And Yet It Moves (but not like that)

UPDATE : Attention readers !  After reading this post, I strongly urge to also read this one ! The creator of the animation described here has made a new version, one that is ten million times better and has correct physics.



There's this annoying space GIF roaming the internet causing trouble. Perhaps you've seen it. No ? Well, here it is.

What it purports to show is the motion of the Solar System through space. But the accuracy of this has been utterly derided as an affront to scientific dignity. Which is a shame, because the video version is really quite nicely done, with good camera movement and a catchy soundtrack. The principle antagonist is notorious "Bad Astronomer" Phil Plait, who wrote a convincing and virulent attack on the video. I decided to investigate for myself.

Like many people, I was at first glance really quite impressed with the video, and didn't have any major objections to it. Obviously the orbits and size of the planets are not to scale (and I think their orbital speeds have be altered too), but that's just to make them visible. Fair enough. But then I read Phil Plait's analysis, and it seems that things are much, much worse than that. Says Plait :

"Sadhu shows the Sun leading the planets, ahead of them as it goes around the galaxy... This is not just misleading, it’s completely wrong. " 

He clarifies :

"Sometimes the planets really are ahead of the Sun as we orbit in the Milky Way, and sometimes trail behind it (depending on where they are in their orbit around the Sun)." [my italics]

The orbits of the major planets of the solar system all lie in a narrow plane (like being in economy class ! hahah.... sorry), which is tilted at about 60 degrees to the disc of stars that forms the Milky Way. Like this :

Credit : Science Minus Details.

We'll return to the tilt in a moment. But first, if the Sun was really leading the planets, then the thing is completely ludicrous (and this is quite a major part of Plait's argument). Yet I'm not so sure the viral gif does show the Sun leading the planets. Having read through the author's website, I can't find any evidence that he suggests this. In fact, some of other videos on his website clearly show that this isn't the case :

It seems to me that the appearance of the Sun leading the planets in the gif is just the result of a projection effect - i.e. that things can look different from different angles. On the other hand, Plait read the source material for Sadhu's model, so maybe there's something in there that's more explicit. I've glanced at it, but couldn't find anything stating this precisely. Actually I couldn't find a whole lot that was even vaguely coherent, but we'll return to this later. For now, just keep in mind that Sadhu is using an alternative model, even though that may not always be evident.

What the gif definitely does not show is the fact that the orbits of the planets are tilted at about 60 degrees to the direction of the Sun's motion. Says Plait :

"In the helical model, he shows the planets as orbiting around the Sun perpendicular to the motion of the Sun around the galaxy; "face-on", if you like.This is wrong. Because the orbits of the planets are tipped by 60°, not 90°, they can sometimes be ahead and sometimes behind the Sun. That right there, and all by itself, shows this helical depiction is incorrect."

There can be no mistaking that Sadhu's video shows the orbits with the wrong tilt. But is that so critical ? Well actually no, not really. Fact is that if you include the tilt, you still see the planets making a "spiral" pattern (technically it's a helix) as they move through space. The overall appearance just isn't that massively different compared to a 90 degree tilt.

So what's the big deal ? What does the author claim in this internet sensation that's so outrageous ? Well, not much. That particular video/gif are actually fairly inoffensive, to my mind. The most basic notion that the planets trace helical paths through space is perfectly correct. What honestly surprises me is that this is so incredibly popular on the internet. If you weren't aware that the Sun orbits the center of the galaxy - which, since the planets orbit it, necessitates that they trace out helical paths - then the education system has seriously failed. But do not despair ! This can be remedied very, very easily.

But we're not done yet. There's a sting in the tail, and it's a big one. The gif doesn't show it, but the video version ends with the worrying remarks that :

"Rotational motion and vortex motion are completely different things."*
"Life spirals." [picture of leaves]
"Life is vortex, not just rotation." [picture of developing ferns, then a flower, the Milky Way, the DNA double helix, etc.]
"The Solar System is part of life. Think about this while racing through space."

* Yes, they are. Plait notes : "They’re different in more than just name; they’re actually very different physical motions with different properties—you can get helical motion without the particles in it interacting, like in the solar system, but in a vortex the particles interact through drag and friction." Basically, claiming that the Solar System is a vortex is simply wrong. Sadhu appears not to have checked the word, "vortex" in a dictionary.

