Understanding of gravity in the Middle Ages

[JohnNemo]

In the Divine Comedy Dante describes travellers journeying through the Earth. He and his fellow travellers go down to the centre and continue and immediately realize that they are going "up". This indicates that gravity was understood to be a force or attraction coming from the centre on the Earth rather than, as we now know, a force coming from every part of the mass of the Earth (though in practical terms this difference in understanding only makes a practical difference if you are inside the Earth - outside the Earth the force under the two understandings would behave the same).

What else did people in the Middle Ages know about gravity? Did they, for example, know that the same force which was responsible for pulling objects on the surface of the Earth down was also responsible for keeping the Moon in orbit?

Note: At the risk of stating the obvious, Dante did not actually travel inside the Earth: he was writing fiction, albeit fiction which sheds light on contemporary beliefs about the Earth and its gravity.

 

[Drakkith]

What else did people in the Middle Ages know about gravity? Did they, for example, know that the same force which was responsible for pulling objects on the surface of the Earth down was also responsible for keeping the Moon in orbit?

No, they had no idea of what kept the Moon in its orbit*. This wouldn't be known until Newton developed his theory of Universal Gravitation in the 1600's. They didn't even have the concept of a "force" as we know it now. Classical mechanics hadn't yet been developed. I remember doing a small presentation on Galileo in school and one of my references talked about science during this time period. I'll try to find that reference, but this was several years ago, so I don't know if I'll be able to.

*Edit: I should elaborate that they did indeed have ideas about what made the planets and the Moon move, but these weren't scientific ideas, they were religious, and they had nothing at all to do with what kept people on the surface of the Earth.

For anyone interested, here's a PDF with a quick overview of astronomy in the middle ages: http://www.ifa.hawaii.edu/users/joseph/4.MiddleAges.pdf

 

[kuruman]

What else did people in the Middle Ages know about gravity?

I have read that in the Middle Ages people believed that angels pushed the Moon in its orbit, the angelic push presumably being in a tangential direction as per the Aristotelian belief that a force is needed to push an object in its direction of motion. Now we know better. We understand gravity well enough to stipulate that the angels push in the centripetal direction.

 

[lekh2003]

Gravity was something of the future. They didn't even have a clue what a force is or whether an object has inertia. How could they possible have any idea of any of the properties of gravity if they had no knowledge of forces and kinematics. The only thing they knew was that you go down, towards the Earth. Furthermore, they came up with a half-baked theory about why it only applied to objects and humans on Earth (the whole angel business).

Also, at the time, the modern model of the solar system had not been developed or accepted. They were under the assumption that the sun and moon orbited the Earth. Even if gravity was introduced to them, then they would have so many blocks in their head to accept it since they had no advancements in any other field hinting at it.

 

[sophiecentaur]

They didn't even have a clue what a force is or whether an object has inertia.

It's hard to understand how they managed to get on with their lives, being so ignorant about stuff that we find so important and obvious. The approach to Scientific matters was in terms of simple rules which treated the world as if it had human attributes. Hence "Nature abhors a vacuum" was the way gas pressure was appreciated. Even when Newton and his chums started sorting things out in a more objective way, they were limited in how to describe things verbally until they discovered that Maths does the job very well. After that, there was no looking back.
We were taught by people who (hopefully) had already been taught the modern Science approach so we can find it hard to appreciate why the ancients were so 'dumb'.
But, then, look at the Climate Change Deniers and Conspiracy Theorists of today. Old attitudes die hard.

 

[lekh2003]

It's hard to understand how they managed to get on with their lives, being so ignorant about stuff that we find so important and obvious. The approach to Scientific matters was in terms of simple rules which treated the world as if it had human attributes. Hence "Nature abhors a vacuum" was the way gas pressure was appreciated. Even when Newton and his chums started sorting things out in a more objective way, they were limited in how to describe things verbally until they discovered that Maths does the job very well. After that, there was no looking back.
We were taught by people who (hopefully) had already been taught the modern Science approach so we can find it hard to appreciate why the ancients were so 'dumb'.
But, then, look at the Climate Change Deniers and Conspiracy Theorists of today. Old attitudes die hard.

