De motu antiquiora

De motu antiquiora ("The Older Writings on Motion"), or simply De Motu, is Galileo Galilei's early written work on motion. It was written largely between 1589 and 1592 but was not published in full until 1890. This work shows Galileo's ideas about how things move during his time in Pisa, before he moved to Padua.

Galileo left this book unfinished and unpublished during his lifetime because he was unsure about some of his ideas and math. The style of writing changes from an essay to a conversation between two people who agree with his views. Later, Galileo used some ideas from De Motu in other books he wrote.

In De Motu, Galileo argued against Aristotle's views on the physics of motion. He used clever arguments to show that Aristotle's ideas about motion did not make sense. Even so, Galileo still believed that the classical elements played a role in how things move.

Galileo also suggested a new way to think about motion. He said that the true weight of an object can only be measured in a void, and that the weight changes depending on the liquid or air around it, called buoyancy. He thought that how objects move when thrown through the air depends on a force that slowly disappears over time.

De Motu is important because it is the first place where Galileo talked about pendulums. He noticed that heavier objects swing for longer than lighter ones, but he thought this was because the force in heavier objects lasts longer, not because air affects lighter objects more.

Publication history

Galileo may have planned to publish De Motu, but he left it unfinished. Today, we have a first draft essay about motion, some rewritten parts of the essay, a dialogue, topics and ideas, and many notes.

Parts of this work were first shared in 1854, and more was published in 1883. The full manuscript came out in 1890 in a big book of Galileo's writings. The first English version of the essay was published in 1960.

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Influences and origins of Galileo's Pisan dynamics

Historians have talked about what ideas helped shape Galileo's early thinking about how things move during his time in Pisa. Some believe his ideas grew from older thoughts about motion from the 1300s.

One thinker, Pierre Duhem, thought Galileo built on ideas from teachers like Jean Buridan and Nicole Oresme. Another, Alexandre Koyré, said Galileo tried to use math to explain these old ideas but ended up starting fresh later on.

Ernest Moody suggested Galileo’s way of thinking came from even older sources, possibly from writers like Avenpace and John Philoponus. He thought Galileo might have learned from people like Giambattista Benedetti, Jerome Borro, and his older coworker Francisco Bonamico, who also talked about how things move.

Distinguishing Galileo's early impressed forces from Buridanist impetus

Galileo had his own ideas about how things move, which were different from older thoughts by a philosopher named Buridan. Buridan believed that once something was moving, it would keep moving forever without stopping if nothing got in its way. Galileo thought differently. He believed that things moving would slowly lose their speed because of air resistance, even if there was no other obstacle.

When it came to the movement of the stars and planets, many ancient thinkers thought angels were pushing the stars to keep them moving. Buridan suggested that if God had set the stars moving when the world began, they would continue moving on their own without needing help. Galileo did not agree with either view. He wondered if the stars would keep moving forever or eventually stop, but he did not give a clear answer.

Synopsis of the essay portion

Galileo wrote his essay in chapters, though the chapters were numbered later by someone else to make it easier to reference. Some of Galileo’s ideas might seem correct, but others contain mistakes due to wrong assumptions or math errors.

Chapter 1: Heaviness and lightness

Galileo starts by explaining heaviness and lightness, which is similar to today’s idea of specific gravity or relative density. Two things are equally heavy if they have the same volume and weight. One thing is heavier than another if it weighs more but has the same volume.

Chapter 2: Heavy substances are by nature located in a lower place and light substances in a higher place

Galileo thought about the universe in a way that placed Earth at the center, with layers like earth, water, air, and fire stacked on top of each other. He questioned why these layers existed and suggested that heavier materials naturally settle closer to the center.

Chapter 3: Natural motion is caused by heaviness or lightness

Galileo said that objects move to their natural place. For example, a stone sinks in water because it’s heavier than the water, while lighter things float.

Chapter 4: Proof that bodies of the same heaviness as the medium move neither upward nor downward

This chapter provides the proof for the above statement.

Chapter 5: Proof that bodies lighter than water cannot be completely submerged

After giving the proof, Galileo concludes that heavier bodies sink and lighter ones float.