I could forgive even these rather hippyish sentiments, if they were no more than that. Alas, they're symptomatic of a much larger problem. Plait's merciless attack is full of sound and fury, but it's also signifying something. Reading more of the author's website, it turns out he is actively promoting quackery. It's on a par with the excellent Space Mirror Mystery* (the idea that everything further away than about 150 million km is just a reflection in a giant mirror), but less funny.
* I was delighted to find that this website is back online. Seriously, read it. It's epic.

From Sadhu's website :

"In this diagram it seems the Solar System travel to the left. When the Earth is also traveling[sic] to the left (for half a year) it must go faster than the Sun. Then in the second half of the year, it travels in a “relative opposite direction” so it must go slower than the Sun. Then, after completing one orbit, it must increase speed to overtake the Sun in half a year. And this would go for all the planets. Just like any point you draw on a frisbee will not have a constant speed, neither will any planet."

Apparently he thinks this is a problem. Worryingly, it suggests that he didn't show the 60 degree orbital tilt not for mere simplicity, but because he doesn't believe it's possible. Which - if true - is utter madness, pure and simple. There's absolutely no reason the planetary speeds have to be constant as they move around the galaxy - the massive gravitational pull of the Sun is keeping them firmly in its orbit, regardless of how those orbits are inclined.

"Secondly, most planets are visible throughout the entire year. In a “flat” model, every single planet would hide behind the Sun at least once a year. They don’t. Now the heliocentric model isn’t entirely flat, but mostly."

Fine. The heliocentric model isn't flat, which perfectly explains why planets aren't eclipsed by the Sun once per year. What need to state this ? Is he really saying that this is a problem in a heliocentric model ...? SERIOUSLY ?

"Fact of the matter is that if the helical model is correct and our Solar System is a traveling[sic] vortex, it will change how we feel about our journey. For me personally the heliocentric model feels like a useless marry[sic]-go-round: after one year we are back to square one. The helical model feels much more like progress, growth, a journey through space in which we never ever come back to our starting point. We are NOT in a big marry[sic]-go-round. We are on a journey."

Planets trace a helical path in space because our Solar System is orbiting the center of the galaxy. Big bloody deal. It's that simple. You don't need a wacky alternative model of the Solar System for this - it's happening anyway ! As for going on a journey though - well no, not really. Every other star is also orbiting the center of the galaxy, so no, we're not actually getting anywhere relative to other star systems.

Then there are some pointless ravings about the Mayan calendar.

He also links the following video. Skip to about 2 minutes in :

This has the bizarre quote that :

 "The planets do not come back on to their [own] path[s]. They don't. If they did, we most likely would have the same set of information over and over and over... like a broken record. And we'd probably get bored. It would be like Groundhog Year."

Then he links a video claiming that the Fibonacci sequence is the fingerprint of God.

None of which changes the fact that his first video/gif has only minor inaccuracies, but at this point I can't help feeling that this was more by luck than judgement.

Then there's his second video. This one is more objectively just plain wrong. He shows the Sun tracing out a corkscrew pattern as it orbits the galaxy, which makes no sense. The Sun simply goes around the center of the galaxy (and up and down a little bit) - nothing else. It's not orbiting anything else at the same time. For it to trace a helix is just nonsense. He seems to have an almost unique case of helix madness.

What of the source material - the alternative model Sadhu uses ? Garbage. Utter garbage. I find it difficult to read more than a sentence or two, because it's verging on incomprehensible. As in almost at the level of TimeCube.

"Three types of time may be recognized
- An absolute time that is universal and has neither a known starting point nor an end point; not even limited to a measurable parameter.
- For living organisms there is a time for birth and a moment for death. The interval is the life span. This time may be measured with parameters like seconds, minutes, days and so on. Mechanical devices may measure fractions and to some extant reliable. In every case some kind of energy source or gear system is involved.
- When one is engaged with some work involvement in another activity may be impossible or result to be unnatural. In such cases personal values decide what course to take up and say “no time” to the other work, however important that may be. This time is highly subjective."

Later :
"The constellations at the background are sufficient evidence to deny the heliocentric orbits for planets. The Sun at 500 light seconds distance, when visible within a cone of 30° maintaining a background of one constellation, say for example Aries, (Hamel at 68ly) the SOLSTICES and EQUINOXES through Zodiac Earth maintains in the opposite constellation at midnight, namely Libra. After six months to maintain heliocentric orbit, the mid day of today should become midnight and the midnight should become midday. This has not taken place!"

