Tag Archives: astronomy

Why did people think the Sun went round the Earth?

Here’s something which puzzled me for a long time, until I happened to come across the answer a couple of years or so ago. I think the book Episodes from the Early History of Mathematics by Asger Aaboe (Cambridge University Press, 1997) provided me with the revelation, but I wouldn’t swear to it.

The puzzle

When people began watching the motions of the Sun, Moon and planets in the sky and measuring them properly—which happened many centuries ago—their movements turned out to be quite complicated, especially if you assumed (as most people did) that the Earth was stationary at the centre of the universe. Everything circled the Earth once a day, but with various wobbles superimposed on the motion.

This led to a rather unsatisfying and complicated picture involving epicycles, in which a planet’s motion in the sky was a combination of two or more movements: its circle around the Earth plus the various wobbles.

Before I go any further, let me point out that the the idea of a flat earth wouldn’t have entered into this: the realisation that the Earth was a sphere happened around 300 BC and Eratosthenes was the first to measure its size—with remarkable accuracy—in about 240 BC.

It was occasionally suggested that things might make more sense if actually everything went round the Sun, with the Earth rotating once a day. Yet, there was great resistance to that explanation. Why? What was the problem? Why not switch to the obvious answer? Why not just do it? Why was the “Copernican revolution” so revolutionary?

One reason was that Copernicus’ system, which based everything on combinations of perfectly circular movements, ended up being even more complex than what had gone before. But there’s another, very simple and logical reason.

The answer

If we look at the night sky we see the Moon, and lots of little points of light. Mostly these keep the same positions relative to each other, simply circling the North pole (or South pole if you’re south of the equator). They behave as though they’re attached to a rigid sphere, making them all rotate together. That was called the sphere of the fixed stars, and was thought of as the outermost part of the universe.

A few of them, though, seem not to be attached to it. They  move around according to  rules of their own, and were given spheres of their own to move them. But apart from that—and from not twinkling—they look pretty much the same as the other points of light. They just happen to be attached to their own spheres instead of the outer one.

How far away were the fixed stars?  Nobody knew, but they couldn’t really be much further away than the moving ones or they’d be too dim to see.

So far so good. But why can’t the Sun be at the centre instead of us? What difference would it make, apart from a little bit of wounded pride?

Putting the Sun at the centre of this picture creates a huge, glaring problem.

The closer you are to something, the bigger it looks. If the Earth went round the Sun, then any given constellation would change size in the sky as we moved towards and away from it. This would be clearly visible. Yet we don’t see it. And that, surely, proves that the Earth isn’t moving. It must be the Sun which moves around the Earth.

The revolution

So if you want the Sun at the centre, you have to explain why the fixed stars always stay the same distance apart in the sky. This entails making the sphere they’re attached to considerably bigger. And then you have to make them bright enough to see, so they’re not little twinkly things any more, but are like other suns in their own right.

But the Sun and its collection of spheres was the entire universe. Saying that the stars are really other suns is like saying that actually there are thousands of universes. It’s mind-boggling. It sounds somewhat insane.

It’s not a matter of demoting the Earth a little from its symbolic position as the centre around which everything revolves, and giving that honour instead to the Sun; it’s a matter of making both the Earth and the Sun utterly insignificant in the scheme of things.

Seen that way, the idea is indeed shocking.

Imagining the unimaginable

NARRATOR: The Hitch-Hiker’s Guide to the Galaxy is a truly remarkable book. The introduction starts like this: ‘Space,’ it says, ‘is big. Really big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the street to the chemist, but that’s just peanuts to space. Listen . . .’ And so on.
. . . After a while the style settles down a bit and it starts telling you things you actually need to know.

Douglas Adams, The Hitchhiker’s Guide to the
Galaxy: the original radio scripts, Pan Books, 2003.

This post was going to be another instalment of my thoughts on science on religion—about the religious implications of living in a rather big universe—but it’s turned into an unashamed attempt at expressing the sheer size of space. The approach used here is the only one so far that’s worked for me; I hope it might work for you too.

By the way it does not involve describing things as “the size of a football field”, “the thickness of a human hair”, “the length of a London bus”, or “the size of a blue whale”. So if your heart was already sinking in anticipation of those, please feel free to cheer up again. It does however involve an orange. I apologise for that.

