham, a town usually subjected to Westerlies dumping Atlantic moisture onto the textile mills, dampening their red brick in a permanent sheen against the sodden sky. On a good day, though, you’d take the vinegar in return for sunlight on the moors. Oldham looks like Joy Division sounds – and I like Joy Division. There was a newsagent on the corner of Kenilworth Avenue and Middleton Road and on Fridays my granddad would take me there and we’d buy a toy – usually a little car or truck. I’ve still got most of them. When I was older, I’d play tennis on the red cinder courts in Chadderton Hall Park and drink Woodpecker cider on the bench in the grounds of St Matthew’s Church. One autumn evening just after the start of the school year, and after a few sips, I had my first kiss there – all cold nose and sniffles. I suppose that sort of behaviour is frowned upon these days; the bloke in the off-licence would have been prosecuted by Oldham Council’s underage cider tsar and I’d be on a list. But I survived, and, eventually, I left Oldham for the University of Manchester.

Everyone has an Oakbank Avenue; a place in space at the beginning of our time, central to an expanding personal universe. For our distant ancestors in the East African Rift, their expansion was one of physical experience alone, but for a human fortunate to be born in the latter half of the twentieth century in a country like mine, education powers the mind beyond direct experience – onwards and outwards and, in the case of this little boy, towards the stars.

As England stomped its way through the 1970s, I learned my place amongst the continents and oceans of our blue planet. I could tell you about polar bears on Arctic ice flows or gazelle grazing on central plains long before I physically left our shores. I discovered that our Earth is one planet amongst nine (now redefined as eight) tracing out an elliptical orbit around an average star, with Mercury and Venus on the inside and Mars, Jupiter, Saturn, Uranus and Neptune beyond. The Sun is one star amongst 400 billion in the Milky Way Galaxy, itself just one galaxy amongst 350 billion in the observable universe. Later, at university, I discovered that physical reality extends way beyond the 90-billion-light-year visible sphere into – if I had to guess based on my 46-year immersion in the combined knowledge of human civilisation – infinity.

This is my ascent into insignificance; a road travelled by many and yet one that remains intensely personal to each individual who takes it. The routes we follow through the ever-growing landscape of human knowledge are chaotic; the delayed turn of a page in a stumbled-upon book can lead to a lifetime of exploration. But there are common themes amongst our disparate intellectual journeys, and the relentless relegation from centre stage that inevitably followed the development of modern astronomy has had a powerful effect on our shared experience. I am certain that the voyage from the centre of creation to an infinitesimally tiny speck should be termed an ascent, the most glorious intellectual climb. Of course, I also recognise that there are many who have struggled – and continue to struggle – with such a dizzying physical relegation.

John Updike once wrote that ‘Astronomy is what we have now instead of theology. The terrors are less, but the comforts are nil’. For me, the choice between fear and elation is a matter of perspective, and it is a central aim of this book to make the case for elation. This may appear at first sight to be a difficult challenge – the very title Human Universe appears to demonstrate an unjustifiable solipsism. How can a possibly infinite reality be viewed through the prism of a bunch of biological machines temporarily inhabiting a mote of dust? My answer to that is that Human Universe is a love letter to humanity, because our mote of dust is the only place where love certainly exists.

This sounds like a return to the anthropocentric vision we held for so long, and which science has done so much to destroy in a million humble cuts. Perhaps. But let me offer an alternative view. There is only one corner of the universe where we know for sure that the laws of nature have conspired to produce a species capable of transcending the physical bounds of a single life and developing a library of knowledge beyond the capacity of a million individual brains which contains a precise description of our location in space and time. We know our place, and that makes us valuable and, at least in our local cosmic neighbourhood, unique. We don’t know how far we would have to travel to find another such island of understanding, but it is surely a long long way. This makes the human race worth celebrating, our library worth nurturing, and our existence worth protecting.

