PHY1033C/HIS3931/IDH 3931 : Discovering Physics:
The Universe and Humanity’s Place in It
Fall 2015
Prof. Peter Hirschfeld, Physics
Announcements
• HW 2 due Sept. 22 (use Almagest passage in
syllabus)
• Lab 2 today 2nd hour
• Reading: Gregory, Chapter 3, pp. 45-51
• http://www1.umn.edu/ships/galileo/library/c
usa3.pdf Sections 156-166 (pp.89-95) (tough!)
• Osiander’s foreword and Copernicus’ preface
Last time
• Aquinas: synthesis of Aristotelian and Christian
doctrine – condemnation of 1277
• Dispute over other worlds and people on them
• Ancient references to earth’s motion
Clicker quickies
The central issue in the medieval debate
over other worlds was about
a. How many angels could fit on the head of a pin
b. Whether other worlds were as large as the Earth
c. Whether other worlds revolved around the Earth
d. Whether God could have created other worlds, and
had He chosen to do so
e. Exactly how many people lived on Saturn’s rings
Ancient advocates of moving Earth
Heraclides (390-310 BCE) rotation of the Earth
Aristarchus (310-230 BCE) earth revolves around a “central fire”
See account by Archimedes (287-212 BCE):
You (King Gelon) are aware the 'universe' is the name given by most astronomers to the
sphere the center of which is the center of the Earth, while its radius is equal to the straight
line between the center of the Sun and the center of the Earth. This is the common account
as you have heard from astronomers. But Aristarchus has brought out a book consisting of
certain hypotheses, wherein it appears, as a consequence of the assumptions made, that the
universe is many times greater than the 'universe' just mentioned. His hypotheses are that
the fixed stars and the Sun remain unmoved, that the Earth revolves about the Sun on the
circumference of a circle, the Sun lying in the middle of the Floor, and that the sphere of the
fixed stars, situated about the same center as the Sun, is so great that the circle in which he
supposes the Earth to revolve bears such a proportion to the distance of the fixed stars as
the center of the sphere bears to its surface.
Ptolemy (2nd cen CE) considered and rejected arguments in favor of the earth’s motion
Aside: Aristarchus calculates ratios of lunar/solar
distances
Because of his error in angular measurement, he was a factor of
20 off! But A+ for the idea.
Medieval arguments against moving Earth
• Jean Buridan (1300-~1358) One can’t decide if
earth is still or rotates based on astronomical
observation. Can decide the question physically:
shoot arrow straight up, see where it lands
• Nicole Oresme (~1320-1382) Rejected Burdian’s
reasoning: atmosphere could be carried with
moving earth. Pointed out sailors on smoothly
moving ship can’t tell they are moving!
But earth doesn’t move because of scripture, he
concludes!
Joshua 6:1-27
During battle of Jericho, God caused the sun to “stop in the sky”
so that Joshua could defeat the Canaanites.
Raphael (1483-1520)
Reading: Nicholas of Cusa (1401-1464)
Concludes earth is in motion for theological reasons
God is “Absolute Maximum,” creation is “contracted maximum”
Creation “unfolds from God,” so all nature is “enfolded” in Abs Max
Humans know by measuring, but can only measure finite things
Hence humans cannot know God, who is utterly unknowable
Still we try to understand --“learned ignorance”
Analogy to help us poor mortals who can never understand God (infinity) perfectly:
As the circumference of a circle increases it approaches a line in imagination
This is an example of “coincidence of opposites”, gives us a glimpse of God
As coincidence of opposites, God is “center and the circumference” of the universe
Earth therefore not fixed in place, but moves, nothing natural is utterly at rest
http://plato.stanford.edu/entries/cusanus/
N. De Cusa raises question of
infinite/finite nature of the universe
Depiction of Giordano Bruno contemplating the infinite in 19th cent. science text.
https://www.youtube.com/watch?v=jCtml57GjYk
See also https://www.youtube.com/watch?v=gtk0gFim-Nk
velocity
Centripetal acceleration
v=speed
r=radius of circle
Copernican system
for public consumption
for calculations
Advantages of Copernican system
•No equant
•Slightly better fit (was not hugely better)
•Internal harmony (one geometry of heavens,
not 7 individual account s of planets)
•Order of planets sure
Copernican system determines the
order of the planets
Disadvantages of Copernican system
• Not appreciably less complex (still plenty of epicycles, etc.)
• Sun the center of stars, not planets
• Physics of motion inconsistent with predicted experience
(if earth in motion)
• Challenge to understanding of scripture (Joshua)
• Lack of parallax