Everything’s too big

Imagining big things is difficult. Beyond a certain size, the numbers all blur into “too big to imagine”. So the task I’ve set myself is rather ambitious. For example, astronomical distances are measured in light years—the distance a beam of light would travel in a year. Even the distance it travels in a second is uncomfortably big for the imagination: 186,000 miles, 23 times the diameter of the earth, or two thirds of the way to the moon. Alarmingly, a light year is over 30 million times as far as that. Our nearest neighbour star, Alpha Centauri, is about 4 light years away. These distances are not small.

I do know, though, what a mile looks and feels like. It’s a little less than the distance from here to the bank; I walk it in about 16 minutes. And I know that a train journey to London takes around 3 hours. People who fly regularly know how long it takes them to get to their destinations. This gives us some inkling, at least, of the size of the Earth: Its diameter is about the same as 8,000 walks to the bank, or 1½ flights to America.

Another way to get a sense of how big the Earth is is too go to the beach, stand at the water’s edge and look out at the horizon. Assuming you’re about six feet tall, the horizon is about two miles away. It’s effectively a six-foot-high bulge created by the Earth’s curvature. Now try to imagine what’s over the horizon. More sea, curving away downwards. Mentally double the distances: 4 miles, 8 miles, 32 miles. Feel the earth’s gravity holding you on the ground. Try to get a grasp of what an 8,000 mile sphere looks like.

My task is to somehow relate the distances of space to the everyday ones we have a grasp of. My tool is the Ordnance Survey map: the one you might use when out walking in the country, or the equivalent map if you live outside the UK. I’m going to use the old, inch-to-the-mile maps, printed at a scale of 1:63,360 (the number of inches in a mile). If you don’t remember them, well, the current 1:50,000 maps are the same idea, but blown up a little bit.

Make it smaller!

OK, here goes.

Step 1

Scale: 1:126,720,000 (1 inch to 2000 miles)

Shrink the Earth to the size of an orange. (Fruit is traditional and compulsory in these comparisons.) In other words, make a model of the Earth on a scale of 2000 miles to the inch. At this scale:

  • the Earth is a squishy ball of not-very-solid rock, 4 inches across.
  • alarmingly, the nice solid crust, on which we live, is less than a thousandth of an inch thick.
  • The Sun is a very hot ball of fire, ¾ of a mile away and 36 feet across.
  • Alpha Centauri is, I’m afraid, still 200,000 miles away: a bit less than the actual distance to the moon.

Well I can imagine an orange and I can imagine a ¾-of-a-mile walk, but if I’m honest, 200,000 miles still doesn’t mean much to me. Time for the OS map . . .

Step 2

Scale: 1:8,028,979,200,000 (1 inch to 126.72 million miles)

Let’s make a map of the scale model, at a scale of an inch to the mile—that is, shrink everything down again. The new situation is:

  • the Earth is now about 1/16,000″ across, or 1/600 of a millimetre. You can see what a millimetre looks like by looking at an ordinary ruler or tape measure.
  • the Sun is ¾ of an inch away, and 1/150″ across, or 1/6 of a millimetre.
  • our friendly neighbourhood star, Alpha Centauri, is now 3.1 miles away—not quite in our neighbourhood, but walkable in under an hour.
  • Our galaxy, The Milky Way, is about 70,000 miles across.

That is, an inch-to-the-mile map of an inch-to-2000-miles map of the Galaxy needs a sheet of paper 70,000 miles across, the Sun is just about big enough to be seen with the naked eye, and the Earth can only be seen with a microscope.

Step 3

Scale: 1:508,716,122,112,000,000 (1 inch to 1.36 light years)

Let’s make another map: an inch-to-the-mile map of our inch-to-the-mile map of our inch-to-2000-miles scale model of our bit of the universe. It turns out it still needs a pretty big sheet of paper, but at least it’s not an unimaginably big one. On this new map:

  • the Earth is almost exactly 10-9 inches across, or 1/40 of a nanometre; ¼ ångstrøm. In other words, it’s smaller than a hydrogen atom, the smallest atom in nature.
  • the Sun is 1/84,000 inches away, and 10-7 inches across.
  • Alpha Centauri is just over 3 inches away.
  • The Milky Way is just over a mile across. It contains about 1011 stars; you could imagine it as a mile-wide snowstorm, with the stars scattered like snowflakes (but considerably tinier).
  • Our neighbour galaxy Andromeda, in the so-called Local Group (ha!), is 26 miles away.