They were pretty backward, but this was a long time ago. I guarantee some chap like you or me 500 years in the future will be laughing at our methods of working with QED or other aspects of modern physics.

 

[russ_watters]

It's hard to understand how they managed to get on with their lives, being so ignorant about stuff that we find so important and obvious.

Your question contains, yet argues against its own answer: it's not important. Not to them. Pretty much all they needed to know is that when the sun rises over there, it's time to plant the crops. They "why" of how the sun got there just didn't matter.

 

[sophiecentaur]

They were pretty backward, but this was a long time ago. I guarantee some chap like you or me 500 years in the future will be laughing at our methods of working with QED or other aspects of modern physics.

I think they will, at least, credit us with learning to do without the idea of an 'agency' at work. That has been one of the very major steps in our Science, imo.

 

[lekh2003]

I think they will, at least, credit us with learning to do without the idea of an 'agency' at work. That has been one of the very major steps in our Science, imo.

I don't understand, what do you mean by "agency"?

 

[sophiecentaur]

Your question contains, yet argues against its own answer: it's not important. Not to them. Pretty much all they needed to know is that when the sun rises over there, it's time to plant the crops. They "why" of how the sun got there just didn't matter.

You are right. If a society is after stability then rocking the Scientific boat is counter-productive. Things at the moment are moving so fast that we are in great danger of total instability. The possibilities of extinction of the race are so much greater these days.
I think hunter gatherers had it about right. They had a pretty short working week, by all accounts, and such humans survived for many thousands of years. A short lifespan is not necessarily a sign of an unfulfilled life - as long as they had come to terms with the 'nasty, brutish and short' idea.

 

[russ_watters]

You are right. If a society is after stability then rocking the Scientific boat is counter-productive. Things at the moment are moving so fast that we are in great danger of total instability. The possibilities of extinction of the race are so much greater these days.
I think hunter gatherers had it about right. They had a pretty short working week, by all accounts, and such humans survived for many thousands of years. A short lifespan is not necessarily a sign of an unfulfilled life - as long as they had come to terms with the 'nasty, brutish and short' idea.

Uh, well, ok, but I don't agree with any of that!

 

[sophiecentaur]

I don't understand, what do you mean by "agency"?

An agency is something or someone who makes things happen. It's a term that I have heard used by philosophers to describe the human explanation of how and why things happen.
Our Parents are the first agency we are aware of. Then we may be taught about a divine agency to explain away things. In today's less godly age, we replace that with the government, the football club, etc etc, unless we think more rationally and try to look past that. But this is not a Philosophical Forum and we must not go down that road.

 

[lekh2003]

An agency is something or someone who makes things happen. It's a term that I have heard used by philosophers to describe the human explanation of how and why things happen.
Our Parents are the first agency we are aware of. Then we may be taught about a divine agency to explain away things. In today's less godly age, we replace that with the government, the football club, etc etc, unless we think more rationally and try to look past that. But this is not a Philosophical Forum and we must not go down that road.

Ooh, I get it. I also agree with your previous statement then.

 

[sophiecentaur]

Uh, well, ok, but I don't agree with any of that!

Good. Such disagreements are healthy.
I am glad that you can be so optimistic about things. Perhaps I have got too old and crabby.

 

[kuruman]

It's hard to understand how they managed to get on with their lives, being so ignorant about stuff that we find so important and obvious.

Actually, it's not that hard. Think about how often one uses the well known fact that the Earth is roughly spherical in shape. Apart from NASA people, pilots and navigators of all kinds and other such specialized professions, the common person manages everyday life quite well under the assumption that the Earth is flat. The same applies to the use of the geocentric system. I cannot imagine how I would describe the appropriate time of the day in the heliocentric system without using "sunrise" and "sunset". So I would say that we get on quite well. We use the approximation to ignore scientific facts when it's convenient and safe to do so. Climate Change Deniers find it convenient, but refuse to accept that the approximation is unsafe for us and our descendants.

 

[sophiecentaur]

Actually, it's not that hard

I guess I meant that, were you to return to that time, you would very rapidly run out of patience or 'understanding'. Worse still, you would need to keep your ideas to yourself or you'w be locked up as a loony.

