Aristotle(384-322 B.C)

• Greek philosopher, scientist, and educator.

• Systematized existing knowledge:

• Made observations

• Collected specimens

• Gathered together, summarized, and classified existing knowledge of the physical world

Aristotle(384-322 B.C)

• Believed the Earth was a sphere because:– The Earth’s shadow on

the Moon during a lunar eclipse is always round.

Aristotle(384-322 B.C)Aristotle

(384-322 B.C)

• Believed the Earth as sphere because:– Stars near the

celestial pole are higher in the sky the farther north you travel.

Aristotle(384-322 B.C)

• Believed the Earth was a sphere because The fact that objects fall to Earth towards its center means that if it were constructed of small bits of matter originally, these parts would naturally settle into a spherical shape by compression

Ships Sailing Over the Horizon

Aristotle on MotionsViolent Motion

Natural Motion

Natural Motion• Is caused by

objects “striving” to get to their proper place.

• The “proper place” is determined by how much of each of the fours elements (Earth, water, air, fire) the object contains.

Smoke (made of Air) strives to get to the Air.

A feather (part Earth, part Air) strives to get to the Earth, but is slowed by also striving to get to the Air.

A rock (made of Earth) strives to get to the Earth.

Violent Motion

• Results from something external pulling or pushing on an object in opposition to its natural motion

• Requires a constant push or pull to sustain the motion

Once in flight, air filling in behind and pushing on the arrow keeps it going

Lifting the rock opposes the rock’s striving to the Earth.

Some Aristotelian Conclusions

• A heavier object (having more earth) will fall to the ground faster than a light object.

• If you stop pushing an object, it will stop moving.

Aristotle’s Universe

• The Earth is stationary and at the center.

• The heavens are 55 concentric crystalline sphere, each carrying celestial objects and rotating.

• Celestial bodies are perfect spheres made of a perfect and unchanging substance (quintessence).

• A “Prime Mover” keeps the heavens in motion.

Eratosthenes(276-195 BC)

At that moment, the shadow cast by a vertical obelisk in Alexandria (5,000 stadia distant) showed that the Sun was 7.2 degrees away from the vertical.

At noon on the Summer Solstice, sunlight struckthe bottom of a vertical well in Syene, Egypt.

Therefore, the Earth’s circumference is5000 x (360/7.2) = 250,000 stadia

How Accurate Was Eratosthenes?

• Historians disagree on how big a Greek “stadium” was.

• Depending on what the correct conversion to the modern units is, Eratosthenes’s estimate was between 1 and 20 percent larger than the modern value.

Why Does It Matter? The panel of experts appointed by

King Ferdinand to review Columbus’s proposal to sail to India was aware of Eratosthenes’s work. Because of this, they ruled that the distance was too great and that he therefore should not go.

Columbus, using either error-laden estimates of others, or using an incorrect conversion factor, believed that the distance was only 1/3 or ½ of the true distance.Queen Isabella overruled the experts and sided with Columbus.

Beginnings of modern, empirical science: Ibn al-Haythem (965-1040 CE)

• Born in Basra, Iraq• Educated in Basra and Baghdad• Worked in Egypt and Spain• Developed the modern theory of optics by taking

careful measurements in repeatable experiments

• Considered various theories of light and designed and carried out experiments to determine which theory might be more accurate

Ibn al-Haythem

• Studied the propagation of light and the nature of colors

• Studied optic illusions and reflections• Studied the refraction (bending) of light rays

when pass from one transparent medium to another (air, water) and studied the angle of incidence and refraction quantitatively

• Dispersion of light into colors• Studied the way the light travels through the

atmosphere and estimated the thickness of the atmosphere

Beginnings of modern, empirical science:

Roger Bacon (1214-1294 CE)• Born in England, worked in Paris and Oxford• Became a Franciscan friar• Vigorously encouraged experimental science • Asserted that the study of the natural world by

observation and exact measurement was the surest foundation for truth.

• Having read al-Haythem’s works, he performed optical experiments with lenses and mirrors, described spectacles

Roger Bacon“The strongest arguments prove nothing so long as the conclusions are not verified by experience. Experimental science is the queen of sciences and the goal of all speculation.”

“For the things of this world cannot be made known without a knowledge of mathematics”

All science requires mathematics. 

Galileo Galilei(1564-1642)

• Defined new concepts in addition to discovering objects

• Simplified investigations through:– Reducing scope (focusing on

one issue at a time)– Idealization (thinking away

complications)• Designed experiments to

test hypotheses• Described the physical

world with mathematics

Galileo’s approach sets the stage for all of modern

science.

Galileo on authority, measurement, and math

In questions of science the authority of a thousand is not worth the humble reasoning of a single individual.

Philosophy is written in this grand book, the universe, which stands continually open to our gaze. But the book cannot be understood unless one first learns to comprehend the language and read the letters in which it is composed. It is written in the language of mathematics, and its characters are triangles, circles, and other geometric figures without which it is humanly impossible to understand a single word of it; without these, one wanders about in a dark labyrinth.

Count what is countable, measure what is measurable, and what is not measurable, make measurable.

Galileo on Falling Bodies

• Experiment: drop objects of various weights from the Leaning Tower of Pisa

• Conclusion: Except for small effects of air resistance, objects of various weights fall together and reach the ground at the same time.

Aristotle observed that there was always a resistive medium

in the world around him.

Galileo imagined what would happen if there was no

resistance.

Galileo on Motion• Experiment: Observe motions of

various objects going down inclined planes.

• Conclude: In the absence of friction, a rolling ball rolling along a horizontal surface:– will neither speed up nor slow down– will never come to rest.

Aristotle: motion requires a push or a pull to keep moving.

Galileo: with no interference, a moving object will kepp moving

in a straight line forwever.

Inertia

• Galileo defined a new word

Inertia: the property of an object that tends to keep it moving straight ahead or that resists changes in it motion

Galileo’s Universe

• Seeing that Jupiter hadmoons orbiting meantthat celestial bodies(such as our Moon)could orbit objects otherthan our Sun.

• Extending the idea of inertiato circular motion meant thatthe Earth could circle theSun indefinitely.

• So Galileo adopted the Copernican model of the universe

“Mathematics is the language with which God has written the universe.”

“Philosophy is written in this grand book - I mean the Universe - which stands continually open to our gaze, but it cannot be understood unless one first learns to comprehend the language and interpret the characters in which it is written. It is written in the language of mathematics, and its characters are triangles, circles and other geometrical figures, without which it is humanly impossible to understand a single word of it.”

Galileo