More Planets: Tiny but dangerous ones.

“So Albert, what's next on this tour of the solar system?”

Well, we're just passing through the asteroid fields between Jupiter and the next planet we'll come to which is Mars.

“So where are all the asteroids? On Star Wars when they went into an asteroid field they were dipping and diving to miss them.”

One day I'll have to see this Star Wars film you keep talking about, but I'm not sure it gives you a very accurate picture of the universe. There are certainly plenty of asteroids it's just that space is very big.

“So how many asteroids are out here?”

I just looked up the number, by last month they had found 378,546 asteroids but they are finding thousands more every month so there may be a million or more out there. The biggest Ceres is much smaller than the earth's moon, around 600 miles across, and that single one contains a third of all the rock and ice in the asteroid belt. Here's a picture showing the Earth's moon and Ceres together.

“Is Ceres another Greek or Roman God?”

Well guessed, Ceres is the Roman God of agriculture. Ceres was the first asteroid they found and initially astronomers thought it was a new planet, until they started finding more and more of them in the same area. For a while they kept giving them names of minor Gods and Goddesses, like Pallas, Juno and Vesta.

“Then what happened?”

They were finding so many that they were running out of names, so the the discoverers starting naming them after their country, family members and even pets.
Now there are so many that most have only numbers or a scientific code. Have a look at THIS PAGE to see if anyone you know has an asteroid named after them .

“OK Albert, back in a minute...”

“Back again. I don't believe it. There are asteroids called Smiley, Doctor Watson and Asterix! Isaac Newton has his own one, but I'm afraid there's no Albert Einstein, just one called Albert.”

I think you missed one. I checked earlier and there is also an asteroid called just Einstein so that means I have two, one more than Newton.

“Oops, sorry, but guess what asteroid number 9007 is called?”

I'm not sure I know that one, sorry.

“James Bond!”

Why?

“James Bond? Double 'O' 7, get it?”

Is this a Star Wars thing again?

“No he's a secret agent, but he is in films too. Once this trip is over Albert I need to sit you down and show you all the great films you missed since you died. Oh, was that an asteroid we just went past?”

That was a fairly typical small lumpy looking asteroid yes.

“Hmm, not very exciting are they? How about getting onto the next proper planet?”

Not so fast young man, patience. They may be small but asteroids can have much more effect on the Earth than any of the monster planets we were talking about earlier.

“Such as?”

Such as killing half the living creatures on Earth for a start.

“Now you're the one talking about the nonsense from films.”

No, it really happened. That's how the dinosaurs died off and it wasn't just them. In the seas even more species were lost than on land.

“This sounds like science fiction.”

No just science but like a lot of science it started with a crazy idea that people found hard to believe. Luis Alvarez and his son Walter Alvarez made the suggestion in 1980 that an asteroid about 6 miles across crashed into the Earth 65 million years ago and caused the dinosaurs to die out.

“How could they possibly prove that?”

All over the world in rocks that were 65 million years old they found a thin layer of clay that had lots of an element called Iridium.

“What does that prove?”

Well there is very little Iridium in most rocks but there is a lot of Iridium in asteroids and meteorites.

“OK, but where's the big hole?”

They found a crater in Mexico that is now buried under the sea that dates from just the right time. They've even found a few other possible craters from around the same time in India and in the North Sea near England, so something big could have broken up and landed in several pieces.

“Oh, so people think it really happened. It's not just a crazy idea?”

No, now it's a considered the most likely reason for how the dinosaurs died out.

“Could it happen again?”

Yes it could. There was an even bigger extinction 251 million years ago when 90% of sea and 70% of land species died out. That might have been an asteroid too. The Earth is always being hit by small things, a hundred tons arrives from space every day but most are tiny particles or dust that burn up as they arrive. But there are bigger asteroids that come near the Earth and could possibly hit it one day.

“Any time soon?”

There seems to be no immediate risk, but I read that a few years ago it was predicted an asteroid called 1997 XF11 might hit Earth October 26 2028. Don't panic, they made a slight error in their calculations so it will miss by a safe margin.

“Phew. So the Earth is safe for now.”

