# Trivia 12

How do we know the Earth Rotates?

Sometimes we think to have a good argument to teach that the Earth rotates by just looking at the sky and watch the stars moving around “us” from East to West, to conclude that the Earth rotates from West to East. That is all right, but it in not a prove that the Earth rotates, it is the consequence of it. The people in the Middle Ages also saw the sky moving from East to West around them, but did not conclude from this observation that the Earth rotated around its axis (what was an axis of rotation anyway?) but thought that the universe rotated around them, announcing and proclaiming that we were the center of the universe.  In modern scientific thinking, we would take that apparent motion of the sky to hypothesize a possible rotation, but have to get the information by other means. There is a famous experiment made by Jean Bernard Léon Foucault (1819-1868). He hang on a long string (67 m) a heavy mass (28 Kg) creating a pendulum from the ceiling of the Pantheon in Paris, and with it, he was able to show how the plane of the swing motion of the pendulum was changing direction, that can only be explain if the Earth rotates. Why? Because the forces acting on the pendulum (the Tension on the string) and the force of gravity (its weight) are in the same plane; no lateral forces were acting on the mass. But because the pendulum has in motion (periodic motion) and due to the angular velocity of the rotating Earth, a fictitious force, called the Coriolis force, acts on the moving mass, making the plane of the pendulum to rotate. Illustration of this phenomenon can be seen in many science museums around the world.  The Coriolis effect can be seen on large masses that move on another body in rotation, like on Earth, and the moving masses of air determining the weather patterns. You might be wondering about the word “fictitious” force. Because, it is and effect of rotation and not a real acting force. Another fictitious force is the “centrifugal” force, that is, another effect of rotation, that in many cases are being interpreted at a reaction force of the centripetal force. Those are not action-reaction pair of forces. The Foucault pendulum is the prove that the Earth rotates (and it must do it around an axis of rotation) and now, we can use this knowledge to explain the motion of the stars around us.

¿Cómo sabemos que la Tierra rota alrededor de su eje?