Climate Change Deniers find it convenient,

Ah yes. Amazing how denying climate change is the province of people who find the idea uneconomical.

 

[JohnNemo]

Now we know better. We understand gravity well enough to stipulate that the angels push in the centripetal direction.

That sounds like something Newton might have mused 300 years ago. Should we not say now that the angels push either tangentially, or in a centripetal direction, depending on the frame of reference of the observer?!

 

[kuruman]

Should we not say now that the angels push either tangentially, or in a centripetal direction, depending on the frame of reference of the observer?!

I would say no if you are talking about traditional reference frames. Depending on that kind of reference frame, the angels generate a centripetal or centrifugal force, but not a tangential force. I would say yes if you are talking about a temporal frame of reference, i.e. Middle Ages vs. Information Age.

 

[jerromyjon]

That sounds like something Newton might have mused 300 years ago

You mean towards the end when he went insane?

Should we not say now that the angels push either tangentially, or in a centripetal direction, depending on the frame of reference of the observer?!

I get what you meant even though it doesn't quite make geometrical sense. I don't know how to say it better but...

Depending on that kind of reference frame, the angels generate a centripetal or centrifugal force, but not a tangential force.

Yeah, something like that.

 

[JohnNemo]

You mean towards the end when he went insane?

I get what you meant even though it doesn't quite make geometrical sense. I don't know how to say it better but...

Yeah, something like that.

@kuruman was making a joke at #3 and I was attempting - not very successfully - to make a joke in response. Difficult to explain the joke - it is a particular type of humour - but my reference to Newton was intended just in connection with centripetal force, not in connection with angels!

 

[JohnNemo]

It's hard to understand how they managed to get on with their lives, being so ignorant about stuff that we find so important and obvious. The approach to Scientific matters was in terms of simple rules which treated the world as if it had human attributes. Hence "Nature abhors a vacuum" was the way gas pressure was appreciated. Even when Newton and his chums started sorting things out in a more objective way, they were limited in how to describe things verbally until they discovered that Maths does the job very well. After that, there was no looking back.
We were taught by people who (hopefully) had already been taught the modern Science approach so we can find it hard to appreciate why the ancients were so 'dumb'.
But, then, look at the Climate Change Deniers and Conspiracy Theorists of today. Old attitudes die hard.

In a different thread @Nugatory posted a link to an article http://chem.tufts.edu/AnswersInScience/RelativityofWrong.htm which is very readable and, it occurs to me, is relevant to the general drift of this thread in giving a slightly more positive evaluation of the progress of scientific understanding.

 

[gmax137]

It's hard to understand how they managed to get on with their lives, being so ignorant about stuff that we find so important and obvious.

And yet, they would probably say the same about us. If most any of us were transported to central Europe, 1000 AD, we would be the frightfully ignorant ones. Unable to grow or find food, clothe ourselves, etc. Most people today cannot tell by looking whether the moon is waxing or waning.

Humans are adapted to their times, they get it from their parents and peers.

 

[Dr_Zinj]

It's hard to understand how they managed to get on with their lives, being so ignorant about stuff that we find so important and obvious. The approach to Scientific matters was in terms of simple rules which treated the world as if it had human attributes. Hence "Nature abhors a vacuum" was the way gas pressure was appreciated. Even when Newton and his chums started sorting things out in a more objective way, they were limited in how to describe things verbally until they discovered that Maths does the job very well. After that, there was no looking back.
We were taught by people who (hopefully) had already been taught the modern Science approach so we can find it hard to appreciate why the ancients were so 'dumb'.
But, then, look at the Climate Change Deniers and Conspiracy Theorists of today. Old attitudes die hard.

It's not "dumb" to question what appears to be bad methodology and execrable modeling, or preconceptions of the researchers biasing their conclusions; on BOTH sides of the question.

 

[StandardsGuy]

What else did people in the Middle Ages know about gravity? Did they, for example, know that the same force which was responsible for pulling objects on the surface of the Earth down was also responsible for keeping the Moon in orbit?

A quick Google search answers at least part of your question: https://en.wikipedia.org/wiki/History_of_gravitational_theory.

