Our Ability to Perceive Speed and Acceleration

When we are aboard a plane flying at cruising speed, we will not be able to feel that the plane is in fact moving at speed of more than 900 km/h. But if the plane changes speed, turns or changes its altitude, we can easily feel it.

Same thing happens with the motion of the Earth. Because of the Earth’s rotation, the surface of the Earths moves at a speed of 1656 km/h near the equator. It also travels around the Sun at a speed of about 107000 km/h. We never feel any of these because the speed is constant, or in other words, the acceleration is zero.

Without sensory input from sight and hearing, we can feel acceleration, but not speed. We feel acceleration through our vestibular system in our inner ear. Because of this vestibular system, we can maintain balance when upright, and determine which way is up even while blindfolded.

So, how do we know the plane is flying at such speed? We can use our sense of sight. We can look out and observe the objects outside are in fact moving, indicating the plane we are in is actually moving.

Again, same thing happens with the motion of the Earth. We can determine the Earth is moving by observing the motion of celestial bodies.

When on a plane, we could have refused to look out the window, but obviously we can’t conclude that the plane is not moving, and that other passengers that look out the window are all wrong.

Not much different from our case as inhabitants of the Earth. Maybe we can’t see the stars, and it is definitely harder to see the stars now than a couple of centuries ago. Maybe we simply can’t understand their motions. But we cannot conclude that the Earth is not in motion.

The Limits of Acceleration That Can Be Perceived By Humans

Nesti et al (2013) summarized the results of several experiments. The conclusion is that the lowest acceleration we can perceive is 0.02 m/s².

Centrifugal Acceleration Caused By Earth’s Rotation and Revolution

Earth’s rotation will result in centrifugal acceleration about 0.03 m/s² near the equator. It is small and practically in the same axis as the Earth’s gravitational acceleration of 9.8 m/s², so we can’t feel it. In many cases, the centrifugal acceleration is in fact already accounted for in the magnitude of Earth’s gravitational acceleration.

The revolution of the Earth around the Sun will result in centrifugal acceleration of about 0.006 m/s². It is far below our limit to perceive it. In addition, we are orbiting the Sun, and thus, in free fall situation with respect to the Sun. The centrifugal acceleration is cancelled by the Sun’s gravitational acceleration.

Acceleration Caused By Earth’s Orbit Eccentricity

According to Kepler’s Law, an object orbits with the shape of a conic section, and its speed changes according to its position in the orbit. In case of the Earth orbiting the Sun, the average acceleration resulting from this orbital motion is about 0.0000635938 m/s².

It is a very small acceleration, way below the limits of our senses to perceive it.

“But an aircraft has roof, unlike the Earth!”

Some flat-Earthers reject the analogy of airplanes because the air inside the plane is ‘separated’ from the outside air, while the atmosphere of the Earth is ‘not separated’ from space.

The reasoning is flawed because Earth’s atmosphere also rotates along with the surface of the Earth. If not, then we should be feeling wind at speeds in excess of 1600 km/h around the equator!

The analogy of being a passenger of a plane is correct because the air in the cabin also moves along the plane. Same thing happens with Earth and its atmosphere.