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A Brief History of Physics

Physics has its roots in the study of Cosmology; an overview from Ptolemy to Newton

Deborah Skapik

on 4 December 2017

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Transcript of A Brief History of Physics

Cardinals of the Inquisition meet and decide to condemn the teachings of Copernicus, so when Galileo publishes a dialog supporting them in 1632, he is placed under house arrest.
Kepler’s description of planetary orbits seemed to work, but still, no one had yet explained why it works...
Copernicus still assumes uniform, circular motions, so he must include epicycles to get orbits right.

The work is not published until after his death, and includes a preface by Andreas Osiander, Lutheran theologian.

The Copernican doctrine is eventually condemned by the Church as heretical in 1616.
Notes on De Revolutionibus
The inability to measure parallax of stars held back the progress of physical science for nearly 2000 years.

So…what did we settle for?

P = 1/D
Where P is the angle measured in units of arcseconds and D is the distance to the star measured in units of parsecs.


1 degree = 60 arcminutes
1 arcminute = 60 arcseconds

1 parsec = 3.26 light years = 3.08 x 10^16 meters
How is parallax useful?
Philosophical objections: Earth is not divine and cannot circle in the “higher” cosmic spheres

Some physical objections:
If Earth is offset from the “cosmic center” then observers on one side of Earth should see a smaller volume of the Universe than on the other side (but they don’t).
Stars should appear to get brighter and dimmer cyclically as we swing toward and away from them.
Objections to heliocentric view
Democritus (~420 B.C.)
Day/night cycle
Phases of Moon
Movements of heavenly bodies (especially retrograde motion)
What must ancient cosmologies explain in one logical system?
kosmos logos
“world knowledge”
forming a view of the universe: the story of how our modern understanding of physics was shaped by science and culture
the universe
P^2 = a^3
P (period) is measured in units of years
a (semi-major axis*) is measured in units of AU

*avg distance of planet from Sun
Kepler’s 3rd Law
Second Law: Equal Areas in Equal Times
First Law: Planetary orbits are ellipses
There is no one centre in the universe.
The Earth’s centre is not the centre of the universe.
The centre of the universe is near the Sun.
The distance from the Earth to the Sun is imperceptible compared with the distance to the stars.
The rotation of the Earth accounts for the daily rotation of the stars.
The apparent annual cycle of movements of the Sun is caused by the Earth revolving around it.
The apparent retrograde motion of the planets is caused by the motion of the Earth, from which one observes.
7 Axioms of the Little Commentary
Nicholas Copernicus 1473-1543, Poland
Tycho Brahe 1546-1601, Prague (Czech Republic)
Galileo Galilei 1564-1642, Italy
Johannes Kepler 1571-1630, Germany
Isaac Newton 1643-1727, England
Cast of Characters
The Magic 100 Years (1543-1643)
…the answer comes…
…but it is hideously complex!
Not even Ptolemy himself believed it to be a physical model—just a mathematical construct.
Huge flaw: this model does not explain why planets move as they do.

We need a cosmology that not only explains HOW (kinematics) but WHY (dynamics).
Ptolemy’s system has predictive power

No real progress is made in understanding the nature of the physical universe until the 16th century…
Ptolemy cements the “geocentric” system, but without physics
Improves Apollonius’ theory, but usually gets the credit for it
Hipparchus (190-120 B.C.) of Nicaea
Provides the first quantitative estimate of Earth’s size
Eratosthenes (276-194 B.C.) of Alexandria
The notion of a spinning Earth is philosophically distasteful and physically preposterous to some
No observable stellar parallax
Other problems with cosmologies that include
a moving Earth
From http://www.math.tamu.edu/~don.allen/history/eudoxus/eudoxus.html
Eudoxus (400-347 BC)
(from Cnidus, now Turkey)
Phases of the Moon
3 Laws of Planetary Motion:

All planets follow elliptical orbits with the Sun at one focus.
Planets in orbit cover equal amounts of area in equal amounts of time.
The period of a planet in orbit is related to its distance from the Sun (farther planets take longer to orbit Sun).
Johannes Kepler (1571-1630)
Diagram from De Revolutionibus
Shakes the dust off
Aristarchus’ heliocentric theory
In 1514 writes his
“Little Commentary” (and circulates it amongst interested scholars)
His major work (which included math proofs), De Revolutionibus orbium coelestium, is not published until 1543 (post-humus!)
Nicholas Copernicus
Advanced by Claudius Ptolemy
of Alexandria (lived ~100-170 AD)

His most significant work was The Almagest, 13 books.
Proposes a complex system to explain the orbits of Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn and stars around the Earth.
The Ptolemaic System
The first to propose “epicycles” and “deferents” to explain why planets move differently than stars.
Apollonius (240-190 B.C.) of Perga
There are only 40 stars within 5 parsecs of Earth.