Well of course it hasn't - it's complete gibberish ! Plait may well be right that somewhere in this mess is a model wherein the Sun leads the planets, but I don't have the time or sheer mental fortitude to read the whole thing. I will note, though, that there's a paragraph where the author rubbishes the conventional explanation for the ozone hole - and God help us all if that goes viral. That, not petty disputes about whether the orbits of planets are tilted by 60 or 90 degrees, is why such quackery deserves to be shot down without mercy.

In conclusion then, the first video and gif of the Solar System as a "vortex" are not really all that bad. Unfortunately, the inaccuracies are not due to some minor over-simplifications, but are symptoms of a some very deep-seated misunderstandings. My feeling is that if your take-home message was only that the Solar System moves through space, and the planets trace out pretty spirally paths, then all is well and no harm done. But if it's leading you to question the heliocentric model, then we're all buggered.


EDIT : Epilogue
Although not directly related, a trailer was recently released for an entire glitzy-documentary called, "The Principle". This claims that not only is the heliocentric model of the Solar System wrong, but that geocentrism - the idea that the Earth is the center of the Solar System - is correct.

This is several steps worse than Sadhu's misguided ideas - it is wrong beyond wrong, on a par with Hollow Earth or Flat Earth gibberish. It appears that they have used stock footage of extremely famous, trusted scientists (Michio Kaku and Lawrence Krauss) and edited them into their documentary. Krauss categorically denies any wilful involvement, Kaku hasn't commented. Captain Kathryn Janeway, who was narrating the trailer, also vehemently denies supporting geocentrism, stating that she was misinformed about the work she was hired to do.

So, to the extremely small minority of readers who think this post was unnecessarily harsh, I say fuck no. Claiming that geocentism is correct is effectively stating that all of science is wrong. That is a lie - and that is not an opinion, it is a demonstrable fact. Such beliefs, if unchecked, threaten to return us to a time of superstition, ignorance and fear. I think we deserve better.

Everyone has the right to freedom of speech, so go ahead and believe in magical pixies and faith healing and crystals and turtles all the way down. That's fine, because by the same token, I've got the right to retaliate.

The Best Space Rock Ever

Sometimes life gives you lemons... and sometimes those lemons are fabulous. Today's lemon is the asteroid 1998QE2, the asteroid that just keeps on giving. For starters, it shares the name of the QE2 ocean liner, and happens to be of comparable size. As, of course, I already demonstrated, but here's a reminder.

Much more awesome video version here.

The asteroid shot to fame about a year back partly due to its name but mostly because it made a very close pass of Earth. That gave planetary researchers a chance to zap it with powerful radar beams, and lo! a moon was discovered. An adorable little moon, which, perfectly, is not a lot bigger than a real-life space station.

Larger version.

All this was known almost as soon as the data came in, when the asteroid shot past Earth at a distance of less than 4 million miles back in May 2013. After that, everyone quickly forgot about it, because it's no more a threat to civilization than the other QE2, or indeed, the other other QE2, and most people don't care about space rocks unless they're threatening to land on us.

Of course the other other QE2's threat factor may have been underestimated.

... that is, except for those plucky rock-zappers who now found themselves with a bunch of data to analyse. Through the magic of science and time travel*, the fruits of their labours can now be revealed. Probably the most awesome thing about radar observations (if you're trying to make graphics out of them) is that they can determine the true 3D shapes of the things they're measuring. Even better, I don't have to do that, because I'm a galaxy hunter, not a rock zapper.

*Well, we've travelled a year into the future. Compared to a year ago, I mean.

                     

These are the shape models from the Arecibo radar observations (not to scale - QE2 is on the left and the moonlet is on the right), generously provided by Sondy (as, in fact, were most of the numbers in the rest of the post, which means I have someone to blame if anything's wrong). By sheer fluke, it turns out that 1998QE2 is pretty spherical, so my earlier renderings weren't complete nonsense.

What this lets us (and by us I mean Sondy) determine is a more accurate measure of the mass and density of 1998QE2 and its moon... which I'm going to call Prince Charles, for obvious reasons. Since one theory of binary asteroid formation involves the larger one spinning so fast it splits in two, thinking of QE2 as the parent may not be entirely inaccurate.

