History and Philosophy of Western Astronomy
History and Philosophy of Western Astronomy
This set of notes by Nick Strobel covers the development of western astronomy
and modern science. I focus on the rise of modern science in Europe, from the
ancient Greeks to Isaac Newton. I do realize that other cultures were also
quite interested and skilled in astronomy (the Mayans, Egyptians, peoples of
India and China come immediately to mind), but I am less knowledgeable of their
history and others on the net will hopefully create documents about astronomy
in those cultures. An excellent resource I used to construct these notes is
Science and the Human Prospect by Ronald Pine. These notes will be in
outline form to aid in
distinguishing various concepts. As a way to condense the text down I'll
often use phrases instead of complete sentences. The vocabulary terms are italicized.
-
- Universe a rational place following universal, natural laws and we are
able to figure out those laws. Open inquiry and critical evaluation is highly
valued. The emphasis was on the process rather than attaining the goal. People
eventually got tired of learning and wanted absolute answers. Science is not
able to give absolute, certain answers. There was disagreement among the
experts and there came to be a crisis in confidence that led to the rise of
the Sophists.
-
- The Sophists taught that truth and morality are myths; relative to the
individual. If most people don't agree with you, then you should conform to the
prevailing views.
-
- Socrates (470-399 B.C.E.) taught that real truth is possible through
collaboration with others.
-
- Plato (427-347 B.C.E.) taught that there are absolute truths--mathematics
is the key. Statements about physical world will be relative to
individual/culture but mathematics is independent. His Four Basic Points:
- There is certainty.
- Mathematics gives us the power of perception.
- Though the physical applications of mathematics may change, the thoughts
themselves are eternal. Another realm of existence.
- Mathematics is thought = eternal and known by anyone.
[Today: Mathematical ideas are free creations of the human mind--tools to map
the world. Experience is the key. Certainty not possible but knowledge still
possible. We can attain reasonable beliefs even though absolute certainty is
not possible.]
-
- When one studies mathematics, one studies the mind of God. Mathematical
symmetries are the language of universal design and harmony. Their faith in
order caused the Greeks to try to find explanation for the seemingly unordered
planets (particularly
retrograde motion). Their
faith in an ordered universe
compelled them to make precise observations and they were sustained by their
belief in the power of reason. Modern scientists have this faith (in one form
or another) in an ordered universe and the power of human reason.
-
- Paradigm--a general consensus of belief of how the world works =
world view.
The Pythagorean Paradigm had three key points about the movements
of celestial objects: 1) Their motion is perfectly circular; 2) The Earth is
at the exact center of their motion; and 3) Their motion is perfectly uniform.
Find a geometric explanation for the apparent motion of the planets (especially
the strange retrograde motion). One key observation: as a planet undergoes
retrograde motion, it becomes brighter.
-
- Some of the observations that convinced the Greeks that the Earth was
not moving:
- The Earth is not part of the heavens.
- The celestial objects are bright points of light while the Earth is an
immense nonluminous sphere of mud and rock.
- There is little change in the heavens: Stars same night after night.
Earth is home of birth, change, and destruction. Celestial bodies have immutable regularity that is never to be achieved on the corruptible Earth.
- Our senses show Earth is stationary:
- a.
- Air, clouds, birds, and other things unattached to Earth are not left
behind as they would be if the Earth was moving. There is no strong wind.
- b.
- If the Earth were moving, then a man jumping from a high point would hit
the Earth far behind from the point where the leap began.
- c.
- Rocks, trees, and people would be hurled from rotating Earth (they knew
about centrifugal force).
-
- Plato's Instrumentalism--An infinite number of theories can be
constructed to account for the observations. We can never empirically answer
what the universe is really like. Scientific theories are tools or calculation
devices and are not to be interpreted as real. Generalizations may
be shown to be false in the future and, also, false generalizations can
actually ``work''.
-
- Aristotle's Realism (student of Plato, lived 384-322 B.C.E.)--scientific,
mathematical tools are not merely tools--they characterize the way the universe
actually is. He thinks that Plato has gone too far. At most one model is
correct. He chooses Eudoxus' crystalline spheres because:
- Most popular and observational evidence supports it (see item A above).
