Why does the Earth spin one way and not the other? There’s a logical reason.


Why does the Earth spin one way and not the other? There’s a logical reason.

The Sun sits, a big bright blob in the centre of the Solar System. Small and smaller balls spin round in concentric circles — or ovals, if you want to be perfectionist.

Sprinkle two rings of still finer stuff — not round, but all manner of rough, jagged and irregular shapes. Add some cold icy comets if you like. And finally, a splash of stars to set he background.

There you have it: your familiar picture of the Solar System at a glance.

But the Solar System is not a glance. Once you’ve learnt where each planet stays, you’ll eventually raise the question: which way do they move?

At first glance, the answer seems simple. The planets move anticlockwise round the Sun, when seen from above.

But what is “above”? How do you decide which is the “top view” of the Solar System, and which the bottom?

Sitting here on Earth, it’s easy to tell up from down. It’s nearly as easy as telling front from back, and certainly easier than telling left from right. Down is the way you fall, and up is the way you don’t: that’s all there is to it.

Or, should I say, that’s all there was to it. Not any more.

In olden days, things were simple. The Earth seemed like a neat, flat place, with humans always “up” and potatoes always “down”. Then came cartography and astronomy (not to mention artificial potato-cultivation), and things became much more complicated.

Zoom out a bit, and you’ll see that the Earth is nothing but a giant ball. People stand all round the surface — or at least, wherever there’s land. “Down”, if anything, is towards the centre of the Earth, and all other directions are “up”.

Then you notice all the millions of other stars, with their billions of other planets, each one with gravity to make a “down” of its own. “Down”, it seems, is in all other directions too.

There’s no “up” or “down” in space, any more than there’s “front” or “back” in a marble. The terms just don’t make sense.

But people needed a reference direction to talk about, if only to discuss which way planets move. So they came up with a simple solution. Slice the Earth in half, they said, across the way it’s spinning.

The half we’re on is “up”, and the other half is “down”.

Because the people in the Northern Hemisphere were many and more dominant, they were the ones who decided which way was “up”. And, that’s the direction you look from if you want to say which way the planets are spinning.

If Australians had called the shots, planets would be spinning “clockwise” because we’d be seeing them from the other end.

Or would they?

In the early days of the Solar System, there were no planets or Sun. (Come to think of it, there were no “days” either). There was just one massive cloud of matter, spinning fast from the previous supernova explosion.

Over time, the cloud collected into lumps. The big lump became the Sun; the others planets: each still going round on its circular path, but also spinning around itself like a top.

That’s why, with a few exceptions, they’re spinning the same way they’re travelling. Call it clockwise or anticlockwise, they’re spinning the same “-wise” as they’re circling around the Sun.

One notable exception is the planet Venus. People think it started out the same way as everybody else, but got knocked upside-down somewhere along the way. So now it moves clockwise compared to the others.

Why do clocks turn the way they do? Why not the other way round? To find the answer, we’ll need to look at the first clock in the world.

When you think of ancient clocks, you may think of pendulums or hourglasses — but before that was an even simpler method. Take a stick. Stick it into the ground. And there you have it: the sundial.

It doesn’t have to be a stick either. It can be anything that’s pointing straight up, including yourself standing. In the morning, the Sun will shine from the east, casting a long shadow to the west of your body. In the evening, the opposite happens. And at noon, with the Sun right overhead, you’ll cast no shadow at all.

Actually, that’s not quite true — unless you’re right on the equator.

That’s when the Sun’s shining directly from above. As you come up North(or up South, as the case may be) the Sun shows up lower in the sky more to the south (or north) than dead centre.

Right at the poles, the Sun is always near the horizon: barely showing itself for half a year, and barely vanishing for the other half, as the Earth makes its slow way round the Sun. Even closer to the equator, the Sun doesn’t go straight overhead. It goes in a slightly tilted semicircle — so even at noon, there’s a slight shadow in one direction.

And that shadow doesn’t just grow longer and shorter: it traces a full circle on the ground.

When people started making clocks, they made them move the same way as sundials did: clockwise.

Of course, if they’d done it in the Southern Hemisphere, they’d have made clocks go the other way and called that clockwise. But they’d also have called their side “up”, and seen the Solar System from the other end, so we’d still see planets moving in the direction opposite to our clocks.

While it seems arbitrary at first, planet and clock movements are intrinsically related. No matter which way you start from, except for special cases like Venus, planets will always move in the direction opposite to clocks. Even if you’re an alien from another star where planets move the other way round.

The Earth doesn’t move anti-clockwise. Clocks move anti-Earthwise.

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