Well...an asteroid called Apophis, the Egyptian god of evil and destruction who lives in the darkness might just cause a problem. On April 13, 2036 this asteroid has 0.0022% chance of hitting the Earth or a 1 in 45,000 chance, but on the bright side there is a 99.9978% chance the asteroid it will just miss us.

“Apophis destroying the Earth. That's straight out of Stargate, now I know you are joking.”

No, straight out of Nasa. Take a look for yourself. http://neo.jpl.nasa.gov/risk/a99942.html

“So we may have to work out how to blow up asteroids after all to save the world.”

We might.

“Is that 13th of April in 20..whatever a Friday by any chance?”

No I think it's a Sunday.

“Phew.”

Now it's time to get some practice blowing up a few asteroids. Have a go at this great Asteroids game I found.

Planets of the solar system: Big ones.

“Albert, that big planet ahead looks blue, is that water?”

Those pretty clouds may contain a bit of water but there is a lot of other poisonous stuff like methane.

“What's that?”

A gas that comes from rotting things and cow dung, amongst other things.

“Oh how did it get out here then?”

It was here when the planets were first made from the gas and dust from blown up stars and bits left over the from big bang.

“Still it's a pretty impressive compared to poor Pluto. What's that planet called?”

That's Neptune, the first real planet we've met in this solar system, but the astronomers have made such a mess with their new definition of a planet that Neptune might not be a planet either.

“But it's huge!”

That's right four times bigger than Earth but one of the definitions of a planet is that it has to have cleared its orbit.

“What does that mean?”

It is supposed to mean that it should be the only thing in its orbit. Any decent sized planet's gravity with make any smaller mini-planet in the same orbit crash into it or get captured as a moon. The problem is that as Pluto cross Neptune's orbit, Neptune seems to fail that test.

“So it's not a planet?”

Oh, it's definitely a planet, scientists just need to get their words straight on defining a planet.

“They're meant to be the smart ones aren't they?”

Scientists are usually smart but often foolish and you can nearly always find something they can't agree on. Even something that seems quite simple can get scientists vexed, but Neptune has been here far long than scientists have been on Earth and Neptune will still be here when that definition is long forgotten.

“So tell me something interesting about Neptune. Is it outside that Goldilocks zone you were talking about last time?”

Oh definitely not, at almost three billion miles from the Sun it's always cold out here. One of Neptune's 11 moons, Triton, is famous for being one of the coldest places in the solar system at -240oC and having a crinkled surface like a cantaloupe melon. Apart from the poisonous atmosphere on Neptune there is also the problem with the weather which seems to be pretty awful too. One the space probes that have been sent to explore out here found a hurricane bigger than the whole Earth and the winds get up to 700 miles per hour.

“So there is not chance of any life down there?”

Well, even though the sun doesn't warm up the surface, the inside of Neptune might be quite hot and people have even suggested there might be vast oceans of water deep down. If there was any life in those seas it would have to be very different to us, the combination of gravity and heat from the centre would make the ocean more like a pressure cooker with a temperature of several thousand degrees centigrade.

“What's the strangest thing about Neptune?”

The wildest idea, possibly in the whole solar system, came from scientists in America in 1999. They recreated the high pressure and high temperature atmosphere of Neptune in a laboratory and found that the methane gas could be turned in diamond dust.

“Diamonds? Are you serious?”

Methane contains atoms of carbon, and diamonds are a form of pure compressed carbon so it makes sense. So the scientists, Benedetti and Raymond Jeanloz, suggested that it might be literally raining diamonds on Neptune and on Uranus too which that is big lump of a planet over there.

“Uranus looks a bit like Neptune, just a bit greener.”

They are almost like twins and made of the same stuff with lots of methane. Uranus most definitely wins the prize for the moons with the best collections of names. If you think what poets on earth achieved with just one moon called "the moon", imagine how lyrically they would have waxed with five moons called Ariel, Umbriel, Titania, Oberon and Miranda.

“Do all these big planets out here have loads of moons? They all seem to have loads.”

The Earth is unusual with only have one moon, these big planets also have rings but the rings on Uranus and Neptune are pretty small.

