The Sun was the easiest to see of these planetai, and its movements were the easiest to understand – or that was how it seemed. Every morning, the Sun rises in the east. It then gets slowly higher in the sky until midday, starts to sink again in the afternoon and finally sets in the west. We don’t see it at night but it’s back in the east by next morning. It seemed obvious that the Sun circled round us.
Less obvious was explaining why the Sun’s position in the sky at midday was much higher in the summer than it is in winter. We now know that the Earth is slightly tilted as it spins round the Sun. When the tilt means we point towards the Sun, we get more hours and a greater density of sunlight – our summer. At the same time we have our summer, the other side of the Earth tilts away from the Sun so is in winter. But ancient civilisations thought the Sun went round the Earth, so they came up with different explanations.
The ancient Greeks knew that if you threw something up into the sky it fell back down to Earth. So, they asked themselves, why didn’t the Sun fall to Earth? They decided that it had to be fixed to something. Since the Sun circled around us, they decided it was fixed to something round. Since the height of the Sun varied through the year, they decided the thing was a sphere, and the position of the Sun on this sphere was not quite aligned with Earth. Since we could see other planets and stars further away than the Sun, the sphere had to be made of something transparent. The only naturally occurring transparent material they knew of was crystal. So, in a series of logical steps, they concluded that the Sun was fixed to a huge crystal sphere, encircling and spinning round Earth.
* * *
‘Logic, my dear Zoe, merely enables one to be wrong with authority.’
The Second Doctor, The Wheel in Space (1968).
* * *
Other ancient people had their own ideas about how the Sun moved round the Earth: the ancient Egyptians thought it was rolled through the sky by a gigantic (and invisible) scarab beetle – just the way that dung beetles in Egypt rolled balls of dung along the ground. In The Aztecs (1964), the Doctor meets people who understand the movements of the Sun enough to know there’s going to be an eclipse – but they think the Sun won’t shine again afterwards unless it’s offered lots of human blood.
These theories might not have been right, but they seemed to fit the observed movements of the Sun. More importantly, understanding that the Sun’s position varied in a regular cycle – what we call a year – meant ancient people knew how the Sun would behave in future.
Until that point, human beings had themselves been wanderers. We hunted and foraged for food where we could find it. In the very first Doctor Who story – An Unearthly Child (1963) – the Doctor meets a tribe of people living a desperate existence in what seems to be the Stone Age. They’re scared and cold and hungry because ‘Orb’ (the Sun) has left the sky. To eat, they must hunt dangerous animals – and do it every day. We’re told many of the tribe have died.
But once people understood Orb’s movements through the sky, life became a lot easier. For example, in about 4000 BC, people settled along the banks of the River Nile in Egypt. The river flooded every year, and the flooded, muddy ground was good for growing crops. Understanding the movement of the Sun and the passing of the year, people knew when the river would flood, when to sow crops and when to reap the harvest.
At first they sowed and reaped by hand, but once the Egyptians started using a strangely shaped bit of wood called a plough they suddenly produced far more food than they needed. That meant they didn’t have to spend every day worrying about where their next meal would come from. They had time to think, experiment and invent things that improved their lives.
The food they grew had to be stored so it wouldn’t go bad, so they invented some of the first clay pots. They made lots of pots and had to know what was in them and who they belonged to, which meant inventing some of the first kind of writing. Keeping track of how much food there was and how to divide it needed mathematics. When the Nile flooded, it could wash away the markers that said who owned which bits of land, so people needed ways of recording those spaces and marking them out again. That needed more complicated maths. Irrigation systems and building projects needed more complex skills – and tools. Weapons were needed to stop anyone stealing the food. Extra food and inventions could be swapped with other peoples – so trade was invented. People could even earn food by doing things that weren’t directly useful: making up stories and songs to entertain everyone else.
This sequence of major inventions took time. There was about 1,500 years between people settling by the Nile and them building what we think was the world’s first large stone building – the step pyramid of Djoser at Saqqara (which you can glimpse in the opening moments of Fourth Doctor story Pyramids of Mars (1975)). But what’s important is that these developments – what we now think of as the beginning of civilisation – were only made possible by first understanding the movement of the Sun.
* * *
‘I’m the Doctor. Who are you?’
‘Organon, sir … Astrologer extraordinary. Seer to princes and emperors. The future foretold, the past explained, the present apologised for.’
The Fourth Doctor and Organon, The Creature from the Pit (1979)
* * *
People studied the movements of the other ‘planets’, too. For example, it was handy to know how the Moon behaved in the days before electric lights. If you were planning a party, you’d want it on a night when there would be a full Moon so your guests could see their way home safely. Understanding the Moon also helped keep track of the passing of time on a scale longer than a day but shorter than a year. If you lived by the sea, understanding the Moon helped you predict the tides.
Some people also thought that events in the sky were connected to events on Earth. If so, knowing how the planets moved and where they’d be in future meant you could predict what was going to happen here. Today, we call that astrology, and it’s different from the science of astronomy. In ancient times, astrology and astronomy were mixed up together.
