Earth's journey through space
The Earth is spinning, and we’re whizzing around the Sun at a dizzying speed; our journey through space is also not circular, and the Earth is tilted on its axis; both of these factors make a world of difference to life here on the planet’s surface. To put some numbers on this, Chris Smith spoke with Cambridge University space scientist Matt Bothwell...
Matt - I guess the answer is it depends which way you look at it. The earth is rotating and we're rotating at about a thousand miles per hour if you're standing at the equator. But the earth is also orbiting around the sun. If you zoom out of it, we're traveling at about a hundred thousand kilometers per hour around the sun. So somewhere between fast and very fast, depending on which way you look at it!
Chris - And of course it doesn't stop there does it because the sun is on its own orbital journey around our galaxy. And our galaxy is on its own orbital journey around its cluster of galaxies.
Matt - Exactly. Yeah. The Milky Way is rotating once every couple of hundred million years. And we are traveling around 200 kilometers per second around the centre of the galaxy, which is just a phenomenally large feat.
Chris - So it's quite hard to actually say how fast we really are moving through space!
Matt - Exactly. You always need to define it relative to something, right. If you're traveling in a car along a road at 60 mph, you're defining your speed relative to the roads, it's all about what you, what you're traveling relative to, I guess.
Chris - But returning to our cosmic neighborhood and how this affects life on Earth for us, why do we have a seasonal pattern to the weather?
Matt - So that's all about the earth's axial tilt. So the earth isn't actually straight up in space. The earth is a bit wonky. We've been knocked off the straight axis by about 23 degrees. That's actually the evidence of an ancient impact. So billions of years ago, the earth was struck by some kind of big body in the solar system. It's the same impact that formed the moon. And ever since then, the earth has been wonky by about 23 degrees. And that's why we have our seasons. So as we go round and round the sun, the Northern hemisphere in one part of the cycle, and then the Southern hemisphere six months later, in the other part of the cycle will be pointing towards the sun and will therefore get more sunlight and will experience summer. So we only have seasons because of this axial tilt.
Chris - And is that axial tilt stably wonky or, or does it wobble around?
Matt - It actually wobbles around. So there is a 26,000 year cycle where the earth's axis wobbles around a bit like a spinning top. If you imagine setting a spinning top going and seeing it kind of slowly rotating around, that's kind of what the earth does. So over about 26,000 years, the earth's axis rotates around. It's called the procession of the equinoxes. And because of that, the night sky actually shifts. So Polaris for example, is the North star right now. It's the star that is above the earth's axis. If we wait a few thousand years, the earth's axis would have shifted around and Polaris will no longer be the North star. About 5,000 years ago when the pyramids were being made, we had a different North star and we will have a different North star again in the future. It's kind of amazing the night sky shifts above our head because of the earth.
Chris - Yeah. It boggles your mind. Doesn't it. Given that the earth is tilted, but that tilt, you're saying wobbles about, does the wobbliness with this 6,000 year cycle translate into climate changes with a similar sort of cycle?
Matt - Yes, it absolutely does. There are these things in climate called Milankovitch cycles, and these are the cycles of the climate that are caused by the earth's motion in space. As the earth travels around the sun, sometimes it's orbit is a bit more squashed and sometimes it's more circular and this the wonkiness of the earth changes over this 26,000 year period. And then, so over hundreds of thousands of years, yes, the climate changes because the earth is orbiting in very slightly different ways around the sun.
Chris - And if we come down and sort of land on the earth surface for a moment, you did start by saying the earth is spinning pretty fast. First of all, why is the earth spinning pretty fast? And what effects does that have?
Matt - So the earth is spinning for the same reason that the earth is orbiting the sun. We've inherited the primordial spin, if you like, from the cloud of gas that formed the solar system. If you go back billions of years before the sun and the planets existed, there was a big cloud of gas and gravity squashed everything down and formed the planets and the sun. In physics we have a thing called conservation of angular momentum. Basically, if something starts spinning, you can't get rid of that spin. And so any little bit of spin that that gas cloud had originally, would be inherited by the sun and the planets. And so that's why the sun is spinning. The planets are spinning and they're all orbiting the sun. It's all the leftover spin from that original gas cloud.
Chris - And what effect does that spin have on things on the earth surface? For instance, the atmosphere, for instance, the oceans and the seas?
Matt - There's an effect called the Coriolis effect, which is basically caused by the fact that at the equator, that's the fastest you can spin and the higher latitude you go, or the lower latitude towards the South pole, you're actually spinning slower and slower. And if you imagine standing near the North pole or the South pole, you'd be going around very, very slowly, and that causes all kinds of interesting effects in the atmosphere. For example, if you have air going from lower latitudes around the equator, up to high latitudes, it's going to be traveling much faster than the air up in the North. And so, um, that is going to cause all kinds of interesting cyclic effects in the atmosphere and the ocean. And so a lot of what the weather is doing is caused by this Coriolis effect, which is basically a result of the earth spinning round.
Chris - And that would be why storms spin characteristically? When you see them on satellite pictures, they spin one way in the Northern hemisphere and the other way in the South hemisphere.
Matt - Exactly. It's wonderful. When you see pictures of Earth from space, you can see cloud formations, which are just showing you these, these forces caused by the earth's rotation just kind of written in the clouds. It's kind of beautiful.