“Now that planet has really nice rings.”

That’s Saturn, it’s mostly made up of gases, but has sixty moons at the last count, and the most amazing set of rings that are more than 170,000 miles across.

“Impressive. What are they made of?"

They're made up of lumps of ice, dust and rocks. For all their size they are very thin, only a few miles thick or even less in places.

“Where do they come from?”

Well, they may be collections of small rocks that never made it to be moon sized, or they might have once been moons that were destroyed by colliding with each other or by the gravity of their planet pulling them apart. Now how about this for a strange fact. Although Saturn is the second largest planet in the solar system and weighs almost one hundred times more than the Earth (a mere 50,000 billion billion tonnes), it would float if you could find a big enough bath to float it in.

"How can a planet that weighs 50,000 billion billion tonnes float?"

All it means is that, like ice or wood or anything else that floats, the density of Saturn is less than water. An iceberg can be very heavy, but as long it weighs less than an equal volume of water it will float.

"And Saturn weighs less than an equal volume of water?"

Precisely. Archimedes, another of the ancient Greeks and one of their best mathematicians, worked all this out and it is still called the Archimedes Principle.

“How could he have worked out that Saturn would float? I thought they didn't know what the planets really were?”

That's right but he worked out the principle that applies to all floating or sinking things. It all started when he was asked to work out if a goldsmith had cheated King Hieron II of Syracuse when making a crown. When the idea came to him in the bath, he is supposed to have run naked down the street shouting Eureka – or I’ve got it. Once the cheating goldsmith had been sorted out, Archimedes developed the idea to explain how things float. Unlike some other ideas dating from ancient Greece, the Archimedes Principle has stood the test of time and is as valid today as when Archimedes leapt out of his bath dripping with water and enthusiasm.

“Now that is one massive planet over there.”

This next one is Jupiter is huge, the biggest planet in the solar system

“What’s that big red spot?”

That swirling red area, reasonably enough called the ‘Red Spot’, is a storm that is more than twice the size of the earth and it’s been there for at least 100 years. So you earthlings shouldn’t complain too much about your weather.

In some ways Jupiter is like a mini solar system. The planet itself is very similar to the sun being mostly hydrogen (about 85%) and most of the rest being helium. It is encircled by four large moons and twelve medium sized ones. In total Jupiter has over 60 moons if you count the very small ones too. The only small thing about Jupiter is its ring; a rather puny affair compared with Saturn or even Uranus.

"If Jupiter is big and made of the same stuff as the sun why doesn't it glow?"

Jupiter is almost as big as the smallest star but it doesn't have enough gravity to make the centre hot enough for nuclear reactions to start. In stars the gravitational forces compress the gases in the centre raising the temperature to the required 10 million degrees or so. Jupiter hasn't got enough gravity to do that.

“What would have happened if Jupiter had been big enough to become a star?”

In many solar systems that's exactly what did happen, there are lots of stars that have companion stars and they rotate around each other. If that had happened in our solar system, I doubt the Earth would be such as a nice place to live in terms of temperature. It might get a bit hot with two suns but it would make for interesting sunsets.

“Like in Star Wars.”

When did the star wars happen, while I was dead?

“Don't worry Albert, it's just a film.”

Planets, planets everywhere but not a drop to drink....until now

“Albert, this solar system we're about to explore, is this the only one?”

Well there was a time when some people thought so. Even the nearest stars are so far off that seeing a planet around another star seemed impossible.

“But you told me that I should try to believe in the impossible.”

Exactly, and that's what astronomers did when they went searching for planets circling far off stars.

“Did they just build bigger telescopes?”

No, they had to think how they could detect something that they couldn't see and they worked out the answer a long time before they found the first planet.

“How did they manage it?”

Well the first way they discovered was to look tiny wobbles in the position of a star?

“How does that help?”

Imagine an ice-skater spinning on the ice. On their own he or she can spin on just one spot.

“So?”

Well, what happens if two skaters are hold hands and spinning. They can't both spin on the same spot can they?

“No, but I'm not sure where you are going with this Albert.”

