The Rotating Earth

Nicholas Oresme

While re-examining Aristotle, Jean Buridan used observation and brainpower to anticipate some of the ideas we attribute to Galileo and Newton. He carried his ideas further when he put his mind to the question of the Earth's movement.

For most scholars of the Classical and early Medieval eras, the Earth was fixed, and the Heavens rotated around the Earth once each day. Buridan didn't like this: the Heavens are so much larger than the Earth; why would God design such an inelegant system? Moving the Earth would be easier.

Ptolemy knew this could not be, because if the Earth were rotating, there would be a constant rushing of wind as the air of the atmosphere passed over the land underneath it. Buridan scoffed at this: the atmosphere would be rotating just as the land does. There was no reason to dismiss the idea that Earth rotated daily.

For Buridan, however, empirical evidence was crucial. Of course, his predecessors argued, the Earth clearly does not move; we can see that. Buridan, however, likened the situation to being in a boat on a river. An observer on a second boat that was tied to the bank would see the first boat moving, but if the observer on the second boat could not see the surrounding landscape, then he would not know which of the boats were moving. The problem, Buridan knew, was that without an outside frame of reference, one cannot tell if it is the Earth or the Heavens that is moving. He needed an experiment, and he thought of one.

...and that's when he made his mistake.

Here was his idea: shoot an arrow straight up above your head. If it comes back down where you are standing, then the Earth is stationary. If the Earth rotated under it, then the arrow would come down somewhere off to the side.

He didn't realize that the same property that moves the atmosphere along with the ground would carry the arrow along as well. It would be Buridan's most brilliant student, Nicholas Oresme (c.1325-1382), who would realize and state that the arrow moves along with the Earth and atmosphere. Lacking a way to definitively prove his ideas, however, Oresme would ultimately fall back on the Bible for guidance on this issue.

Nicholas of Cusa

American History has a charming anecdote that the head of the US Patent Office once declared there was nothing left to invent. The background for this fable is explained here. Someone in 2010 said that we create as much data in 2 days as in all of 2003. Our ability to find and learn new things looks like it will never match the things there are to learn.

With the amount of knowledge being gained rapidly accelerating, why don't we know everything yet? Will we know everything?

Nicholas of Cusa (c.1400-1464) had an opinion on this. He outlined the problem in the opening remarks of his De Docta Ignorantia (On Learned Ignorance):

It so far surpasses human reason, however, to know the precision of the combinations in material things and how exactly the known has to be adapted to the unknown that Socrates thought he knew nothing save his own ignorance, whilst Solomon, the Wise, affirmed that in all things there are difficulties which beggar explanation in words;

He argued, however, that every living creature is

endowed with suitable faculties and activities; ... there is in them a discernment that is natural and in keeping with the purpose of their knowledge, which ensures their natural inclination serving its purpose and being able to reach its fulfillment.

If that is true, he says, then limits to our ability to know things must be built-in by our Creator for a purpose. And that purpose is that, at the end of our reasoning, when reason fails, we must turn to faith for the ultimate answers.

For this, Nicholas has been declared an anti-intellectual by at least one prominent modern scholar. But this misses his point: until we reach the limits of what we can figure out, we must travel as far as we can with reason. Just how far Nicholas of Cusa let his reason take him (and in ways that were appreciated by men like Bruno, Copernicus, Galileo and Kepler) I will explore this week.

The Oxford Calculators

The men before Galileo

In the first half of the 1300s, a handful of scholars at Oxford University, most of whom were at Merton College, applied observation and careful thought to what had been proposed by Aristotle and other Classical philosophers.

Their first innovation was to distinguish between kinematics (motion of bodies regardless of the forces acting upon them) and dynamics (the study of the forces that initiate or affect motion). Their use of observation, experimentation, and mathematics led them to understand that Aristotle was wrong when he said the motion/speed of a falling body was determined by its weight (which, to Aristotle, was affected by the proportions in the object of the four elements; water and earth tended to make it fall faster, while a greater amount of fire or air made it fall more slowly). They understood that two bodies of different weights accelerated similarly; they were able to prove this with math. This knowledge spread to universities on the continent. Nicholas Oresme, a bishop in France, and Giovanni di Casali, a Franciscan in Italy, were each inspired to produce graphs and diagrams to explain this motion.

Despite this dissemination of information, it is assumed that these works were not likely to have been known by Galileo (1564-1642), who gets the credit in physics texts for developing the theory of falling bodies.