The reason is something called ‘inertia.’ The Earth always spins toward the east. Suppose you’re about to take off in an airplane, in that same direction. Whether you realize it or not – as you sit on the runway – your plane is already moving at the same speed Earth spins.

And – because of ‘inertia’ – you inside your airplane must maintain the same speed of Earth’s spin…unless something acts on you to stop or change it.

Consider this. If you ask for a cup of coffee on a fast-moving train, you know the coffee will go into your cup – not go streaming off behind you as the train moves forward. The coffee goes in your cup because it has acquired the same motion as the train. That’s inertia.

Because of inertia, everything on Earth moves right along with it as our world spins on its axis, as it moves in orbit around the sun, and as the sun moves around the center of the galaxy. We don’t notice any of these motions of Earth because we’re moving, too.

What this means is that – in order to figure out the actual speed of your airplane relative to some fixed point in space – you would have to add the airplane’s speed to the speed you already had sitting on the runway … the speed of Earth’s spin!