Building on these ideas, my view is that we humans represent an isolated island of meaning in a meaningless universe, and I should immediately clarify what I mean by meaningless. I see no reason for the existence of the universe in a teleological sense; there is surely no final cause or purpose. Rather, I think that meaning is an emergent property; it appeared on Earth when the brains of our ancestors became large enough to allow for primitive culture – probably between 3 and 4 million years ago with the emergence of Australopithecus in the Rift Valley. There are surely other intelligent beings in the billions of galaxies beyond the Milky Way, and if the modern theory of eternal inflation is correct, then there is an infinite number of inhabited worlds in the multiverse beyond the horizon. I am much less certain that there are large numbers of civilisations sharing our galaxy, however, which is why I use the term ‘isolated’. If we are currently alone in the Milky Way, then the vast distances between the galaxies probably mean that we will never get to discuss our situation with anyone else.

We will encounter all these ideas and arguments later in this book, and I will carefully separate my opinion from that of science – or rather what we know with a level of certainty. But it is worth noting that the modern picture of a vast and possibly infinite cosmos, populated with uncountable worlds, has a long and violent history, and the often visceral reaction to the physical demotion of humanity lays bare deeply held prejudices and comfortable assumptions that sit, perhaps, at the core of our being. It seems appropriate, therefore, to begin this tour of the human universe with a controversial figure whose life and death resonates with many of these intellectual and emotional challenges.

Giordano Bruno is as famous for his death as for his life and work. On 17 February 1600, his tongue pinioned to prevent him from repeating his heresy (which recalls the stoning scene in Monty Python’s Life of Brian when the admonishment ‘you’re only making it worse for yourself’ is correctly observed to be an empty threat), Bruno was burned at the stake in the Campo de’ Fiori in Rome and his ashes thrown into the Tiber. His crimes were numerous and included heretical ideas such as denying the divinity of Jesus. It is also the opinion of many historians that Bruno was irritating, argumentative and, not to put too fine a point on it, an all-round pain in the arse, so many powerful people were simply glad to see the back of him. But it is also true that Bruno embraced and promoted a wonderful idea that raises important and challenging questions. Bruno believed that the universe is infinite and filled with an infinite number of habitable worlds. He also believed that although each world exists for a brief moment when compared to the life of the universe, space itself is neither created nor destroyed; the universe is eternal.

Although the precise reasons for Bruno’s death sentence are still debated amongst historians, the idea of an infinite and eternal universe seems to have been central to his fate, because it clearly raises questions about the role of a creator. Bruno knew this, of course, which is why his return to Italy in 1591 after a safe, successful existence in the more tolerant atmosphere of northern Europe remains a mystery. During the 1580s Bruno enjoyed the patronage of both King Henry III of France and Queen Elizabeth I of England, loudly promoting the Copernican ideal of a Sun-centred solar system. Whilst it’s often assumed that the very idea of removing the Earth from the centre of the solar system was enough to elicit a violent response from the Church, Copernicanism itself was not considered heretical in 1600, and the infamous tussles with Galileo lay 30 years in the future. Rather, it was Bruno’s philosophical idea of an eternal universe, requiring no point of creation, which unsettled the Church authorities, and perhaps paved the way for their later battles with astronomy and science. As we shall see, the idea of a universe that existed before the Big Bang is now central to modern cosmology and falls very much within the realm of observational and theoretical science. In my view this presents as great a challenge to modern-day theologians as it did in Bruno’s time, so it’s perhaps no wonder that he was dispensed with.

Bruno, then, was a complex figure, and his contributions to science are questionable. He was more belligerent free-thinker than proto-scientist, and whilst there is no shame in that, the intellectual origins of our ascent into insignificance lie elsewhere. Bruno was a brash, if portentous, messenger who would likely not have reached his heretical conclusions about an infinite and eternal universe without the work of Nicolaus Copernicus, grounded in what can now clearly be recognised as one of the earliest examples of modern science, and published over half a century before Bruno’s cinematic demise.

OFF CENTRE

Nicolaus Copernicus was born in the Polish city of Torun in 1473 and benefited from a superb education after being enrolled at the University of Cracow at 18 by his influential uncle, the Bishop of Warmia. In 1496, intending to follow in the footsteps of his uncle, Copernicus moved to Bologna to study canon law, where he lodged with an astronomy professor, Domenica Maria de Novara, who had a reputation for questioning the classical works of the ancient Greeks and in particular their widely accepted cosmology.