The formation mechanism of the other Prince Charles was rather
different, but no less terrifying.

First, here are the asteroids to scale and at their correct separation. The texture is, of course, entirely fictional :

The new measurements put QE2 at 3.2 km across, while Prince Charles is 800m in diameter - slightly larger than the initial estimates, but they're both still pretty puny. They're separated by 6.4 km, center to center - or perhaps more meaningfully, Charlie would be just 4.4 km (less than 3 miles) above the surface of QE2. It would span a whopping 10 degrees in the sky - this is massive. The Moon spans just 0.5 degrees of the sky from the surface of the Earth.

Left : Earth's moon, seen from the Earth. Right : Prince Charles seen from 1998QE2.

The real fun, though, comes from the mass. Previously I stated that the mass of the QE2 was equivalent to about 300,000 cruise ships. That has to be lowered a bit to about 225,000 ships. This matters. For starters, the escape velocity of QE2 is about 1 m/s. Which means you could jump off it - easily. Thanks to the moon, this means you could jump from one world to another. Anyone growing up in this crazy environment would learn orbital mechanics at the same time they were learning to walk, though walking would be nearly impossible. Surface gravity would be less than 0.003% of Earth - drop a ball on 1998QE2 and it would take about 90 seconds to hit the floor.

But the most fun implication from the mass is the density - 0.7 kg per cubic meter. That's less than water. And that means this asteroid, with the same name as a large famous ship, would float. I love this rock.

And yes, those are QE2-sized ocean liners.

Except that it wouldn't - not like that. That's the naive, dramatic version. Since the density of the asteroid is only about 70% that of water, 70% of its volume would be underwater. Instead of a mile-high cliff sticking out of the water, what would actually be visible would be more like a small, low island.

Of course the water transparency wouldn't be anything like this - it would be, at best, twenty times less than this.

Sadly though, even this isn't accurate. The asteroid is made of rock, not ice, so to have such a low density it's probably a rubble pile - a loose collection of rocks and gravel, with lots of empty spaces. Instead of bobbing about in the ocean, it would collapse in a huge unsightly heap,a bit like unleashing a truly ginagorous ("titanic", you might say) bath bomb. Enclosing it in a giant plastic bag might work, but it would still collapse and at best, what you'd get would look like a monstrous jellyfish.

So here we have a world with the same name as a famous ship, of such low density that it would float (though only for a little while before disintegrating), and which is so small you could jump off of it and land on its moon - which could legitimately be confused with a space station. If there's a more awesome asteroid, I'd very much like to hear about it.

Metro Ninjas Just Want To Give You Stickers And Maybe Also Fish

Another, "let's all laugh at the hapless expat" post

Every Czech is born with an innate superpower. Actually, they're all ninjas... well, one-trick ninjas, at any rate. Their unique skill is the ability to exit a Metro train slightly faster than expected. Suppose I stand about one foot from the doors. In Britain, this would be a clear signal that I'm about to exit through said doors. Here, it means I'm standing by a door. Consequently, I invariably find that the moment I'm about to exit, someone else is already standing in the gap. Such an event is beyond the pale in the British queuing system, leaving me staring gormlessly every single time.

Prague has an unfortunate problem with graffiti which rather spoils many otherwise respectable areas. Only the most touristy-buildings appear to be granted any form of protection. In most cases, at least, I have no idea what the graffiti actually says. Yet for some reason I cannot understand, the words, "Read" and "Amok" appear to be some of the most common words found. I can only assume that the graffiti artists are hell-bent on a literacy promotion campaign.

The polite graffiti doesn't stop there. "Kiss" is written almost everywhere (a remarkably prudish instruction given the city's lack of scruples), "fish" is less common but not rare (I have no idea if it's advice to eat fish, go fishing, or get a pet fish). So far I've only spotted one instance of "ALFA", but since this is clearly a call for proposals to observe with the ALFA receiver, I take it as powerful evidence that the Prague graffiti squad is literate, educated, and probably a bunch of STEM activists (which also neatly explains why no-one is tackling the graffiti).

Because it's very important that everyone know about fish.

It's an unusual way to announce a call for proposals, I'll grant you...