It will become clear don't worry. Two skaters holding hands will spin in a circle, with the middle of the circle between them. So imagine one very large, fat skater and a tiny little one, if they held hands they would spin in a circle too, but the centre of that circle would be close to the fat skater.

“OK, but we are talking about discovering planets aren't we?”

Of course we are and that is how they discovered the first planets outside our solar system. Imagine you are looking at the fat and tiny skater from a distance, you might only be able to see the fat one but you could tell there had to be another skater there because they would be wobbling slightly. So that is what astronomers did, they looked for stars that seemed to be wobbling from side to side in space. Some people claimed to have found wobbling stars that must have planets going around them as long ago as 1855, but it wasn't until 1988 that three Canadian astronomers, Bruce Campbell, G. A. H. Walker, and S. Yang found the first definite star with a planet in the constellation Cepheus.

“Where's that?”

It's a constellation that looks like a house up near the pole star. This famous star with the first detected planet is the called Alrai (or gamma Cephei) and in terms of the constellation it makes up the top of the roof. It is one brighter stars near the pole star so in Autumn and Winter if you live in the northern hemisphere look up above and a little bit north and you'll see it.

“Will I be able to see it wobbling?”

It might be twinkling like any other star but the wobble is so tiny it can only be detected in photographs taken by the largest telescopes on earth.

“Is that the only planet they've found so far?”

Not at all, once they found the first one other astronomers started looking more seriously. There are now over two hundred planets detected and they are finding 40 or more every year now. The fact they are finding so many planets tells us that the universe is probably full of stars with planets.

“So where do they all come from these planets?”

Come from? The view these days is that both the sun and the planets formed at a similar time from a swirling disc of gas and interstellar debris. As things started to cool down gravity helped to collect most of the gas into the centre which became the sun. Gravity also collected the left overs and debris into planetoids and then planets.

“Are they aliens on these planets?”

No-one knows, but most of the planets that have been discovered so far are huge planets very different to earth so it's unlikely they have anyone like us on them. But they have just discovered a planet in the Goldilocks zone, where earth-like life could exist.

“The Goldilocks zone?”

From Goldilocks and the three bears, where Goldilocks found a porridge that was just right, not too hot and not too cold. If a planet is too close to its star it will be too hot and if it is too far away from the star it will be too cold. So there is a zone around every star that is now called the Goldilocks zone where the temperature would be just right for life, somewhere between 0 and 40 degrees Celsius. The planet they have just discovered around a star called Gliese 581, is in just the right place and seems to be a rocky planet like earth and only a little bit bigger.

“What's the planet called?”

It's scientific name is Gliese 581c, but it has been given the name Ymir. It also has two other planets, so it is really like another solar system.

“So the aliens would be called Ymirians then?”

You and your aliens! Well, we don't know for sure that there is any water on Ymir, but as Xavier Delfosse, one of the team that discovered it said, “on the treasure map of the universe one would be tempted to mark this planet with an X.”

“So is anyone sending a space ship there?”

It's still 20.5 light years away so there is no way humans could get there at the moment, but a radio or TV signal could get there in 20.5 years. So the TV pictures from the moon landings would have reached them in about 1990. Perhaps if they exist and are impressed by our first attempts at space travel they might send us a message back.

“When could that message reach us?”

If they didn't spend too long thinking about it, a message could get back to us by about the year 2010..

“Are you serious?”

Not very often, but it is nice to think that it is at least possible.

Seeing the invisible, from the largest to the strangest telescope on Earth.

Of course, astronomers now make telescopes that can see the invisible.

“How can a telescope see things that aren't visible?”

Light is just a small part of a much larger spectrum, the electromagnetic spectrum.

“Why is it called that?”

It's called the electromagnetic spectrum because a Scotsman, called James Clerk Maxwell, in 1873 discovered that magnetic and electrical fields both behave as waves and travel at precisely the same speed as light. Maxwell then went on to produce a theory that encompassed light, electrical waves and magnetic waves: hence electromagnetic. The range of all electromagnetic waves is huge; from x-rays and gamma rays with wavelengths of one hundred trillionth of a metre to radio waves with wavelengths of hundreds of miles. Do you remember what a wavelength is?