The classical view of the universe was based on Aristotle’s not unreasonable assertion that the Earth is at the centre of all things, and that everything moves around it. This feels right because we don’t perceive ourselves to be in motion and the Sun, Moon, planets and stars appear to sweep across the sky around us. However, a little careful observation reveals that the situation is in fact more complicated than this. In particular, the planets perform little loops in the sky at certain times of year, reversing their track across the background stars before continuing along their paths through the constellations of the zodiac. This observational fact, which is known as retrograde motion, occurs because we are viewing the planets from a moving vantage point – the Earth – in orbit around the Sun.

This is by far the simplest explanation for the evidence, although it is possible to construct a system capable of predicting the position of the planets months or years ahead and maintain Earth’s unique stationary position at the centre of all things. Such an Earth-centred model was developed by Ptolemy in the second century and published in his most famous work, Almagest. The details are extremely complicated, and aren’t worth describing in detail here because the central idea is totally wrong and we won’t learn anything. The sheer contrived complexity of an Earth-centred description of planetary motions can be seen in Ptolemy’s Model, which shows the apparent motions of the planets against the stars as viewed from Earth. This tangled Ptolemaic system of Earth-centred circular motions, replete with the arcane terminology of epicycles, deferents and equant, was used successfully by astrologers for thousands of years to predict where the planets would be against the constellations of the zodiac – presumably allowing them to write their horoscopes and mislead the gullible citizens of the ancient world. And if all you care about are the predictions themselves, and your philosophical prejudice and common-sense feeling of stillness require the Earth to be at the centre, then everything is fine. And so it remained until Copernicus became sufficiently offended by the sheer ugliness of the Ptolemaic model to do something about it.

Copernicus’s precise objections to Ptolemy are not known, but sometime around 1510 he wrote an unpublished manuscript called the Commentariolus in which he expressed his dissatisfaction with the model. ‘I often considered whether there could perhaps be found a more reasonable arrangement of circles, from which every apparent irregularity would be derived while everything in itself would move uniformly, as is required by the rule of perfect motion.’

The Commentariolus contained a list of radical and mostly correct assertions. Copernicus wrote that the Moon revolves around the Earth, the planets rotate around the Sun, and the distance from the Earth to the Sun is an insignificant fraction of the distance to the stars. He was the first to suggest that the Earth rotates on its axis, and that this rotation is responsible for the daily motion of the Sun and stars across the sky. He also understood that the retrograde motion of the planets is due to the motion of the Earth and not the planets themselves. Copernicus always intended Commentariolus to be the introduction to a much larger work, and included little if any detail about how he had come upon such a radical departure from classical ideas. The full justification for and description of his new cosmology took him a further 20 years, but by 1539 he had finished most of his six-volume De revolutionibus, although the completed books were not published until 1543. They contained the mathematical elaborations of his heliocentric model, an analysis of the precession of the equinoxes, the orbit of the Moon, and a catalogue of the stars, and are rightly regarded as foundational works in the development of modern science. They were widely read in universities across Europe and admired for the accuracy of the astronomical predictions contained within. It is interesting to note, however, that the intellectual turmoil caused by our relegation from the centre of all things still coloured the view of many of the great scientific names of the age. Tycho Brahe, the greatest astronomical observer before the invention of the telescope, referred to Copernicus as a second Ptolemy (which was meant as a compliment), but didn’t accept the Sun-centred solar system model in its entirety, partly because he perceived it to be in contradiction with the Bible, but partly because it does seem obvious that the Earth is at rest. This is not a trivial objection to a Copernican solar system, and a truly modern understanding of precisely what ‘at rest’ and ‘moving’ mean requires Einstein’s theories of relativity – which we will get to later! Even Copernicus himself was clear that the Sun still rested at the centre of the universe. But as the seventeenth century wore on, precision observations greatly improved due to the invention of the telescope and an increasingly mature application of mathematics to describe the data, and led a host of astronomers and mathematicians – including Johannes Kepler, Galileo and ultimately Isaac Newton – towards an understanding of the workings of the solar system. This theory is good enough even today to send space probes to the outer planets with absolute precision.

