The proofs of the immobility
(or mobility ) of the Earth
Thetraditional proofs of the immobility of the earth are of two kinds:
A) the sensible evidence that the sun and the whole celestial vault daily describe a circumference around the earth, together with the circumstance that if the earth really moved eastward, an object falling from a height shouldfall, contrary to evidence to the west of it.
B) the missed observation of the stellar parallaxes (picture 1).
The defenders of the mobility of the Earth, in primis Galileo, answered the two previous objections in the following way: A) they observed that sensible evidence can deceive and appealed to a principle of inertia, according to which an object falling from a height keeps the same velocity that had been set to it. Moreover,as it was observed later, as the velocity of the same object is bigger than the velocity of the lower strata of the atmosphere where it falls, it even makes it fall to the east of the height. B) They appealed to the inadequacy of the available instruments (as a matter of fact stellar parallaxes are fully detected by present telescopes). Further proofs of the motion of the earth, which were unimaginable both during the classical age and in the XVI century, were given by the development of mechanics and consisted of experiments like Foucault's pendulum : the plane of the pendulum whose stand rotates around the vertical, remains fixed in the space if it is considered by an inertial observer, but it rotates in the opposite direction to the rotation of the earth if it is considered by a terrestrial observer (pictures 2 and 3).

Fig 1 Lunar Parallax: A and B are two observers, L is the Moon, L1 and L2 the two apparent positions of the Moon

Fig 2-3 Pendulum of Foucault
Go to Galileo