While you are reading this article you are rotating around the Earth’s axis with speeds comparable to that of sound: to be precise, 1180-1280 km/h to the latitudes of Italy. Yet, despite the very high speed, we don’t feel this movement at all. We feel the rotation very well when we take a bend in the car or when we get on a carousel, yet we don’t feel the Earth spinning: why?
An answer that we often read around – but it’s wrong – claims that we don’t notice the earth’s rotation because this movement occurs at constant speed and we are moving with the Earth. The logic is the same as when we are on a plane and we do not feel the speed when the plane proceeds in a straight line without any kind of disturbance: inside the aircraft we do not feel any acceleration (because there is no acceleration) and therefore the sensation is essentially indistinguishable from being still. In physics this concept is called Galilean relativity: systems in uniform rectilinear motion are indistinguishable from a physical point of view from systems at rest.
This explanation, however, leaves time behind: if we were on a carousel that rotates at a constant speed, we would certainly feel the rotation! The reason is easy to say: Rotating systems are not in uniform rectilinear motion (by definition!) therefore Galilean relativity does not apply to them. In rotating systems, in fact, we find accelerations (apparent, but we won’t go into technicalities here) that do not exist in systems in uniform rectilinear motion. The most obvious example is thecentrifugal accelerationwhich pushes everything away from the center of rotation. It is this acceleration that we feel when we are rotating! But then why don’t we feel it even though we are on the surface of a planet that is rotating wildly on itself?
The fact is that the centrifugal acceleration due to the Earth’s rotation, although it may appear enormous due to the high speed, it is almost completely negligible compared to the force of Earth’s gravity that keeps us firmly on our planet. Calculator in hand, one latitude of 45° (that of Northern Italy, so to speak) the acceleration is equal to 0.0238 m/s2. In simple terms, we are talking about an acceleration 416 times weaker than gravity that pushes us down.
At the equator, where centrifugal acceleration is greatest due to the larger rotation circumference, things don’t change much: 0.0337 m/s2, 294 times weaker than the gravitational one. In short, we are talking about accelerations that are too weak to be felt, further “extinguished” also by the Earth’s atmosphere, which with its friction contributes to preventing any movement.
Even if it is not perceptible with our senses, the Earth’s rotation still leaves very clear clues, such as the motion of the Foucault’s pendulumwhose first public demonstration took place on8 January 1851. To celebrate this anniversary, January 8th is celebrated throughout the world Earth Rotation Day.