At first sight it is difficult to understand why Ptolemy’s contrived mess lasted so long, but there is a modern bias to this statement that is revealing. Today, a scientifically literate person assumes that there is a real, predictable universe beyond Earth that operates according to laws of nature – the same laws that objects obey here on Earth. This idea, which is correct, only emerged fully formed with the work of Isaac Newton in the 1680s, over a century after Copernicus. Ancient astronomers were interested primarily in predictions, and although the nature of physical reality was debated, the central scientific idea of universal laws of physics had simply not been discovered. Ptolemy created a model that makes predictions that agree with observation to a reasonable level of accuracy, and that was good enough for most people. There had been notable dissenting voices, of course – the history of ideas is never linear. Epicurus, writing around 300 BCE, proposed an eternal cosmos populated by an infinity of worlds, and around the same time Aristarchus proposed a Sun-centred universe about which the Earth and planets orbit. There was also a strong tradition of classic orthodoxy in the Islamic world in the tenth and eleventh centuries. The astronomer and mathematician Ibn al-Haytham pointed out that, whilst Ptolemy’s model had predictive power, the motions of the planets as shown in the figure here represent ‘an arrangement that is impossible to exist’.

The end of the revolution started by Copernicus around 1510, and the beginning of modern mathematical physics, can be dated to 5 July 1687, when Isaac Newton published the Principia. He demonstrated that the Earth-centred Ptolemaic jumble can be replaced by a Sun-centred solar system and a law of universal gravitation, which applies to all objects in the universe and can be expressed in a single mathematical equation:

The equation says that the gravitational force between two objects – a planet and a star, say – of masses m₁ and m₂ can be calculated by multiplying the masses together, dividing by the square of the distance r between them, and multiplying by G, which encodes the strength of the gravitational force itself. G, which is known as Newton’s Constant, is, as far as we know, a fundamental property of our universe – it is a single number which is the same everywhere and has remained so for all time. Henry Cavendish first measured G in a famous experiment in 1798, in which he managed (indirectly) to measure the gravitational force between lead balls of known mass using a torsion balance. This is yet another example of the central idea of modern physics – lead balls obey the same laws of nature as stars and planets. For the record, the current best measurement of G = 6.67 × 10⁻¹¹ N m²/kg², which tells you that the gravitational force between two balls of mass 1kg each, 1 metre apart, is just less than ten thousand millionths of a Newton. Gravity is a very weak force indeed, and this is why its strength was not measured until 71 years after Newton’s death.

NEWTON’S LAW OF GRAVITY

F

Force between the masses

G

Gravitational constant

m₁

First mass

m₂

Second mass

r

Distance between the centres of the masses

This is a quite brilliant simplification, and perhaps more importantly, the pivotal discovery of the deep relationship between mathematics and nature which underpins the success of science, described so eloquently by the philosopher and mathematician Bertrand Russell: ‘Mathematics, rightly viewed, possesses not only truth, but supreme beauty – a beauty cold and austere, like that of sculpture, without appeal to any part of our weaker nature, without the gorgeous trappings of painting or music, yet sublimely pure, and capable of a stern perfection such as only the greatest art can show. The true spirit of delight, the exaltation, the sense of being more than Man, which is the touchstone of the highest excellence, is to be found in mathematics as surely as in poetry.’

Nowhere is this sentiment made more clearly manifest than in Newton’s Law of Gravitation. Given the position and velocity of the planets at a single moment, the geometry of the solar system at any time millions of years into the future can be calculated. Compare that economy – you could write all the necessary information on the back of an envelope – with Ptolemy’s whirling offset epicycles. Physicists greatly prize such economy; if a large array of complex phenomena can be described by a simple law or equation, this usually implies that we are on the right track.