Such a hypothesis is lent credence from a second observation. In other countries, gangs of youths might stalk the streets getting wasted and/or high, shouting profanities at luckless innocent bystanders. They might well profess their allegiance to their socially deviant clique by means of branding, probably tattoos. Not here. They might be wasted, yes, but instead of shouting profanities they're more likely - apparently - to hand you a sticker. Which presumably also serves the function of a tattoo.

Any attempts to look threatening are completely undone by the name
of the gang. Bless them for trying.

I previously reported on the ubiquitous presence of what Pratchett would call "houses of negotiable affection", a term unrelated to haggling over the price of wi-fi, since it's free. Further to this, I should note that it is literally impossible to walk through the center of Prague on any day of the week after about 10pm and avoid invitations to such... err, facilities (moreover, it doesn't matter if you're a man, woman, single, couple or a group - they're very modern and/or desperate and ask everybody). Polite refusals do little to discourage the questioning :

"Hey, do you wanna go to a -"
"No thank you."
"Well where are you going to go instead ?"
"Home, I only came out to buy some milk*. [gritted teeth] Perhaps another time."
"Aww, why not tonight ? Plleeeeease ? I'll be your friend !!"

* This had the virtue of being true : no local shops stay open after 9pm.

Perhaps I exaggerate, but only slightly. There are only limited options : face annoying delays persuading half a dozen people (or more) that you just want to be left alone, be extremely rude, or give in. Let's assume the latter two are not options.

Fortunately, being massively introverted, I already have some experience avoiding talking to people I don't want to talk to. Cardiff does not have Prague's particular problem, but it suffers in the daytime from inescapable charity muggers (aka "chuggers") and various people with clipboards conducting surveys. By far the most stupid approach any of them adopted was to ask for directions :
"Excuse me, do you know where Burlington's is ?"
"No - sorry !"
And then you're free to carry on walking without having offended anyone. Unfortunately most of them were more intelligent than that, but I discovered years back that a foolproof answer was to look eager, point my index finger in the air in a meaningful way and say, "Hang on a minute !" and carry on walking. It worked for a little while, until they wised up to it.

I haven't tried using that one in Prague, but one evening, after enduring three determined attempts to persuade me that I really wanted some negotiable affection, I hit upon a new approach :
"Hey -"
"Oh, dydw i ddim yn siard Saesneg !" [I don't speak English]

In the course of about ten minutes I used this no less than three times and received looks of utter, priceless bewilderment. After the third it was all I could do to keep a straight face. Welsh is simply not in their repertoire. Which is a mercy, because it isn't in mine either, so the rest of the conversation would probably be something like :
"Siarad Cymraeg ?"
"Err... rydw i'n hoffi coffi ! Dydw i ddim yn esiau Ysgol Uwchrardd Yr Eglwys Newydd yfory ! Ble mae fy nghad ?" [I like coffee ! I don't want to go to Whitchurch High School tomorrow ! Where is my dad ?]

And so on and so forth. If it stops working, I'll probably have to give them a sticker and persuade them to join my awesome gang.

Unknown Unknowns

About a month ago, our property agency was taken over by the estate agency who found us the apartment. A few days before our rent was due, the estate agent we originally dealt with contacted us to tell us about the new arrangements - basically, there weren't any, except that now we would pay the estate agency - let us call them company X, even though that's not their real name - instead of the property agency, company Y (our landlords).

Company Y, it seemed, was now either owned by or taken over by company X, and company X owned all their property. The agent herself (let's call her Jim, for no reason) would now be our contact for any problems with the apartment. And indeed she inquired as to any issues there might be, and suggested without prompting that the new company would try and install washing machines where possible. And various other details : the new company had an Italian owner, they regretted they wouldn't be able to do much about the graffiti on the outside of the building, etc.

For this one time, we should pay Jim in cash since the electronic banking facilities for the new company weren't set up yet. Everyone was still trying to get used to the new systems in place. The terms of the existing contracts would be honoured; essentially nothing would change except for who we paid the money to and contacted in the event of a problem.

Hindsight is a wonderful thing, but readers should understand a few things at this point. Firstly, this was the same agent who originally showed us the apartment, who arranged the contract with our property agency, who was present at the property agency office while we signed the contract and handed over the first month's rent and deposit. It was obvious that the two parties knew each other, at least to some degree, so the credentials seemed impeccable. Also, I spent the last two years paying my rent directly to my landlord, who was my next door neighbour - so I have zero previous experience with property and estate agencies.