However, we can’t stop there. What really brought home to me the appalling vastness of space was a map of approximately a million galaxies (just the ones brighter than a certain value) in a small section of the sky. It presented a lumpy grey appearance; the grey was simply the visual result of printing a dot for each galaxy. Not only were the Sun and Earth utterly tiny and insignificant among the 1011 stars of our galaxy; the Milky Way itself was insignificant among the many millions of galaxies in the observable universe.

Think what this means. To find the Earth, you first have to find the right galaxy–the correct dot on the lumpy grey map. That’s not exactly easy. Then, having scaled this tiny dot up so that it’s a mile across, you have to find something smaller than a hydrogen atom, too small to see even with an electron microscope. You scale this up to a decent size, and all of humanity is living on a thin layer less than 1/1000 of an inch thick . . .

The size of space is, in itself, still unimaginable; what I’ve done is to shrink some of it to an imaginable size, by what I hope are imaginable steps. If I’ve succeeded, maybe you now at least have a sense of how unimaginable it is.

I said above that this means we’re totally insignificant. Actually, that’s not the only way to see it. I’ll go into that in another post. But it seems clear that we can’t claim to be any more significant than any other populated planet in the unimaginably big universe. Claiming that it’s all centred on us seems so impossibly arrogant that it simply doesn’t make sense.

At this point a lot of people would look at the world’s religions, which make rather a lot of the importance of humanity to God, and say “Well that proves it. Religion is impossible.” What they really mean is that religion which puts human beings, or a particular tradition’s ideas, at the centre of the Universe is impossible. But that is only one kind of religion . . .

More on that in another post.


An exercise in astrolexicography

Plutoids and plutinos . . .

When the former planet Pluto was demoted to the status of “dwarf planet” fairly recently, two new words were defined by the International Astronomical Union: plutoid and plutino. If you ask me, these would be damn good words whatever they meant: they belong to that group of words which seem to exist as much because they’re fun to say as because they’re needed.

Several weeks ago one of my contacts on Twitter, @Exoplanetology, came up with the word exoplutoid, meaning a plutoid in a planetary system other than our own.

Should you wish to know, a plutino is an object which, like Pluto, orbits the Sun twice for every three orbits made by Neptune. (This is called a 2:3 resonance, and the object remains trapped in that orbit.) A plutoid, roughly speaking, is simply a dwarf planet which orbits the Sun further out than Neptune does.

I suppose an exoplutoid might be a dwarf planet in another star system, further from its star than the last convincing planet.

Nice words. Are there more?

Plutonyms in the dictionary

Let’s proceed with caution. A look at the dictionary reveals that a number of pluto- words already exist. Furthermore, not all of them are anything to do with Pluto. Plutocrats, being plutocratic in a plutocracy, get their name from the Greek word ploutos, which means wealth.

In geology, plutonic relates to rocks which have solidified from a molten state at the fiery depths associated with the god Pluto and his underworld, and a pluton is a “body of instrusive igneous rock”. Geology also uses the word plutonism in this connection.

In chemistry, the element plutonium has nothing to do with plutonism; the elements uranium, neptunium and plutonium take their names (rather nicely) from Uranus, and Neptune and Pluto, which were all planets at the time.

Plutogenous neologisms

Given the existence of all these words already, are we to conclude that Pluto has contributed all it can to the English language? I think not!

There are still plenty of Pluto-related situation requiring words. Some of the situations are more “serious” than others. But all need words, and it is my pleasure to present them to you. They are grouped by function rather than alphabetically. Use and enjoy.

similar in material or structure to Pluto.
exoplutoid, exoplutino:
a body in another planetary system analogous to a plutoid or plutino in ours.
originating from, or generated or caused by, Pluto and its status. For example, plutogenous fisticuffs might result from a heated discussion about its classification. See plutonym, below.
removal of Pluto or a Pluto-like object, e.g. from a list of recognised planets or (as a more advanced engineering project) from a planetery system
relating to the creation of Pluto-like objects, i.e. to plutogenesis.

a word created with reference to Pluto and its status; that is, one which enters the language as a plutogenous neologism.
the study of plutonyms.
the creation of a dictionary or glossary of plutonyms
an inability to remember what Pluto is officially classified as these days.
suffering from or relating to plutamnesia.
someone who suffers from plutamnesia.
1. condition of accidentally using the wrong plutonym, e.g. calling a plutoid a plutino or describing plutogenous situation as plutogenic. The corresponding adjective is paraplutotic.
2. erroneous identification of an object as Pluto.

Got any more? Post them here and I’ll do the plutolexicographer’s job of gathering them together, time and energy permitting. Especially if they’re good.