The quest for elegance and economy in the description of nature guides theoretical physicists to this day, and will form a central part of our story as we trace the development of modern cosmology. Seen in this light, Copernicus assumes even greater historical importance. Not only did he catalyse the destruction of the Earth-centred cosmos, but he inspired Brahe, Kepler, Galileo, Newton and many others towards the development of modern mathematical physics – which is not only remarkably successful in its description of the universe, but was also necessary for the emergence of our modern technological civilisation. Take note, politicians, economists and science policy advisors of the twenty-first century: a prerequisite for the creation of the intellectual edifice upon which your spreadsheets, air-conditioned offices and mobile phones rest was the curiosity-driven quest to understand the motions of the planets and the Earth’s place amongst the stars.

AT THE CENTRE OF THE SOLAR SYSTEM

Matching the observations of the wandering stars – the planets – of the night sky with the idea that the Earth was at the centre of the solar system required extremely complex models. In the case of Venus, combining the Earth at the centre with the observations meant that Venus had a circular orbit around a point midway between the Earth and the Sun, so-called epicycles, with all the other planets having similar complicated orbits around various points scattered around the solar system. Placing the Sun at the centre of the solar system, with the planets arranged in their familiar order, with the Moon orbiting the Earth, gave a much simpler system.

CHANGING PERSPECTIVE

1968 was a difficult year on planet Earth. The Vietnam War, the bloodiest of Cold War proxy tussles, was at its height, ultimately claiming over three million lives. Martin Luther King Jr. was assassinated in Memphis, prompting presidential hopeful Bobby Kennedy to ask the people of the United States ‘to tame the savageness of man and make gentle the life of this world’. Kennedy himself was assassinated before the year was out. Elsewhere, Russian tanks rolled into Prague, and France teetered on the edge of revolution. As I approached my first Christmas, my parents could have been forgiven for wondering what kind of world their son would inhabit in 1969. And then, as Christmas Eve drifted into Christmas morning, an unexpected snowfall decorated Oakbank Avenue and Borman, Lovell and Anders, 400,000 kilometres away, saved 1968.

Apollo 8 was, in the eyes of many, the Moon mission that had the most profound historical impact. It was a terrific, noble risk; a magnificent roll of the dice; a distillation of all that is great about exploration; a tribute to the sheer balls of the astronauts and engineers who decided that, come what may, they would honour President Kennedy’s pledge to send ‘a giant rocket more than 300 feet tall, the length of this football field, made of new metal alloys, some of which have not yet been invented, capable of standing heat and stresses several times more than have ever been experienced, fitted together with a precision better than the finest watch, carrying all the equipment needed for propulsion, guidance, control, communications, food and survival, on an untried mission, to an unknown celestial body, and then return it safely to Earth, re-entering the atmosphere at speeds of over 40,000 kilometres per hour, causing heat about half that of the temperature of the Sun – almost as hot as it is here today – and do all this, and do it right, and do it first before this decade is out’. If I heard that from a leader today I’d be first on the rocket. Instead I have to listen to vacuous diatribes about ‘fairness’, ‘hard-working families’, and how ‘we’re all in it together’. Sod that, I want to go to Mars.

To set Apollo 8 in context, Apollo 7, the first manned test flight of the Apollo programme, was flown by Schirra, Eisele and Cunningham in October 1968. Apollo 8 was supposed to be a December test flight for the Lunar Lander, conducted in the familiar surroundings of Earth orbit, but delivery delays meant that it was not ready for flight and the aim of meeting Kennedy’s deadline looked to be dead. But this wasn’t the twenty-first century, it was the 1960s and NASA was run by engineers. The programme manager was George Low, an army veteran and aeronautical engineer who knew the spacecraft inside out and had the strength of character to make decisions. Why not send Apollo 8 directly to the Moon without the Lunar Lander, proposed Low, allowing Apollo 9 to test-fly the LEM (Lunar Excursion Module) in Earth orbit in early 1969 when it became available and pave the way for a landing before the decade was out? Virtually every engineer at NASA is said to have agreed, and so it was that only the second manned flight of the Apollo spacecraft lifted off from Kennedy on 21 December, ten short weeks after Apollo 7, bound for the Moon. The crew later said that they estimated their chance of succeeding to be fifty-fifty.

Borman: Oh my God!

Look at that picture over there.

Here’s the Earth coming up.

Wow, is that pretty.

