Devika turned to the Doctor. His face reflected the light from below. 'Commander Sanford is speaking to IASA Command on Earth about how we best protect this life form. We'll do better, I promise.'

The Doctor's expression didn't change. 'I'll be watching, so make sure you do. You got lucky today. Very lucky.'

'It was OK in the end, right?' said Clara.

'Just. But here's the thing.' The Doctor turned to face them, tall frame highlighted against yellow Venusian cloud and the inky black carapace of s.p.a.ce above. 'Humanity needs to buckle up. You're expanding, setting off through the stars. I envy you the discovery, but out there...' He opened his arms wide, gesturing into s.p.a.ce. 'Out there, beyond the Solar System, there are even more strange, fantastical, dangerous things to discover.'

The Doctor looked at each of them in turn, Devika and Rick hanging on his every word. Clara smiled as he continued.

'You thought today was scary you just see what's waiting out there.' And he grinned.

'I've had a life with you for 19 years, but then I met the Doctor, and all the things I've seen him do for me, for you, for all of us. For the whole stupid planet and every planet out there.'

Rose Tyler, Doomsday (2006)

Until 24 August 2006 a month after the first broadcast of the Tenth Doctor episode, Doomsday we didn't know what a planet was.

The word 'planet' means 'wanderer' and dates back thousands of years to the ancient Greeks. But their ideas about what they saw when they looked up in the sky were very different from what we understand now. They thought the Earth was at the centre of the cosmos, surrounded by a series of huge, revolving, crystal spheres. Each of the first seven spheres had a planet fixed to it and these seven 'planets' included the Sun and Moon. On the eighth and outermost sphere were fixed all the stars.

OK, the ancient Greeks got it wrong, but as Doctor Who shows us when the TARDIS lands in the past people in history weren't stupid. They were just as clever and inventive as we are. Today, we have ever more powerful telescopes to look deep into s.p.a.ce. We send s.p.a.cecraft and robots to explore the planets near us such as Venus and gaze further out into the universe. But for a long time, all people could use to explore the heavens was the naked eye and some clever thinking.

Imagine a room with a wheel fixed on the wall. The wheel is side-on, so it looks like a disc a circle. The circle is spattered with blobs of white paint. Turn off the lights and you can't see the circle but the paint blobs glow in the dark. They look like a pattern of stars. Then the pattern of blobs starts to move, all together, spinning round and round.

What must be happening? The obvious answer is that the wheel is turning. It doesn't matter that you can't see the wheel in the darkness; you can work it out easily from the way the blobs move. If you'd never seen the wheel if you'd only entered the room when the lights were already off you'd still quickly work out that the glowing blobs were fixed to something that was spinning round.

What you probably wouldn't think is that the blobs were keeping still and you were the one who was spinning. That would be ridiculous! How could you be turning upside down but not feel you were moving at all?

Except, of course, that that is exactly what's happening when we look up at the stars. They slowly turn through the night sky, as if they're fixed on something like a wheel. But they look like that because our planet Earth is turning on its axis. We're the ones going round.

'It's like when you're a kid. The first time they tell you the world's turning and you just can't quite believe it because everything looks like it's standing still. I can feel it. The turn of the Earth. The ground beneath our feet is spinning at a thousand miles an hour, and the entire planet is hurtling round the Sun at 67,000 miles an hour, and I can feel it. We're falling through s.p.a.ce, you and me, clinging to the skin of this tiny little world, and if we let go...'

The Ninth Doctor, Rose (2005)

Frankly, a lot of what we now know about other planets and our own is a bit weird and unsettling. It's certainly not obvious. So how did we puzzle it out? Unlike the Doctor, we can't feel the turn of the Earth beneath us. We don't have a TARDIS to take us to other planets for a quick look round.

But what does the Doctor do when he lands on an alien world where something strange is happening? He explores, looks for clues and asks awkward questions (sometimes getting himself in trouble with whoever's in charge). That's also how, over thousands of years, we puzzled out what planets really are.

Let's go back to the beginning. Why did the ancient Greeks think the planets were fixed to gla.s.s spheres?

As we saw, it looks as if the pattern of stars is slowly turning round and round, fixed to something like a wheel that we can't see. But the stars aren't the only lights in the sky. There are the Sun and the Moon. Then there are a whole lot of things that, with the naked eye, look like the other stars but don't behave in the same way. Instead of turning round and round as part of one fixed pattern, these stars seem to 'wander' about from night to night. The ancient Greek word for wanderers was 'planetai'.

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 s.p.a.ce (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 b.a.l.l.s 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 pa.s.sing 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 s.p.a.ces 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 pa.s.sing 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 s.p.a.ce, so that birds can't fly in a straight line? (We know now that s.p.a.ce 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 gla.s.s 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 s.p.a.ce 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.