Why don’t we feel the Earth’s rotation?
What organ inside our body allows us to feel rotation? And how many sense do we have? Most people say just 5: Taste, smell, feel, hear and see. But this is wildly incorrect – human have many more senses: We really have
smell (olfacoception or olfacception)
temperature sensing (thermoception)
proprioception – knowing the relative position of parts of our body, even with our eyes closed. “It is provided by proprioceptors in skeletal striated muscles, and in tendons (Golgi tendon organ), and the fibrous capsules in joints.” – Wikipedia
Sense of angular momentum acceleration and linear acceleration – equilibrioception.
“Allows an organism to sense body movement, direction, and acceleration” – The organ for this is vestibular labyrinthine system, found in both of the inner ears.” – Wikipedia
Non-human animals also have other senses that human either totally lack, or that we possess only in a minor form which we usually don’t notice, such as:
Electroreception (or electroception) is the ability to detect electric fields.
Sound wave reception that builds up a 3D image – echolocation
“Bats and cetaceans, have the ability to determine orientation to other objects through interpretation of reflected sound (like sonar).” – Wikipedia
Magnetoception (or magnetoreception) is the ability to detect the direction one is facing based on the Earth’s magnetic field.
Polarized light direction/detection is used by bees to orient themselves, especially on cloudy days. Cuttlefish can also perceive the polarization of light.
Our inner ear senses angular acceleration
Q: If we are whizzing around the earth’s axis, and around the sun. why don’t we feel it?
A: We do not feel uniform motion: we feel forces, and forces are more closely associated with acceleration than with motion. If the sea is smooth and the ship’s motion also smooth, you may not even notice that it is moving, though you will notice the waves if present. The waves accelerate the ship up and down, so it exerts variable forces on your feet. They accelerate you, and you feel the variable forces doing that.
The acceleration due to the Earth’s rotation, at Sydney’s latitude, is 28 mm.s−2. This requires a force that is 0.3% of your weight, and it doesn’t vary quickly. From this calculation, you wouldn’t expect to feel the Earth rotating.
Due to its orbit around the sun, the acceleration is 7 mm.s−2. The speeds may be high, but the accelerations are trivial. In motion, you don’t feel speed, you feel the forces associated with acceleration.
…there’s an acceleration due to the spin of the earth around its axis. At the equator it’s about 0.03m/s^2, decreasing to zero at the poles. So if the earth were a perfect sphere, you’d weigh about 0.3% less at the equator than at the poles, meaning that a 160 lb person would feel about half a pound lighter at the equator than at the poles. That’s small, but not minuscule. If the earth were spinning faster, you’d feel it a lot–if a day were only 1.4 hours long, you’d be weightless at the equator. Faster than that, and you’d fly right off.
Away from the equator, this acceleration points away from the earth’s axis, while the gravitational field points towards the center of the earth. Since these are different directions at higher latitudes, another effect of the spin of the earth is to change the direction of gravity. In other words, what you perceive to be straight down isn’t exactly towards the center of the earth, it’s tilted a bit (a fraction of a degree) toward the equator.
In reality, the earth isn’t actually spherical, so the strength of gravity at the earth’s surface varies for a number of other reasons. The earth is slightly squished (oblate), which also makes the gravitational field weaker at the equator (roughly speaking, this is because you’re further from more of the earth’s mass). Local elevation and nearby mineral composition also make measurable differences. These effects make it harder for someone without sensitive instruments to disentangle the small effect of the earth’s spin.