Because even when you’re flying off to a remote island to get away from it all, you can’t escape being part of “it all.” Your airplane is attached to Earth almost as tightly as the mountains below are, except that the airplane is (we hope) at a higher altitude.
Your pilot would be the first to assure you that the plane is firmly attached to the air. And since the air is attached to Earth, you might say that we’re all in the same boat, sailing merrily eastward along with the surface of Earth at around 1,000 miles per hour (1,600 kilometers per hour).
(The ground’s speed is actually 1,040 miles per hour [1,670 kilometers per hour] at the equator; that’s the circumference of 24,900 miles [40,100 kilometers] divided by 24 hours. But it’s slower as we go north or south on the globe because the circular paths get smaller.)
You do, of course, see the ground “moving” beneath you as you fly. But it’s your own airplane’s motion that you’re seeing, not the ground’s. It’s the same as seeing the trees “speed backward” as you speed along the highway in your car. That’s a very important point to realize: There is no such thing as absolute motion. All motion is relative. Nothing can be said to be moving or not moving without specifying “relative to what?” Motion is motion only when it is compared with some independent reference point (Techspeak: a frame of reference).
To the trees, you and your car are moving, but to you and your car, the trees are moving. Who’s right? If you had been born in your car a second ago, you’d swear that it was the trees that were moving, intuitively and egotistically using yourself as the reference point. It is only with experience that we learn to accept reference points outside of ourselves.
If each driver took himself or herself as the reference point, the trees would be “moving” every which way at all kinds of speeds, because every self-centered person’s reference point would be moving in a different direction at a different speed. Stationary trees, however, are so much easier to deal with, so we humans have agreed to take the trees and the land they’re attached to as our stationary references.
But let’s stand back and take a bigger view of Earth. When we say that a palm tree on the equator is moving along with the ground at 1,040 miles per hour (1,670 kilometers per hour), we have to ask, “Relative to what?” Well, how about relative to the center of Earth? That’s the only point on or inside the whole globe that isn’t moving around in circles. In other words, we’re taking the center of Earth as our “stationary” reference point.
But whoa! Let’s stand back even farther. The whole planet is moving around the sun at 10,600 miles per hour (17,100 kilometers per hour) relative to the center of the sun, which we can now take as our new reference point.
But the sun itself is moving relative to other stars. And the stars are moving relative to the center of our galaxy. And our galaxy … And on and on, literally ad infinitum.
Before we get too dizzy, let’s get back into the airplane. Sitting there, anyplace on the plane is your assumed reference point, so you see Earth “moving backward” with the (forward) ground speed of the plane. But remember that you and your little bag of peanuts and that screaming baby across the aisle are all moving together at approximately Earth’s rotational ground speed, relative to the center of the Earth. I say “approximately” because if you’re flying eastward in the same direction as Earth’s rotation, the plane’s speed (relative to the center of Earth) is added to Earth’s rotational speed; if you’re flying westward in the opposite direction of Earth’s rotation, the plane’s speed is subtracted from Earth’s rotational speed. If you’re flying northeast or south by southwest, consult your high school trigonometry teacher. Can you say “vector”?
I said that the plane is firmly attached to the rotating Earth because it is firmly attached to the air and the air, in turn, is firmly attached to Earth. Well, not exactly.
Air is a fluid, meaning that it isn’t rigid; it flows. So as Earth turns, the air can’t precisely keep up; it drags and slops around a bit like a puddle in a rowboat. Although the plane is indeed firmly held by the air, the air is somewhat loosely held by Earth. That’s not to say that we’re in any danger of losing our atmosphere; gravity holds that whole layer of air down quite firmly.
But within that layer, the air is a churning, blowing, moving mass, and local irregularities can still kick your airplane around with tail winds, head winds and coffee-splattering bumps that make you feel as if you’re not very well attached to anything.