Chapter 6: An analogy between bodies moving naturally and the weights of a balance

Galileo compared natural motion to weights on a balance. He explained that when a lighter object is pushed under water, the water pushes back, similar to how a balance works.

Chapter 7: The cause of speed and slowness of natural motion

Galileo explored why some objects move faster or slower. He said that the speed depends on the weight of the object and the medium it moves through, not on things like the object’s shape.

Chapter 8: Different bodies moving in the same medium maintain a ratio of speeds different from what is said by Aristotle

Galileo disagreed with Aristotle’s ideas about how fast objects fall. He argued that objects of the same material fall at the same speed, regardless of their size, and gave examples to support his view.

Chapter 9: In view of all the above, bodies moving naturally are reduced to the weights of a balance

Galileo looked at the force needed to hold objects underwater and compared it to the weights on a balance. He talked about how the speed of motion relates to the weight of the object and the medium.

Chapter 10: Proof that, if there were a void, motion in it would not take place instantaneously

Galileo argued against the idea that motion in a vacuum would happen instantly. He believed that motion would still take time, just like in other places.

Chapter 11: Disproving Aristotle’s claim that air has weight in its own place

Galileo disagreed with Aristotle’s idea that air has weight. He explained that air doesn’t feel heavy when we swim, and that objects move based on their weight compared to their surroundings.

Chapter 12: Disproving Aristotle’s claim that absolutely light and absolutely heavy exist; and even if they did, it would not be earth and fire

Galileo questioned Aristotle’s ideas about the heaviest and lightest things in the universe. He argued that fire and earth aren’t necessarily the heaviest or lightest, and gave reasons why.

Chapter 13: Proof that differences in weights and motions are determined only in a void

Galileo said that to understand the true weight and speed of objects, we would need a vacuum where there’s no medium affecting them.

Chapter 14: A discussion regarding the ratio of the speeds of bodies moving along various inclined planes

Galileo looked at how objects move down sloping surfaces. He tried to find the force needed to push an object up an incline and used that to understand how fast it would fall down.

Chapter 15: An argument that rectilinear and circular motions have a ratio to each other

Galileo disagreed with Aristotle’s claim that straight-line and circular motion can’t be compared. He argued that even though they’re different, they can still have a relationship in terms of measurement.

Chapter 16: The question of whether circular motion is natural or forced

Galileo wondered if the motion of a spinning sphere at the center of the universe is natural or forced. He concluded that it’s neither and left the question of whether it would keep spinning or stop for another chapter.

Chapter 17: The agency by which projectiles are moved

Galileo disagreed with the idea that air pushes projectiles along. Instead, he believed that a force is imparted to the projectile when it’s thrown, and this force keeps it moving.

Chapter 18: A demonstration that the motive force is gradually diminished in a moving body

Galileo argued that the force keeping a projectile moving gets weaker over time, and that motion can’t continue forever with the same speed and force.

Chapter 19: The cause of acceleration of natural motion towards the end of motion

Galileo tried to explain why falling objects speed up as they fall. He suggested that the force pushing the object decreases over time, but his explanation had errors.

Chapter 20: A demonstration that, at the apex of projectile motion, an interval of rest does not occur

Galileo argued against the idea that a thrown object stops for a moment at the highest point of its path. He believed it passes through rest instantly.

Chapter 21: It is proved that if natural motion could be extended without limit, it would not become swifter without limit

Galileo disagreed with the idea that an object would keep speeding up forever if it kept moving. He thought that eventually, the force would run out, and the object would reach a steady speed.

Chapter 22: An explanation as to why, at the beginning of their motion, less heavy bodies move more swiftly than heavier ones

Galileo explained that lighter objects start moving faster because they have less weight to overcome. He compared this to how heat fades from hot iron.

Chapter 23: An explanation why objects shot at a greater angle from the horizon move closer along a straight line than at lesser angles

Galileo discussed how the angle at which an object is shot affects its path. He argued that the force imparted to the object is stronger when it meets more resistance, like when fired at a steeper angle.