There was no doubt that the earth rotated around a central axis. But there was no way to demonstrate that it actually did. No concrete proof.
Not until January 3, 1851 that is, when Leon Foucault found that long sought proof with the help of an iron ball swinging from a 67 meter wire, suspended first from the dome of the Paris Observatory and a week later from the Pantheon in Paris.
His life presents nothing worth telling
except the discoveries he made
~ Editor, ‘Collected Works of Leon Foucault’
The basic premise of his experiment was that a rotating object once set in motion would retain its plane of swing with respect to the fixed reference frame, in this case the point of suspension. In simpler terms, the pendulum will only align itself to one plane. Therefore any change in the orientation at ground level betrayed the motion of the earth!! It was that simple!
This remarkable man who was bad at math and dropped out of medical school was credited with several more discoveries in his very short life! Here is a list.
And below is a trailer of Foucaults picture book biography that I borrowed the title from