 

[JohnNemo]

A quick Google search answers at least part of your question: https://en.wikipedia.org/wiki/History_of_gravitational_theory.

I can't find anything in that article about the Middle Ages. It seems to skip from Antiquity to Modern.

 

[BWV]

Abu al-Fath Abd al-Rahman Mansour al-Khāzini or al-Khāzini defined gravity as a universal force directed towards the center of the Earth.

Two pages from Book of the Balance of Wisdom
Al- Khāzini originally a Greek slave who flourished in Merv at the beginning of the 6th/7th century and who continued the study of mechanics and hydrostatics in the tradition of al-Biruni and the earlier scientists.

In his books The Book of the Balance of Wisdom completed in 1121 in Merv al-Khāzini he doevd his work in eight treatises:
*Theories of centers of gravity according to various Greek and Arabic scholars
*Further discussion on centers of gravity; mechanism of steelyard
*Comparative densities of various metal and precious stones, according to al-Biruni
*Balances designed by various Greek and Arabic scholars
*The water balance of Umar Khayyam – its design, testing and use
*’The balance of Wisdom; determination of the constituents of alloys
*Weights of coinage
*The steelyard clepsydra

Source: http://historyofsciences.blogspot.com/2017/02/al-khazini-concept-of-gravity.html

 

[Hypercube]

It's hard to understand how they managed to get on with their lives, being so ignorant about stuff that we find so important and obvious.

They were pretty backward, but this was a long time ago. I guarantee some chap like you or me 500 years in the future will be laughing at our methods of working with QED or other aspects of modern physics.

Historians typically employ a principle that says we do not judge historical figures by today's standards. If we were, many such figures, e.g. Abraham Lincoln who was ahead of his own time, would be judged racist and backwards by modern standards.

But I agree, who knows how we would seem to someone three centuries into the future.

 

[Dr Wu]

Interestingly enough, the mediaevals (along with the rest of our predecessors) most likely had a far more familiar relationship with the naked-eye heavens than we're able to have in our modern light-polluted night skies! True, we're now worlds away from all that man-centred crystal spheres nitwittery, and believing the universe revolved around us - and amen to that. But then as Yuval Harari points out in 'Sapiens', the Scientific Revolution began with its first, and arguably its most important discovery: the discovery of ignorance. . . that enlightening moment when we so-called wise apes finally realized that we've been walking around for so long with our eyes wide shut - and what's more being big enough to admit it.

Still, if a reasonably educated representative of the Middle Ages were to be miraculously deposited into our modern high-tech world, and ignoring for the moment the massive cultural shock this act would provoke, he or she would surely be correct to ask us searching questions about our own blind spots. That's to say how science has coldly relegated us to the role of passive observers confronting a disinterested and purposeless universe, etc etc. Personally, I wouldn't have it any other way; but I think I would get it if our mediaeval visitor were to cry out: "What kind of hellish torment is this?" And that's allowing for the everyday wonders of novocaine. End of sermon.

 

[sophiecentaur]

Still, if a reasonably educated representative of the Middle Ages were to be miraculously deposited into our modern high-tech world, and ignoring for the moment the massive cultural shock this act would provoke, he or she would surely be correct to ask us searching questions about our own blind spots. That's to say how science has coldly relegated us to the role of passive observers confronting a disinterested and purposeless universe, etc etc. Personally, I wouldn't have it any other way; but I think I would get it if our mediaeval visitor were to cry out: "What kind of hellish torment is this?" And that's allowing for the everyday wonders of novocaine. End of sermon.

It's amazing how quickly people adapt to a change of paradigm if it's presented in an attractive way. Anthropologists report that members of 'primitive' societies take very little time to use modern AV equipment and transport. A mediaeval visitor would, I'm sure, adapt very fast to many ideas and, let's face it, your average Joe Public doesn't bother overmuch about fundamentals of Science. The tendency is to use a few buzzwords and get on with life in much the same way as they did three hundred years ago. The apparent sophistication of familiarity with the mobile phone and TV can be misleading.