So we parted with our cash and got a signed receipt (naturally) though no kind of new contract. I would have expected to have signed something regarding the new ownership and maybe a termination of the previous contract, even if the terms of the new one were identical except for the owner's names. I guess we expected something to be forthcoming. Jim left us her contact details (although we already had these). I really don't remember if she mentioned there would be anything else to sign or not.

Days went by. A week. Life went on. The only odd thing was that our neighbours hadn't heard anything about the new arrangements, and we were given to believe that the new agency owned the whole building. Weird. Perhaps we'd misunderstood something.

Two weeks passed. Any sort of doubt had now gone completely. If I don't pay rent for a few days, I would hope that the landlords wouldn't immediately shout at me, and might be courteous enough to give benefit of the doubt and not say anything at all. That would be perfectly legitimate in my view (on our contract, it says we should pay the rent no later than 5 days after the due date). After a week, I would expect to start hearing polite, "can you pay us some money, please ?" messages. After two weeks I would anticipate justifiably angry "Where is our money ?" demands.

Three weeks passed and still we heard nothing from anyone. I could not imagine that any company expecting to receive regular rent would wait a full three weeks and more before even saying anything to the tenants. Then one afternoon in work came a phone call.
"Hello, is this... William Taylor ?"
"Yes, this is Rhys Taylor, William is my middle name."
"Err... yes. You rent the apartment at... XXXX ?"
"Yes, that's me."
"We have not received any rent for March from you."
I then proceeded to describe the events to an utterly dumbfounded agent. More phone calls followed. The property agency was apparently completely and totally unaware of the new arrangement, quite convinced that they still owned the entire building (which is generally not something it's easy to be mistaken about). They seem surprised, naturally enough, that we would pay money into an account not stipulated in our contract* (I was surprised that they had taken over three weeks to ask for their rent, though I didn't tell them this).

* Silly though it sounds, this hadn't occurred to me. I guess I was imagining that if the ownership of the whole building had changed, and the PA had been taken over by another company, then this was a situation that couldn't be covered by the contract.

Of course I did tell them that it was Jim who told us all this and they tried to contact her. She wasn't responding to them, but when I phoned her a few minutes later she responded instantly. More phone calls followed. Almost surprisingly, the two parties did indeed establish direct contact - at this point I was half-expecting that Jim would simply lie and/or run away. Not so. Both seemed in agreement that Jim would bring the money she'd taken directly to the PA.

Since this happened just before 4pm, when the PA office closes, I waited until 10am the next morning to call and check what was going on. There hadn't been any progress, but half hour later they called me back. Everything had now been settled - Jim had delivered the money as promised. We should continue to pay our rent to the original property agency, as before, and if Jim contacted us again, we should ignore anything she said. Which seems entirely reasonable to me. Though it's hard to tell on the phone, they seemed as ignorant as to what the hell was going on as I was.

I haven't since tried to contact Jim again and I don't intend to. Having signed no property contract except with the original property agency, it seems pretty clear that they are still our landlords. What I can't understand is... well, anything else.

It's pretty much beyond the realms of possibility that our property agency could have been taken over by another company and their building swiped from under their noses without them knowing about it. That just doesn't make any sense at all. Yet, if Jim is trying to pull a fast one, she is both an accomplished liar and yet also curiously unintelligent. I mean, what kind of scam artist leaves a signed receipt, valid contact details and gives the money back when asked ?? And why on earth would anyone fake being a landlord ?

Moreover, a month's rent here isn't a huge sum of money. You could buy a very nice sofa with it, or perhaps a week's holiday somewhere not too expensive. Did Jim just need a short-term loan or something ? And what did she think would happen when the property agency noticed we hadn't paid our rent ?

I suspect this weird little debaclé isn't over. But I've got a contract telling me who my landlord is, and it isn't Jim, and no documents of any kind telling me I should do anything Jim says. Any further dealings will be handled face-to-face at the property office, so there won't be any doubt as to who owns the building and who's gone stark raving mad. Apparently it's quite easy to get those confused.

EDIT : The managing director of the company emailed me a notification that Jim is no longer working at said company and acted without authorization. I'm a little disappointed that I'll probably never understand what the heck Jim was trying to do, but the matter is apparently now closed.