“I think so, it's the distance between the peaks a wave.”

That’s right. For light, the whole range of colours is squeezed into a surprisingly small range of wavelengths. Even though light and sound are completely different types of waves, if you think of light in musical terms, the whole range of visible light, every colour, would fit into a single octave. Do you know what an octave is?

“An octave is the eight notes of a scale isn’t it? Doe a deer a female deer, Ray a drop of golden sun, Me a name I call myself and all that stuff until you get back to Doe.”

Exactly. Now from one end of an octave to another the wavelength of sound changes by a factor of two. Going up it halves and going down in doubles. . If you think of a piano keyboard, which only covers a fraction of the range of possible pitches of musical notes, then this can give you a range of frequencies for sound which is eight times bigger than that for the whole of visible light. In other words, if you could construct a light keyboard the long wavelength light on this keyboard would look red and be like the low pitched notes in sound terms. The short wavelength light would be blue and be like the high pitched sounds with the whole of the rainbow in between. But all the light you can see would only cover eight white keys or thirteen if you count the black ones too.

“Every colour would fit into thirteen keys on a light piano? But that's impossible there are millions of different colours.”

To a completely colour blind man there are only three.

“What are you talking about Albert?”

You may see million of colours but someone with no colour vision at all will see only white, black and gray. There appears to be so many colours because our eyes is much better at detecting fine variations in light than our ears are at detecting differences in the pitch of a sound. If we were bats we might have decided to put a thousand notes in an octave and thirteen keys for light might be too many to describe all the colours a bat can see.

“OK. So where does this electromagnetic stuff fit in?”

If there was an electromagnetic piano its keyboard would be almost eight times longer than a normal keyboard and would have over 750 keys. Visible light would still represent only thirteen of these keys (that's counting the black ones as well), spanning only one octave in a range of over 63 octaves. A normal piano keyboard covers only 7 octaves or so.

“So what does this have to do with telescopes?”

Astronomers can now make telescopes that can detect the whole of this electromagnetic spectrum from x-ray telescopes which are good for detecting black holes and exploding stars in distant galaxies to radio telescopes which can detect pulsars. The biggest single telescope in the world is a radio telescope.

“How big is it?”

It is 305 metres across and in Arecibo, Puerto Rico. The most interesting thing about this telescope is that it was made inside the crater left by a meteorite.

“There are craters on Earth? I thought they only existed on the moon?”

Oh they exist here too but most meteorites burn up in the atmosphere. There is one in Arizona, the Barringer Crater, which is almost a mile across. It looks just like a crater on the moon.

“So why are there so few craters on Earth?”

There have been lots of craters on Earth but the atmosphere burns up smaller meteorites and the weather hides evidence of the rest. The moon doesn't have an atmosphere or any weather so all the craters for the last few billion years are piled one on top of the other. Remember the voyager probes and their golden discs with information about Earth? Well when the Arecibo telescope was upgraded in 1974 a radio message was sent into space towards a tightly packed group of stars in the constellation Hercules. This message, that included coded information about the structure of DNA, the shape of human beings and the solar system.

“Are these scientists crazy, giving away all our secrets to aliens?”

Well even though radio waves travel at the speed of light, the message won't reach its intended target until the year 26,000. So it will be a little while yet before anyone hears the message and decides to pay us a visit. But that's not the most unusual type of telescope. The strangest telescopes look particles called neutrinos which are nothing to do with light or the electromagnetic spectrum.

“Neutrinos? Never heard of them."
Well you have now. There are more neutrinos in the universe than any other particle, knocking us photons into second place. A neutrino could not only pass straight through your body but go through the whole earth. A neutrino telescope has to be the strangest type of telescope in the world. In 1964 a man called Raymond Davis junior decided to go in search of neutrinos.

"Why? Just because they exist?”

When he started no one was sure if they existed or not. They had been invented by theoretical physicists who worked out that they ought to exist before anyone had even detected one.

“So why bother try to find particles that someone has made up?"

They are important because scientists' best theory of how nuclear fusion works inside the sun predicted that the sun should produce a lot of them.

"I thought you said that Earth scientists had already worked all that out, even if they were a bit slow about it."