Anders: Hey, don’t take that,

it’s not scheduled.

Borman: (laughing) You got a color film, Jim?

Anders: Hand me that roll of color quick, will you …?

Lovell: Oh, man, that’s great!

Precisely 69 hours, 8 minutes and 16 seconds after launch, the Command Module’s engine fired to slow the spacecraft down and allow it to be captured by the Moon’s gravity, putting the three astronauts into lunar orbit. Newton’s almost 300-year-old equations were used to calculate the trajectory. This was a spectacular, practically unbelievable engineering triumph. Less than a decade after Yuri Gagarin became the first human to orbit the Earth, three astronauts travelled all the way to the Moon. But the mission’s powerful and enduring cultural legacy rests largely on two very human actions by the crew. One was the famous and moving Christmas broadcast, the most-watched television event in history at that time, when distant explorers read the first lines from the Book of Genesis: ‘We are now approaching lunar sunrise, and for all the people back on Earth, the crew of Apollo 8 has a message that we would like to send to you,’ began Anders. ‘In the beginning God created the heaven and the Earth. And the Earth was without form, and void; and darkness was upon the face of the deep.’ Borman concluded with a sentence clearly spoken by a lonely man 400,000 kilometres from home. ‘And from the crew of Apollo 8, we close with goodnight, good luck, a Merry Christmas – and God bless all of you, all of you on the good Earth.’

The mission’s most potent legacy, however, is NASA image AS8-14-2383, snapped by Bill Anders on a Hasselblad 500 EL at f/11 and a shutter speed of 1/250th of a second on Kodak Ektachrome film. It was, in other words, a very bright photograph. The image is better known as Earthrise. When viewed with the lunar surface at the bottom, Earth is tilted on its side with the South Pole to the left, and the equator running top to bottom. Little landmass can be seen through the swirling clouds, but the bright sands of the Namib and Saharan deserts stand out salmon pink against the blackness beyond. Just 368 years and 10 months after a man was burned at the stake for dreaming of worlds without end, here is Earth, a fragile crescent suspended over an alien landscape, the negative of a waxing Moon in the friendly skies of Earth. This is an unfamiliar, planetary Earth, no longer central; just another world. When Kennedy spoke of Apollo as a journey to an unknown celestial body, he meant the Moon. But we discovered Earth and, in the words of T. S. Eliot, came to know the place for the first time.

OUTWARDS TO THE MILKY WAY

Newton’s laws are the keys to understanding our place in our local neighbourhood. Coupled with precision observations of the motion of the planets and moons, they allow the scale and geometry of the solar system to be deduced, and their positions to be calculated at any point in the future. The nature and location of the stars, however, requires an entirely different approach because at first sight they appear to be point-like and fixed. The observation that the stars don’t appear to move is important if you know something about parallax, as the ancients did. Parallax is the name given to a familiar effect. Hold your finger up in front of your face and alternately close each of your eyes, keeping your finger still. Your finger appears to move relative to the more distant background, and the closer your finger is to your face, the more it appears to move. This is not an optical illusion; it’s a consequence of viewing a nearby object from two different spatial positions; in this case the two slightly different positions of your eyes. We don’t normally perceive this parallax effect because the brain combines the inputs from the eyes to create a single image, although the information is exploited to create our sense of depth. Aristotle used the lack of stellar parallax to argue that the Earth must be stationary at the centre of the universe, because if the Earth moved then the nearby stars would be observed to move against the background of the more distant ones. Thousands of years later, Tycho Brahe used a similar argument to refute the conclusions reached by Copernicus. Their logic was completely sound, but the conclusion is wrong because the nearby stars do move relative to the more distant background stars as the Earth orbits the Sun, and indeed as the Sun orbits the galaxy itself. You just have to look extremely carefully to see the effect.

Amongst the thousands of stars visible to the naked eye, 61 Cygni is one of the faintest. It’s not without interest, being a binary star system of two orange K-type dwarf stars, slightly smaller and cooler than the Sun, orbiting each other at the lethargic rate of around 700 years. Despite the pair’s relative visual anonymity, however, 61 Cygni has great historical significance. The reason for this quiet fame is that this faint star system was the first to have its distance from Earth measured by parallax.