- His physics and theory of motion necessitates geocentric
(Earth-centered) universe. Things naturally move to the center of the Earth and
the only way to deviate from that is to have a force applied to the object.
- Agrees with Pythagorean paradigm of uniform, circular motion (see item F
of previous section above).
-
- Ptolemy (85-165 C.E.) greatly extends Apollonius' and Hipparchus' models which use epicycles (planet has smaller circular
motion--epicycle--around a point which is in a circular orbit around the
Earth). Epicycles are an important part of the model and explain why planets
are brighter as they retrogress. He adds some refinements to explain the
details of the observations:
- Eccentrics--true center of planet motion (not the Earth!)
- Equant--planet moves uniformly in relation to it (not the Earth!)
These refinements are incompatible with Aristotle and Pythagorean paradigm--a
planet on an epicycle would crash into its crystalline sphere and the motion is
not truly centered on the Earth. So adopt instrumentalist view--this strange
model is only an accurate calculator to predict the planet motions but the
reality is Aristotle's model. Ptolemy was successful in having people adopt his
model because he gathered the best model pieces together, used the most
accurate observations and he published his work.
-
- Paradigm by 16th century: Man is God's special creation of the
physical
universe. The Earth is the center of a mathematically planned universe and we
are given the gift of reading this harmony. There is an unbounded faith in
power of reason to solve physical problems.
-
- Nicolaus Copernicus (1473-1543 C.E.) Any model must account for the
observations and have circular, uniform motion. The Ptolemaic model does not do
that. Also, the Ptolemaic model is not elegant (un-Godlike). Copernicus was a
neoplatonist (belief that combines elements of Christianity and Platonism)
which led him to:
- believe that the Sun is a material copy of God--God is the creative force
sustaining life and the Sun gives us warmth and light;
- adopt Aristarchus' heliocentric (Sun-centered) model. Model has
same accuracy as Ptolemaic one but is more elegant. It explains retrograde
motion more naturally than the geocentric models--the looping is only an
optical illusion due to the relative positions of the planets to the Earth and
the different speeds of the planets in their orbits.
- However, other people accept his model as an instrument only. No
parallactic shift observed in stars (viewing stars from two opposite sides of
Earth's orbit should make the stars appear to shift due to our new vantage
point). If there was actually a parallactic shift but it was very small, then
the stars would be very far away--God would not waste that much space so
there must be no parallactic shift at all. Copernicus thought his model was
reality.
- He retains the Aristotelian notion that planets were striving to fulfill
the goal of perfect motion--circular. His model still use epicycles, though they are now only a minor feature.
-
- Tycho Brahe (1546-1601 C.E.) revives Heroclides' model. His naked eye
observations were the most accurate observations yet.
- All planets revolve around the Sun except the stationary Earth. Sun, Moon,
and stars revolve around the Earth.
- His model is mathematically equivalent to Copernicus' model but does not
violate Scripture and common sense. Brahe was not a neoplatonist.
- If the Earth moved, then stars are at least 700 times farther away from
Saturn than Saturn is from Sun. God would not waste that much space in a
harmonious, elegant universe.
-
- The hierarchical structure of the Church's authority is inextricably bound
with the geocentric cosmology.
- ``Up'' means ascension to greater perfection and greater control.
- God and heaven exist outside the celestial sphere. Gradation of existence
and control from perfect existence to the central imperfect Earth.
- God delegates power to angels to control the planet movements and to guide
the various earthly events.
- Plants and animals serve humans. Humans serve God through ecclesiastical
hierarchy of the Church.
-
- Giordano Bruno (1548-1600 C.E.) revives Democritus' view that Sun is one
of an infinite number of stars. This infinite sphere is consistent with
greatness of God. Heliocentric universe is the reality.God gives each of us an
inner source of power equal to all others, so no justification for domination
and servitude. His model had definite political ramifications that threatened
the church's political authority.
-
- Galileo Galilei (1564-1642 C.E.): 1609 points the telescope to the sky (the telescope was originally used as a naval tool to assess the strength of
the opponent's fleet from a great distance). He finds new things:
- New stars never seen before (makes Bruno's argument more plausible).
