illusion of
relative movement
. You will have met the same kind of illusion often enough. You are in a train , standing at a station next to another train. Suddenly you seem to start ‘moving’. But then you realize that you aren’t actually moving at all. It is the second train that is moving, in the opposite direction. I remember being intrigued by the illusion the first time I travelled in a train. (I must have been very young, because I also remember another thing I got wrong on that first train journey. While we were waiting on the platform, my parents kept saying things like ‘Our train will be coming soon’ and ‘Here comes our train’, and then ‘This is our train now’. I was thrilled to get on it because this was
our
train. I walked up and down the corridor, marvelling at everything, and very proud because I thought we
owned
every bit of it.)
The illusion of relative movement works the other way, too. You think the other train has moved, only to discover that it is your own train that is moving. It can be hard to tell the difference between apparent movement and real movement. It’s easy if your train starts with a jolt, of course, but not if your train moves very smoothly. When your train overtakes a slightly slower train, you can sometimes fool yourself into thinking your train is still and the other train is moving slowly backwards.
It’s the same with the sun and the Earth. The sun is not really moving across our sky from east to west. What is really happening is that the Earth, like almost everything in the universe (including the sun itself, by the way, but we can ignore that), is spinning round and round. Technically we say the Earth is spinning on its ‘axis’: you can think of the axis as a bit like an axle running right through the globe from North Pole to South Pole. The sun stays almost still relative to the Earth (not relative to other things in the universe, but I am just going to write about how it seems to us here, on Earth). We spin too smoothly to feel the movement, and the air we breathe spins with us. If it didn’t, we would feel it as a mighty rushing wind, because we spin at a thousand miles an hour. At least, that is the spin speed at the equator; obviously we spin more slowly as we approach the North or South Pole because the ground we’re standing on has less far to go to complete a circuit round the axis. Since we can’t feel the spinning of the planet, and the air spins with us, it’s like the case of the two trains. The only way we can tell we are moving is to look at objects that are not spinning with us: objects like the stars and the sun. What we see is the relative movement, and – just as with the trains – it looks as though we are standing still and the stars and the sun are moving across our sky.
A famous thinker called Wittgenstein once asked a friend and pupil called Elizabeth Anscombe,
‘Why do people say it was natural to think that the sun went round the Earth rather than that the Earth turned on its axis?’
Miss Anscombe answered,
‘I suppose because it looked as if the sun went round the Earth.’
‘Well,’ Wittgenstein replied, ‘what would it have looked like if it had looked as if the Earth turned on its axis?’
You try and answer that!
If the Earth is spinning at a thousand miles an hour, why, when we jump straight up in the air, don’t we come down in a different place? Well, when you are on a train travelling at 100 mph, you can jump up in the air and you still land in the same place on the train. You can think of yourself as being hurled forwards by the train as you jump, but it doesn’t feel like that because everything else is moving forwards at the same rate. You can throw a ball straight up on a train and it comes straight down again. You can play a perfectly good game of ping-pong on a train, so long as it is travelling smoothly and not accelerating or decelerating or going fast around a corner. (But only in an enclosed
relative movement
. You will have met the same kind of illusion often enough. You are in a train , standing at a station next to another train. Suddenly you seem to start ‘moving’. But then you realize that you aren’t actually moving at all. It is the second train that is moving, in the opposite direction. I remember being intrigued by the illusion the first time I travelled in a train. (I must have been very young, because I also remember another thing I got wrong on that first train journey. While we were waiting on the platform, my parents kept saying things like ‘Our train will be coming soon’ and ‘Here comes our train’, and then ‘This is our train now’. I was thrilled to get on it because this was
our
train. I walked up and down the corridor, marvelling at everything, and very proud because I thought we
owned
every bit of it.)
The illusion of relative movement works the other way, too. You think the other train has moved, only to discover that it is your own train that is moving. It can be hard to tell the difference between apparent movement and real movement. It’s easy if your train starts with a jolt, of course, but not if your train moves very smoothly. When your train overtakes a slightly slower train, you can sometimes fool yourself into thinking your train is still and the other train is moving slowly backwards.
It’s the same with the sun and the Earth. The sun is not really moving across our sky from east to west. What is really happening is that the Earth, like almost everything in the universe (including the sun itself, by the way, but we can ignore that), is spinning round and round. Technically we say the Earth is spinning on its ‘axis’: you can think of the axis as a bit like an axle running right through the globe from North Pole to South Pole. The sun stays almost still relative to the Earth (not relative to other things in the universe, but I am just going to write about how it seems to us here, on Earth). We spin too smoothly to feel the movement, and the air we breathe spins with us. If it didn’t, we would feel it as a mighty rushing wind, because we spin at a thousand miles an hour. At least, that is the spin speed at the equator; obviously we spin more slowly as we approach the North or South Pole because the ground we’re standing on has less far to go to complete a circuit round the axis. Since we can’t feel the spinning of the planet, and the air spins with us, it’s like the case of the two trains. The only way we can tell we are moving is to look at objects that are not spinning with us: objects like the stars and the sun. What we see is the relative movement, and – just as with the trains – it looks as though we are standing still and the stars and the sun are moving across our sky.
A famous thinker called Wittgenstein once asked a friend and pupil called Elizabeth Anscombe,
‘Why do people say it was natural to think that the sun went round the Earth rather than that the Earth turned on its axis?’
Miss Anscombe answered,
‘I suppose because it looked as if the sun went round the Earth.’
‘Well,’ Wittgenstein replied, ‘what would it have looked like if it had looked as if the Earth turned on its axis?’
You try and answer that!
If the Earth is spinning at a thousand miles an hour, why, when we jump straight up in the air, don’t we come down in a different place? Well, when you are on a train travelling at 100 mph, you can jump up in the air and you still land in the same place on the train. You can think of yourself as being hurled forwards by the train as you jump, but it doesn’t feel like that because everything else is moving forwards at the same rate. You can throw a ball straight up on a train and it comes straight down again. You can play a perfectly good game of ping-pong on a train, so long as it is travelling smoothly and not accelerating or decelerating or going fast around a corner. (But only in an enclosed
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