# Do pilots consider Earth's rotation?

18 November 2012

## Question

Hi Dr Chris, I live in central South Africa - Free State, and don't receive the radio signal for 702 but down load the pod casts and listen to them then in the car, i have a question that i wonder if you can shed some light on...

When flying up to Europe or down here to South Africa its about a 10 hr flight, when starting the flight, the destination airport will not be in the same place as when you arrive, does the pilot set the destination where it is (at the start of the flight) or where it will be in 10 hrs time (at the end of the flight) or does it not really matter due to wind currents caused by the spinning of the Earth -
meaning that the aircraft will be caught up in the wind currents, staying with the Earth as it spins?

Thanks for your feedback and a great show..
Best Regards Bruce.

## Answer

Dominic - That's a fascinating idea, isn't it?

I always like the idea that perhaps you could take off in a helicopter in London and just float in the air for 6 hours or so and then descend and land in New York, and that would be a very cheap and easy way to cross the Atlantic.

In fact, any aircraft is traveling through the Earth's atmosphere and the Earth's atmosphere is moving with the rotation of the Earth.

Unfortunately, the air that the aeroplane is travelling through is also rotating. You don't have to take any special account for the fact that that the Earth is rotating because you're being carried with that medium.

This is because there is friction between the air in the atmosphere and the ground below.

That friction will start the air rotating with the Earth. At the top of the atmosphere, there's no friction there with space because there's nothing above the atmosphere. So, with time, the atmosphere just picks up the Earth's rotation.

## Comments

### air friction

Please explain how air (a gas) can have friction with the ground.

### The same way air has friction with any moving body...

When the planet moves, its surface moves relative to the air surrounding it. The air molecules are being hit and pushed out of the way by the moving surface. This imparts momentum to these molecules, which in turn hit other molecules and transfer some of that momentum to those other molecules. Over time, the air molecules end up moving in the same direction as the planet is rotating.