- Pits and craters on Moon and spots on Sun. Earth not only place of change
and decay!
- Moons of Jupiter. Mini-model of heliocentric system. The moons are not
moving around the Earth but are centered on Jupiter.
- Venus goes through complete set of phases. The gibbous and full phases of
Venus are impossible in the Ptolemaic model but possible in Copernican model
(and Tychonic model too!). Powerful weapon against the hierarchical structure.
Argues that the heliocentric model is not a mere instrument but is reality. His
experiments show that. His ideas not derived from thought/reason alone
but used the guidance of nature (experiments).
-
- The struggle between Galileo and the Church was not a battle between
science and religion but was part of a larger battle over different conceptions
of the proper routes to knowledge, God, and world view.
-
- Johannes Kepler (1571-1630 C.E.): Religious individualist. He does not go along
with the Roman Catholic Church or the Lutherans. He has an ardent mystical
neoplatonic faith. Uses Tycho's data. All previous models were inaccurate so
others have not yet ``read the mind of God.''
-
- Since an infinite number of models are possible, he has to choose. He was
convinced ahead of time of heliocentrism.
-
- Kepler tries to refine Copernicus' model. After years of failure, he is
finally convinced of an revolutionary idea: God uses a different mathematical
shape than the circle. Goes against 2,000 year old Pythagorean paradigm! Even
Galileo disagrees with Kepler. Planetary orbits are ellipses with the Sun at
one focus.
-
- To account for the planets' motion (particularly Mars') among the stars,
he states that the planets must move around the Sun at a variable speed.
Another break with Pythagorean paradigm! The Sun-planet radius vector sweeps
out equal areas in equal times.
-
- Finally, (one planet's sidereal period/another planet's sidereal period)2
= (one planet's average distance from Sun/another planet's average distance
from Sun)3
OR
(sidereal period in years)2
= (semimajor axis of orbit in A.U.)3
.
This simple mathematical equation covering all observations throughout time
proves to Kepler that the heliocentric system is real. Actually, the first two
laws were sufficient, but the third law is very important for Newton.
-
- Often a mathematical idea/model is discovered with no apparent application
to the physical world until many years later. This aspect of pure, basic
scientific research is not popular among government officials who want answers
NOW!
-
- Creationary resonance between human mind and laws of nature.
Kepler's view: God creates humans with the gift of reading the mathematical
harmonies of God's mind. It is only a matter of time for someone to discover
God's plan.
-
- Evolutionary resonance between human mind and laws of nature. A
modern view held by some that given the infinite experiments of evolution, it
is inevitable that creatures will eventually evolve capable of reading the laws
of nature. Scientific progress is inevitable.
-
- Is creativity actually a logical process in disguise? Most people today
believe one's religious/philosophical beliefs are merely along for the
inevitable revolutionary ride. Others believe that there are many technically
capable paths by which the universe can be modeled. Kepler's Neoplatonism was
not logically necessary for the discovery of the planetary laws of motion, but,
historically, it may have been absolutely necessary.
-
- Self-corrective enterprise of science is a messy one. How do we separate
reasonable ideas from the infinite number of merely conceivable ideas?
-
- Every age has its paradigms. Philosophical considerations intrude on
scientific, creative process. These beliefs are crucial in providing direction to
our inquiries and fuel for our creativity mill. We have a faith that there is some
order in the universe and this faith keeps us striving to solve the cosmic
problems.
-
- Facts have little meaning without ideas to interpret them. Science is a
human discipline. No machine-like objectivity. Often crucial facts supporting
an idea come after a commitment. Is it all individual whim; relative?
-
- Reality kicks back. We can ignore the discrepancies between
nature's truth (observations) and our theories of what should happen only for
so long. Nature eventually wins. Our ideas are crucial to understanding the
world but they eventually yield to the facts. Science makes us confront the
world.
-
- Kepler's elliptical orbits make it impossible to maintain the idea of
planetary motion as a natural one requiring no explanation. What kept them in
their orbits? Spinning Earth: what keeps objects from flying away when thrown
in the air? What keeps us from being hurled off?