The nearest of these is Proxima Centauri, at a distance of 1.33 parsecs. What is its parallax?
The nearer the star, the larger the parallax.
from http://www.malaspina.org/gif/aristarchus1.jpg
A scientist, not a philosopher
Presents a method for finding relative sizes of Sun, Moon and Earth
Is one of the first to propose that Earth rotates and revolves around the Sun
Aristarchus (310-230 B.C.) of Samos
Online works of Aristotle
This is a detail from the fresco The School of Athens by Raphael taken from
Aristotle (384-322 B.C.)
…but they didn’t usually do experiments…
Greek thinkers are probably the earliest to seek natural (rather than supernatural) explanations for natural phenomena.
Babylonian tablet showing a list of eclipses from 518 to 465 BC.

From http://www.livius.org/k/kidinnu/kidinnu.htm
pre-400 B.C. Babylonian, Chinese, Indian Astronomy; European megalithic sites
A brief history of scientific inquiry
How did we arrive at modern physical theories?
Moon has mountains
Spots on the Sun move
Moons orbit Jupiter
Milky Way is made up of countless numbers of stars
Phases of Venus are correlated with its angular size
1610 Siderius Nuncius
Supporting observations come from Galileo Galilei (1564-1642)
Tycho Brahe (1546-1601)
Each major body orbits on a small circle called an epicycle.
Each epicycle moves along a deferent, an eccentric circle with its center offset from Earth.
From a special point called the equant, the motion of the planet appears uniform.
Ptolemy introduces epicycles
Total solar eclipse
November 23, 2003
Jay Pasachoff
Partial solar eclipse
South Africa
April 19, 2004
Jay Pasachoff
Including Olmec to Aztec, Mayan
Advanced numeric systems
Keeping calendars (annual cycles, tracking Venus was very important)
Some evidence they understood motions in the solar system very well!
Meso-American civilizations
(1200 BC-1000 AD)
Newton is the one who at last explains WHY planets orbit the Sun (and not the other way around). He develops the Universal Law of Gravity and fully explains the dynamics of the Solar System.
Isaac Newton (1643-1727)
icosahedron (5 triangles)
octahedron (4 triangles)
tetrahedron (3 triangles)
dodecahedron (pentagons)
Quintessence (?)
Plato (428-348 B.C.)
Apply Kepler’s 3rd to known planets
what's that??
Go to http://psych.hanover.edu/krantz/motionparallax/motionparallax.html

This is equivalent to measuring the diameter of a dime held at a distance of 6 kilometers!
To explain these things, Plato, Aristotle, etc. advanced a picture of an immobile Earth surrounded by nested, perfect spheres.
interested mainly in observing, recording, predicting
In Maya mythology, each cardinal point was assigned a specific color and a specific jaguar deity (Bacab). They are as follows:

Hobnil - Bacab of the East, associated with the color red and the Kan years.
Can Tzicnal - Bacab of the North, assigned the color white and the Muluc years,
Zac Cimi - Bacab of the West, associated with the color black and the Ix years.
Hozanek - Bacab of the South, associated with the color yellow and the Cauac years.
let's look deeper to see how this conclusion about the nature of the universe was drawn
Ptolemy's epicycles
Flash animation of Ptolemeic system:
Even more on where Copernicus got his ideas!
why this date???
1 AU=1 astronomical unit=Earth-Sun distance
there are ~1000 megalithic sites in the British Isles, dating ~2000 BC

the oldest!
Next total solar eclipse visible from North America: April 2024
How would this look different if you were driving in a circle?

Early physics is largely a series of human attempts to understand
by applying geometric principles to the natural world.
interested in a Cosmology that is uniform and orderly; felt motion should be circular
taught that all motion results from the nature of the moving object or a sustained push/pull
click this to see what I think about planets!
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