We know ancient people kept careful note of the movements of the planets. In Room 55 of the British Museum in London, there’s a clay tablet covered in small, triangular marks. This cuneiform writing details sightings of Venus over 21 years between 724 and 704 BC. (Some archaeologists think the tablet is a copy of sightings recorded 1,000 years before that!)
These sightings showed that the planets moved slowly through the sky in curved paths – just as you’d expect if they were attached to crystal spheres we couldn’t otherwise see. But there was something odd when you examined the records, too. Every so often, the outer planets – Mars, Jupiter and Saturn – seemed to double back on themselves in little loops. Why?
Sometime around 250 BC, the ancient Greek astronomer Aristarchus came up with an explanation for this strange movement. He said that if the Sun was at the centre of the cosmos and the Earth circled round it we would go round the Sun faster than the outer planets. The ‘loops’ were the effect of us overtaking them. Aristarchus was right, but at the time people didn’t think so – for sensible, logical reasons. After all, if the Earth is moving, why don’t we feel it? (We now understand that we don’t feel motion but changes in motion – what’s called inertia.) Why isn’t there a constant wind as we move through space, so that birds can’t fly in a straight line? (We know now that space is empty, and the atmosphere moves with the Earth.) If we’re moving round the Sun, why don’t the other stars get bigger as we get closer to them and smaller as we move away? (We now know they do, but the difference is tiny because the stars are a very, very long way from us – as we’ll see in Chapter 2.) What’s more, Aristarchus couldn’t demonstrate his idea to people using computer graphics. To even understand the problem of the loops in the first place, you had to know some tricky maths.
When a Polish astronomer, Nicholas Copernicus, suggested in AD 1543 that the Earth moved round the Sun, plenty of people still did not believe it – or understand the maths.
But by now the printing press had been invented, and books could get to those people who could follow the argument. Argument is probably the right word. It didn’t help that political and religious leaders had for centuries told their people that the Sun circled round the Earth. Questioning that system seemed like questioning them – which meant you were guilty of treason or heresy. For exactly that reason, Copernicus didn’t share his ideas until he was on his deathbed, but a monk called Giordano Bruno was burnt at the stake in 1600 for agreeing with Copernicus and even suggesting that there might be planets circling other stars.
(The Doctor Who story The Ribos Operation (1978) touches on this period of history when Unstoffe meets Binro the Heretic, a poor man tortured for suggesting that his planet circles its Sun and that the points of light in the sky are not ice crystals but other, distant suns.)
In 1609, German astronomer Johannes Kepler published New Astronomy. This book agreed with Copernicus about the planets circling round the Sun, but Kepler went even further. He used very accurate recordings of the movements of the planets going back many years to show that planets didn’t go round the Sun in a perfect circle but in an oval shape called an ellipse. He spotted, too, that a planet’s speed also varied as it went round.
Again, what we now call Kepler’s laws of planetary motion involve some complicated maths and not everyone could follow them. But the same year his book was published, an Italian astronomer, Galileo Galilei, pointed a new invention called a telescope up towards the sky.
A telescope uses curved glass to bend light so that things seen through it appear bigger than they are. Looking through a telescope on 7 January 1610, Galileo spotted small stars beside Jupiter. Over the next few nights, these stars still looked close to Jupiter but were in different positions. He soon realised that these four small ‘stars’ were circling round Jupiter – they were Jupiter’s moons, and proof that not everything in space circled round the Earth.
Galileo was also the first to see the rings of Saturn (though only dimly; he didn’t know what they were) and found that Venus had ‘phases’ like the waxing and waning of the Moon. That matched something Copernicus had said: in an Earth-centred system, Venus could only show two phases; in a Sun-centred one, it would be like the Moon.
The people in charge at the time didn’t like Galileo’s ideas: he was arrested and they stopped his book being published – at least in countries they controlled. But despite their best efforts, his ideas quickly spread. For the first time, people who couldn’t understand the complex maths started to follow the argument, for one important reason. The telescope was a simple enough device that they could copy the experiments Galileo set out in his book. They could see for themselves.
* * *
‘My dear chap, I’m a scientist, not a politician.’
The Third Doctor, Day of the Daleks (1972)
* * *
That’s an important principle of science as we understand it today: we should be able to repeat experiments and see the proof for ourselves. Just like the Doctor, scientists don’t believe something just because they’re told to, even if the person telling them is very clever or important. They insist on seeing evidence.
From that, we get a system for carefully testing new ideas, called the scientific method:
» First we gather good evidence.
» Using that evidence, we come up with an idea – called a model – of what is going on.
» We use the model to make predictions, and see – with more evidence – if we’ve got it right.
» We might have to modify our model, or throw it out and start again. Or we might just have got it right.
» We share our model with other scientists, who – often in a process called peer review – try and find problems with it. Sometimes it takes years to find a problem with a model, which then turns out to be completely wrong.