Well they had come up with what seemed like the right idea, but in science you need to start with the right theory, or hypothesis, and then do experiments or observations to check that the theory is correct. If they could measure that the sun was releasing the right amount of neutrinos is would help to show their theories were right. The problem with neutrinos is that they will fly straight through a normal telescope, through the head of the astronomer and nearly all of them will even pass straight through the earth

"So how do you detect something that can do that?"

It's not easy but this was Raymond Davis's approach. He found a deserted gold mine almost a mile underground and built a huge tank filled with 400,000 litres of dry cleaning fluid. The sun is thought to pump out almost 20 million trillion trillion trillion neutrinos every day and of these only one neutrino a day was expected to interact with one of the chlorine atoms in the fluid and change it into an atom of the gas argon. Having pumped the best part of a million dollars into this old gold mine they only found one neutrino every two or three days. Finding one argon atom in tank of 400,000 litres should have impressed most people, but theoretical physicists aren't impressed by any experiment that gives an answer that disagrees with their theories like this one did. Allowing for the difficulty in detecting neutrinos, this machine and others like it only found a third of the expected amounts of neutrinos. Lots of neutrino detectors have been built but most of the neutrinos are still missing.

“What happened to them?”

Well once they realised it was a problem with current ideas about neutrinos rather than the telescopes they changed their theory. The new theory is that the sun makes the expected amount of neutrinos but on their way to earth they change type to a sort that can't be detected by neutrino detectors.

“Is that cheating?”

No, it's science. You start with a theory that predicts that you ought to find a particular thing. You go looking for that thing and if you find it you start believing that theory but if you find something different it is time to create a new theory.

How the least read book in history changed our view of the universe forever

“So when did people work out that the sun was in the middle of the solar system and the earth moved around it?”

Oh, that took a while, about two thousand years from the time that the ancient Greeks decided that everything moved around the earth.

“Two thousand years?”

Not a long time in terms of the age of the universe. It wasn't that no one had suggested an alternative. Back in the time ancient Greece, Aristarchus of Samos suggested that the earth moved around the sun around 270 BC. Even before him the ancient Indian astronomers put the sun at the centre of the universe in around 900BC. A man called Nikolaus Krebs said it in the 1400’s, but it was another hundred years before the the idea really started to catch on.

“What happened then?”

A polish astronomer called Niklaus Koppernigk published a book that changed people's view of the universe for good.

“How come I've never heard of him?”

Perhaps because people usually call him by the Latin version of his name,
Copernicus.

“I think I've heard of him. So what did he say about the universe?”

He put the Sun in the centre and then worked out a system to explain the movements of all the planets. The fancy word for this is Heliocentric and Geocentric is where the Earth is at the centre. He didn't get it exactly right but it was an important move in the right direction.

“So there must have been a big splash when he discovered that.”

Not really. Nothing happened for thirty years because he was too scared to announce it.

“Scared of what?”

The Church. At that time the Church was very powerful and totally opposed to anyone disagreeing with their view of the universe. The real problem was that he was in the Church and had the post of Canon.

“Tricky one. I can see why he looked so worried in this picture.”

So tricky that he only published his theory that placed the sun at the centre of things when he was almost on his death bed.

“So he didn’t have much time to enjoy the glory after it was published.”

True but it caused such a stir with the Church that he was probably right to wait until just before he died. Very few people actually read the book because it was banned by the Church just after its publication until 1835. By the time it could be published everyone believed the ideas anyway so no-one really bothered to read the book De Revolutionibus Orbium Coelestium which in English means On the Revolution of the Celestial Spheres. So it would probably rate as the world’s most famous and least read book.

“So how did the idea spread if the book was banned so fast?”

There is nothing like banning a book to make people seek out the few copies that were printed. One of the most important turning points was that Copernicus's ideas were taken up by greatest minds of the times, men like Galileo.

“The same man who tried to measure the speed of light with the two lamps and two assistants that you told me about earlier?”