Friedrich Bessel is best known to a physicist or mathematician for his work on the mathematical functions that bear his name. Pretty much any engineering or physical problem that involves a cylindrical or spherical geometry ends up with the use of Bessel functions, and, in blissful ignorance, you will probably encounter some piece of technology that has relied on them in the design process at some point today. But Bessel was first and foremost an astronomer, being appointed director of the Königsberg Observatory at the age of only 25. In 1838, Bessel observed that 61 Cygni shifted its position in the sky by approximately two-thirds of an arcsecond over a period of a year as viewed from Earth. That’s not very much – an arcsecond is one 3600th of a degree. It is enough, however, to do a bit of trigonometry and calculate that 61 Cygni is 10.3 light years away from our solar system. This compares very favourably with the modern measurement of the distance, 11.41 ± 0.02 light years. Parallax is so important in astronomy that there is a measurement system completely based on it, which allows you to do these sums in your head. Astronomers use a distance measurement known as a parsec – which stands for ‘per arcsecond’. This is the distance of a star from the Sun that has a parallax of 1 arcsecond. One parsec is 3.26 light years. Bessel’s measurement of the parallax of 61 Cygni was 0.314 arcseconds, and this immediately implies that it’s around 10 light years away.

Even today, stellar parallax remains the most accurate way of determining the distance to nearby stars, because it is a direct measurement which uses only trigonometry and requires no assumptions or physical models. On 19 December 2013 the Gaia space telescope was launched on a Soyuz rocket from French Guiana. The mission will measure, by parallax, the positions and motions of a billion stars in our galaxy over five years. This data will provide an accurate and dynamic 3D map of the galaxy, which in turn will allow for an exploration of the history of the Milky Way, because Newton’s laws, which govern the motions of all these stars under the gravitational pull of each other, can be run backwards as well as forwards in time. Given precise measurements of the positions and velocities of 1 per cent of the stars in the Milky Way, it is possible to ask what the configuration of the stars looked like millions or even billions of years ago. This enables astronomers to build simulations of the evolution of our galaxy, revealing its history of collisions and mergers with other galaxies over 13 billion years, stretching back to the beginning of the universe. Newton and Bessel would have loved it.

Stellar parallax, when deployed using a twenty-first-century orbiting observatory, is a powerful technique for mapping our galaxy out to distances of many thousands of light years. Beyond our galaxy, however, the distances are far too great to employ this direct method of distance measurement. In the mid-nineteenth century, this might have appeared an insurmountable problem, but science doesn’t proceed by measurement alone. As Newton so powerfully demonstrated, scientific progress often proceeds through the interaction between theory and observation. Newton’s Law of Gravitation is a theory; in physics this usually means a mathematical model that can be applied to explain or predict the behaviour of some part of the natural world. How might we measure the mass of a planet? We can’t ‘weigh’ it directly, but given Newton’s laws we can determine the planet’s mass very accurately if it has a moon. The logic is quite simple – the moon’s orbit clearly has something to do with the planet’s gravity, which in turn has something to do with its mass. These relationships are encoded in Newton’s law, and careful observation of the moon’s orbit around the planet therefore allows for the planet’s mass to be determined. For the more mathematical reader, the equation is:

where a is the (time-averaged) distance between the planet and the moon, G is Newton’s gravitational constant and P is the period of the orbit. (This equation is in fact Kepler’s third law, discovered empirically by Kepler in 1619. Kepler’s laws can be derived from Newton’s law of gravitation.) Under the assumption that the mass of the planet is far larger than the mass of the Moon, this equation allows for the mass of the planet to be measured. This is how theoretical physics can be used to extract measurements from observation, given a mathematical model of the system. To measure the distance to objects that are too far away to use parallax, therefore, we need to find a theory or mathematical relationship that allows for a measurement of something – anything – to be related to distance. The first relationship of this type, which opened the door to all other methods of distance measurement out to the edge of the observable universe, was discovered at the end of the nineteenth century by an American astronomer named Henrietta Leavitt.

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