As we celebrate the Earth completing another lap around the Sun, let’s take a moment to imagine what life would be like in a world without years – a world that somehow ceased to orbit its star. Admittedly, it’s a strange question, but its’s one that I’ve been obsessively wondering about lately. Not because it’s of any particular relevance, but simply because it’s amusing (at least to me) and fun to think about.
What would happen to us if a giant space finger were to gently stop the Earth in its orbit?
Here, try it out for yourself. Press ‘start’ in the simulation below (created by Michael Dubson and the folks at Phet Interactive Simulations / University of Colorado). You should see a planet orbiting the Sun.
Now press ‘reset’, and drag the circle with the letter ‘v’ to shrink the planet’s speed . Then press ‘start’ again. What happens? (While you’re playing with this, you might enjoy trying out some of the different scenarios in the drop-down menus, and watching the gravitational ballet that ensues.)
If you slowed down the planet enough, you should see it crash into the Sun.
To see why, let’s first remember why things stay in orbit. Every child looking at the sky has at some point wondered, “why doesn’t the moon fall down?” The answer is beautifully simple, yet it took a mind as brilliant as Isaac Newton’s to work it out. (Perhaps a sign of genius is coming up with simple answers to children’s questions.)
Newtons’ response to the child’s question would have been – the moon does fall. It’s constantly falling. Being in orbit is a state of always falling, and always missing what you’re falling towards. In The Hitchiker’s Guide to the Galaxy, Douglas Adams describes the secret to flight. “The knack”, he writes, “lies in learning how to throw yourself at the ground and miss”. As it turns out, this is also a great description of what it means to orbit something.
Here’s how Newton explained it. Imagine a cannonball is fired from a height. If you fire the cannonball with more speed, it’ll travel further before it hits the ground. The faster the cannonball, the further it travels.
But wait – the Earth is round. That means that if you shoot the cannonball with enough speed, then by the time it would have hit the ground, it’s travelled far enough that the ground has curved away beneath it. So the cannonball continues to fall towards the ground, and the ground continues to curve away from it. It’s now in a state of perpetual free fall – the cannonball is in orbit!
(Newton’s idea is masterfully explained in this wonderful Radiolab segment.)
So the only thing that makes an orbit different from plain-old falling is having enough speed to miss the thing you’re falling towards. Think dropping a cannonball with zero speed versus shooting it into orbit. And for the same reason, if the Earth were robbed of all of its orbital speed, it would fall straight into the Sun. It would no longer have the speed it needs to miss the Sun.
How long would this ‘Earthfall’ take? Continue reading