Hallelujah, It's Raining Moon

If you're one of the long-suffering followers who only signed on for the science outreach, fear not ! This week I present the thrilling sequel to Blue Marbles (actually this perfectly logical extrapolation wasn't my idea but was suggested to me by some dude calling himself "Joshua K"). "Blue marble", of course, refers to the photograph of the Earth taken by the crew of Apollo 17. Fortuitously, treating the planets as marbles makes a convenient way to compare their sizes and masses. Scroll down to the bottom if you're after the video.

Having already done the gas giants, this time I'm looking at a more-or-less random selection of the Solar System's more solid worlds. Screw Venus - sure it's hot enough to melt lead all the frickin' time, the pressure from the atmosphere is the equivalent to being nearly a kilometre under water, and most of that atmosphere is carbon dioxide... but it's almost exactly the same size and mass of the Earth. This makes it incredibly boring. Let's move on.

Mars presents a more interesting comparison. About half the diameter of Earth, it would take about 9 times the mass of Mars to equal Earth. Mars is about the same distance away from us as Venus, but in the opposite direction - away from the Sun. While the temperature can be bloody cold, it can also be surprisingly warm. The thin atmosphere does not mean your eyes will pop out, no matter what Arnie says. He was right to demand that you be given "eeah" though, because you'd most likely die of oxygen starvation after a few minutes. Less dramatic than eye-sucking, and at least you'd enjoy the 1/3rd Earth gravity for a while.

Mercury isn't much to look at, but it's almost as hot as Venus and colder than Mars, constantly. It has no atmosphere to spread the heat around, so the side facing the Sun is roasting hot and the other side would make a taunton wish for thermal underwear. Mercury is just over a third the size of Earth, but you'd only need 18 of them to equal Earth's mass. This is pretty dense - in comparison you'd need 67 of the similar-sized Ganymede (Jupiter's largest moon) for the same mass. Mercury's high density could be because it's really just the core of a larger planet, with the lighter material having been blasted into space by a titanic collision billions of years ago.

Which brings us to the MOOOOOON ! Seriously, I don't care if you're at work, stand up and say the word, "Mooooooooon !" in a loud, dramatic voice. It'll make you feel better, I promise. If it doesn't, you probably have no soul and don't like the Indiana Jones movies. Err... anyway, the Moon, like Mercury, is a world of extremes. 120 C in the day, -150 C at night. But, like Mercury, it's really just a big rock, so let's move on to somewhere more interesting*.

* Don't misunderstand me. I would maim and injure innocent people by the dozen for a chance to go to the Moon. It's all relative.

Europa ! Yeah, that's pretty interesting. Jupiter has four moons that anyone cares about, and sixty-odd smaller ones that just generally faff about. The smallest of the moons worth paying attention to is Europa, which is fractionally smaller than our own Moon but quite a lot lighter... being partly made from water. Beneath its Hoth-like surface may lurk a liquid ocean. It's even spouting jets of water into space. This, to borrow from the great Bill Bryson again, is a "You must go at once, take my car !" discovery. Or it bloody well should be, anyway.

But I'm not here to blather on about alien fish. Let's skip over the methane oceans of Titan and the nitrogen volcanoes of Triton and head for the world best known for making the astronomy community look like a bunch of morons : Pluto. Even smaller and lighter than our Moon, the discovery of numerous similar objects lead to a redefinition of the world "planet"*. But Pluto is actually a pretty neat place in its own right. Cold, dark and incredibly lonely, it sometimes gets warm enough to thaw some of the frozen nitrogen and methane and produce a thin atmosphere. It also has a giant moon, Charon, which would appear at least seven times wider in the sky than out own Moon does.

*This was because astronomers felt it would be too difficult to cope with more than nine planets because they'd never remember all their names. And that's how we ended up with the world "planet" now having the most ridiculous definition possible. That's what you get for listening to Neil de Grasse Tyson.

We'll know a lot more about Pluto next year when the New Horizons probe whips past. A few months before that, we'll also get our first look at Ceres, the Solar System's largest asteroid. About 950 km across, this tiddly little world is roughly the size of Britain. Its surface gravity is over 30 times less than on Earth, so leaping 10 or 20 metres in the air would be no problem. A world-class bowler could throw a cricket ball about 5 miles, before realising that cricket would be even more boring in low gravity. Golf would be even more tedious, though pole-vaulting and basketball would become a lot more interesting.

Of course, you can also see all of this in animated form, because it's always fun to make it rain planetoids.