 

[Mark Harder]

Blame it all on Aristotle. Aristotle is one of the greatest geniuses ever to set pen to papyrus. He wrote books on: physics, ethics, drama, biology, plus I think politics and poetry. There's a documentary, "Aristotle's Lagoon" as I recall, that offers the opinion that Aristotle's thought was so definitive that it stifled science for centuries to come. A BBC documentary as I recall, and highly recommended for the folks interested in the history of biology. His general outlook was that bodies move according to natural processes toward a proper goal, or 'telos'. Drop a ball and it falls toward the center of the Earth, where it 'belongs'. I don't know what he thought of heavenly bodies and their eternal circling around the Earth, never seeming to reach their goal, if there is one. Nevertheless circling around the Earth was their state of proper motion; and yes, the Sun revolved around the Earth - isn't it obvious? He recognized that there were forms of motion that seemed to violate these teleological rules. You can take a ball and throw it upwards, for example. He called these modes 'violent motion' (If I recall. I'm a bit fuzzy of the details on many of these points.) Violent motion was caused by force. You don't have to exert force on a ball when you drop it, you do have to exert a force on a ball to throw it upward, but once it is free of the influence of the force, it resumes it's natural motion toward the center of the Earth. A key aspect of his physics is the principle that force produces velocity - not acceleration as Newton showed us 1800 years later. It's often said that medieval science was conducted through philosophical contemplation in the absence of experimental observation. Though there was a lot of idle speculation, there were some experiments that said Aristotle was right. For instance, experimental evidence says that the Earth does not rotate. Stand at the edge of a high precipice, Mark the spot directly beneath a ball that you hold in your hand. Now drop the ball. So, if the Earth rotates and the ball is free to follow its natural trajectory, then the ball will land to the West of the mark since the mark has moved eastward along with the Earth. People did this experiment and of course the ball always landed on the spot directly below its point of release. Since the Earth is not rotating, the motion of heavenly bodies must be attributed to their revolution around the earth. What's wrong with that? Why wouldn't a 13th century natural philosopher be convinced that the Sun revolves around the Earth? Once they accept Aristotle's law of motion, that is.

 

[sophiecentaur]

Stand at the edge of a high precipice, Mark the spot directly beneath a ball that you hold in your hand.

I wonder how you would determine where that spot is and the accuracy of your chosen method. I would suggest that dropping the ball would be a probable way - or a plumb line. Did you calculate the likely difference between the answers from the two methods. Latitude would affect things, too.

 

[Mark Harder]

I wonder how you would determine where that spot is and the accuracy of your chosen method. I would suggest that dropping the ball would be a probable way - or a plumb line. Did you calculate the likely difference between the answers from the two methods. Latitude would affect things, too.

Dropping a ball to mark the spot would be a circular argument. I imagine they had plumb bobs in those days. I think they would consider the line to the weight moves with the earth. If you move the top of the line by exerting an unnatural force on it, it moves in the direction of motion of the suspension point.
I 'm afraid I can't answer your question about the precision of the measurement. I don't know if they took that into consideration. They were capable of measuring the relative angular motion of the stars and moon, but I believe they would need trigonometry to convert angular to linear measurements, unless they used Euclid and similar triangles. Still, that would require knowing the distance to the reference object in space. By the middle ages, Euclid was known and the diameter of the Earth was known. However, the discipline of mathematical physics took a while. I believe Galileo is generally considered the founder of math phys. because he derived laws governing falling objects, for instance, from experimental measurements - inclined planes in this case. It's possible that earlier experiments were strictly qualitative, not quantitative. In which case, experiments like the falling ball are subject to experimental error and lack of precision, as you point out.

 

[sophiecentaur]

Dropping a ball to mark the spot would be a circular argument.

That's why I queried what you wrote. A falling ball will be subject to the fictional Coriolis force, which will take it Eastwards as its height reduces. The amount of movement will be small - by a factor of height / Earth's radius. What you say about astronomical measurements could be relevant; the Maths would have been no problem for them. I am always stunned at how Keppler and others worked to such accuracy but angles between point sources are probably a lot easier to measure reliably (averaging out atmospheric disturbances etc.) than the equivalent on Earth. I would expect air currents to produce errors which are greater than what's being measured unless the ball is very massive.