That’s him. Galileo almost single handedly overturned the classical Greek view of the universe. Aristotle and Ptolemy represented the unquestionable truth to most people at this time. This truth required the existence of perfect, unchanging crystalline spheres, with the Earth at the centre and everything revolving in perfect circles. Galileo charged through this peaceful image of perfection like a bull in a renaissance china shop and disprove some of the most cherished ancient beliefs about the universe.

“How did he manage to do all that?”

He heard of man in Holland who had made an instrument that could far away things much clearer.

“What kind of instrument?”

A telescope. It was just a simple tube with two hand polished lenses and Galileo being good at almost everything made himself one. As soon as he did, he pointed it at the night sky. He became expert at designing telescopes and made an amazing series of discoveries about the skies above. The Greeks said the sun was this perfect glowing ball, but he discovered spots on the sun so that showed that idea was wrong. Though of course believers in the old ideas said the problem was with his telescopes.

“Was it?”

Not at all, the Sun really does have spots but that wasn't all Galileo discovered. He found that the moon had mountains and craters that cast shadows, so the moon wasn't a perfect sphere. He found that they were far more stars than can be seen with the naked eye and the milky way, the local galaxy, is in fact made up of thousands of faint stars. He also found that Jupiter had four moons revolving around it.

“Why was that important?”

Well one of the arguments that the Earth had to be at the centre was that if it were moving through space then the moon would be left behind. So when Galileo found moons around Jupiter that removed this argument, even if the Earth did stay still how could they explain how Jupiter's moons could move with it?

“But didn't they know about gravity?”

They knew things fell if you dropped them but Isaac Newton hadn't been born yet so there was no understanding that things like the moon would be affected by gravity.

“So they accepted the idea of the Sun being at the centre of the Universe then?”

Oh they didn't give up that easily. Believers in the old established ways of thinking soon invented answers for Galileo's findings, but these were a little far fetched. Ahah, they said, yes there may appear be mountains and valleys on the moon, but the valleys are filled with a pure, transparent crystal material so the moon is really a perfect sphere.

“That's a daft idea.”

People will often jump through hoops rather than change their minds. But as the new astronomy revealed ever more about the universe, the authorities adopted a different tack. They took to the pulpits and denounced the dangerous and heretical views of these renaissance madmen. Galileo was hauled up before the ecclesiastical courts and forced to renounce his dangerously mistaken views. To save his neck he stood up and declared that the Earth remained still and everything else moved.

“Why did he change his mind?”

Oh, he didn't. In true schoolboy tradition, no sooner had he said the words than he famously muttered under his breath at the end of his trial,. ‘..but it does move’.

“So what happened to him?”

Galileo was placed under house arrest for the last years of his life but that didn't stop him writing and while he was locked up at home he produced his last book - Dialogue Concerning the Two Chief World Systems. Galileo’s book was written as conversation just like this blog. After that the idea of a telling people about science with imaginary conversations fell out of fashion for five hundred years.

“So did Galileo and Copernicus get it all right, was there anything else to discover about the movements of the planets?”

No matter how much we think we know there is always more to discover. Copernicus finally made the world believe that the Earth spun on it is axis like a top and moved around the Sun, which is quite an achievement, if you watch a sunrise or sunset it is much easier to believe the sun is really rising or falling than the ground underneath you is spinning. But he didn't get it all right, he was still attached to the ancient Greek love of circles and ended up with a very complicated system. To explain the movements of the planets the ancient Greek plan needed the planets to rotate around the Earth and do little circles in space too called epicycles. Copernicus's system was just as complicated and in fact the only thing he really got right was putting the sun at the centre of the solar system.

“Who worked out how the planets moved properly then?”

A man called Johannes Kepler who was born in 1571 in Württemberg, Germany. Worked out that the planets don't move in circles but ellipses, sort of egg shaped orbits. From this he created his three laws of planetary motion even though he didn't know why these laws were true. He just worked them from studying the movements of the planets.

“So who did that bit?”

Isaac Newton. Remember his law of gravity? Well when Newton worked out gravity it turned out that his equations could be used to show why Kepler's laws were true.

“So they got it all right?”

Not even then, there was a wobble in the planet Mercury that still couldn't be completely explained.

“So who worked that one out?”

Oh, someone not a million miles from here.

“You!”