One Doctor Who writer, Douglas Adams, explained it like this: the scientific method ‘rests on the premise that any idea is there to be attacked. If it withstands the attack then it lives to fight another day and if it doesn’t withstand the attack then down it goes.’
After Galileo’s discovery – and people being able to check it for themselves – science became a fashionable hobby. In some countries, there were clubs where you could go for a night out to watch the latest experiments be performed. One such club, the Royal Society, was founded in 1660. A few years later, a debate among its members about the movements of the planets prompted Isaac Newton to write up his own ideas.
Newton was a genius, but he’s been described as a sorcerer as much as a scientist. It’s thought that it was his interest in magic – and the ability to apply force from a distance – that led him to come up with the theory of gravity. But, in the scientific method, it doesn’t matter where his ideas came from, only that they fitted the evidence. And gravity more or less did.
Very simply, Newton worked out that bodies in the universe – for example, planets – are drawn towards each other, and the strength of the attraction is related to both their combined mass and how far they are apart. His explanation was complex, but he supplied a simple mathematical formula that allowed people to test if he was right.
Science was still fashionable a century later when the musician William Herschel caught the bug for astronomy. In 1781, from his garden in Bath, he spotted through his telescope a star that didn’t behave like the others. At first, he thought it was a comet, but then realised he’d discovered an entirely new planet – Uranus.
But the movement of Uranus didn’t quite match Newton’s simple formula. Which meant either Newton was wrong or that another large object was out there in space, and the effect of its gravitational pull on Uranus was the reason its orbit was odd. Sure enough, in 1846 another new planet was discovered – Neptune. It’s orbit was odd, too, so astronomers searched for another planet, even further out in space. Pluto was discovered in 1930 – and was still too small to explain the odd orbit. It was discovered in the 1990s that we had overestimated Neptune’s mass; the correct numbers matched the oddness of the orbit.
There were problems with the theory of gravity closer to home, too. Mercury – the closest planet to the Sun – was never quite where Newton’s formula said it should be. Then in 1916 Albert Einstein suggested that the Sun was so big it warped space and time, bending it like a lens in a telescope bends light. In 1919, new observations of Mercury showed it exactly where Einstein – not Newton – said it would be, proving Einstein’s theory of relativity.
When Doctor Who landed on its first alien world in December 1963 (Skaro, home planet of the Daleks), we knew in reality of nine planets, all of them in orbit round our Sun. Pluto was a planet – the Doctor even calls it one when he visits in The Sun Makers (1977). But we still didn’t know what a planet was. So, what changed?
In November 2003, astronomers spotted what was reported in the press as the tenth planet – Sedna. More discoveries soon followed: Haumea in 2004, Eris and Makemake in 2005. Were these all planets, too? Eris seemed to be larger than Pluto, but if we made the decision based on size then surely Ceres counted, too. Ceres, discovered in 1801, is the largest rock in the asteroid belt between Mars and Jupiter – and had originally been thought to be a planet before more of the asteroid belt was discovered.
So, after much debate, in 2006 the International Astronomical Union passed Resolution 5A, setting out three things that define a planet. A planet:
» Is in orbit round the Sun
» Is big enough that its own gravity makes it (nearly) spherical
» Doesn’t share its orbital path with other objects.
* * *
The Tenth Planet and Vulcan
Before Einstein came up with relativity, there were other theories about why Mercury didn’t move exactly as Newton’s formula said it should. As with the odd orbits of Uranus and Neptune, it was suggested that there might be another, as yet undiscovered planet affecting its gravity. But why hadn’t we seen this other planet?
One explanation was that this planet was so close to the Sun that it was hidden by its glare. That’s why the theoretical planet was named Vulcan after the Roman god of fires and forges (as we learnt in The Fires of Pompeii (2008), Vulcan is also where we get the word ‘volcano’). Another explanation was that Vulcan was always on the far side of the Sun from us, moving round it at the same speed that we were. The only way that could happen was if it shared our orbit – a twin planet, just like Earth, always hidden from view.
Though Vulcan turned out not to exist, it influenced two Doctor Who stories in 1966. In The Tenth Planet, Earth’s long-lost twin, Mondas, is home to the Cybermen. The very next story, The Power of the Daleks, is set on a planet called Vulcan. By coincidence, that same year a new science fiction series began on American television. Star Trek’s Mr Spock was also from a planet called Vulcan.
* * *
Ceres, Pluto, Haumea, Makemake and Eris were recognised as ‘dwarf planets’ – though the IAU was clear that that meant they are not planets.
At the same time we were losing one planet, we were discovering thousands more.
In 1992, astronomers announced the discovery of two planets round a star called PSR 1257+12. As of February 2015, we’ve discovered 1,523 planets, while the space observatory named after Kepler has found a further 3,303 that await confirmation by more tests. We found these planets by measuring tiny but regular wobbles in the light from distant stars. Using Kepler and Newton’s laws, we can use these wobbles to work out the size of the planets and how far they are from their suns.
The Scientific Secrets of Doctor Who Page 3