-
- The notion of a greater God and infinite universe with no special places
leads to René Descartes (1596-1650 C.E.) idea of inertia: body at rest
remains at rest, and one moving in a straight line maintains constant speed
and same direction unless deflected by a ``force'' (Newton's 1st law of
motion). Note that this is a change in the definition of a force from
that given by Aristotle.
-
- Isaac Newton (1641-1727) defines force: Force = mass * acceleration (Newton's 2nd law).
-
- Newton completes his theory of dynamics with his third law of motion:
every force on a body by some object has an equal and opposite force exerted by that body on the object (in a closed system).
-
- Using Kepler's third law and his 2nd law, Newton finds that the
attractive force between planets and Sun is Force = k * (planet mass) /
(distance)2. Using
his 3rd law of motion, Newton derives his gravity force law: F_grav = G *
(mass Sun) * (planet mass) / (distance)2.
He unifies celestial and terrestrial events (a revolutionary step!) and
completes the process started by Copernicus. His law of gravity also explains
Kepler's 1st and 2nd laws.
-
- But what is gravity? Newton adopts an instrumentalist view: the
scientist's job is to capture observations in precise mathematical equations;
explain the ``how'' not the ``why''. Only things verified by experience of the
world are admissible into science.
-
- With Newton, there is no longer a hierarchical-teleological (designed for
some purpose) man/universe/God view. Now have a perfect machine universe, based
on mathematics, set in motion by God long ago. God is the reference point for
absolute space and time. Newtonian mechanics requires an absolute coordinate
system to keep things sensible (this also gives God something to do).
-
- Reality is completely reduced to material objects. Ideas, thought,
feelings, and values are secondary. Newtonism undercuts the role of God and
religion and the validity of science: science is just a subjective perspective.
Descartes saw the need to rescue thoughts, ideas and values.
Descartes' mind-body dualism: world of thought and spirit exists independent
of, but parallel to, material world. There is a correspondence between the
God-inaugurated, mathematical thoughts of scientists and the motions in the
physical world. Descartes said that mathematical ideas work so well because
there is a pre-established parallelism between the physical world and the
human mind. What is real does not depend on us--this is probably the
actual completion of the Copernican revolution and is soon called ``common
sense'' (how about that for a paradigm shift!).
-
- Gravity is one of four fundamental forces in nature.
- Strong nuclear--short range (only within the atomic nucleus) and
the strongest of the forces. It holds the atomic nucleus together.
- Weak nuclear--weaker and also short range (nucleus range). Responsible for
the decay of nuclei (radioactivity).
- Electromagnetic--electric and magnetic fields are aspects of the same
thing (first shown by James Maxwell--lived 1831-1879 C.E.). Can be attractive or
repulsive, has an infinite range, it only works for things with charge and
is much stronger than gravity.
- Gravity--weakest of all of the forces but is important for astronomy
because:
- a.
- All atoms feel it.
- b.
- It does not depend on the chemical composition of the material.
- c.
- It is always attractive.
- d.
- It only depends on the number of atoms (mass) and the distance--it is
additive (more mass means more gravity) and it has an infinite range.
It is quite remarkable that only four forces can explain all physical
phenomena everywhere in the universe. We can actually determine the inner
workings of far away objects from terrestrial physics! Meteorites, cosmic
rays, solar wind, and moon rocks tell us that the same laws of physics observed
on the Earth are observed elsewhere. The best evidence for this universality
comes from light (electromagnetic radiation) arriving from the far reaches of
space bearing messages of the conditions of those far away places.
-
- Science reduces observations to the fundamental processes and unifies
them. The
goal of science is to unify seemingly disjoint facts into a coherent whole or
framework. Might the four fundamental forces be manifestations of one super
force? James Maxwell showed that electricity and magnetism are simply
manifestations of the electromagnetic force. In the 1970s, the electromagnetic
and weak nuclear forces were found to be manifestations of the ``electroweak
force''--they behave in the same way at very high temperatures/energies. We're
trying to unify the strong nuclear force with the others--the theories are
called Grand Unification Theories (GUT's). The strong and electroweak forces
would behave in the same way at extremely high temperatures/energies. We're
also trying to merge gravity with the rest of the forces (super-GUT's?).
For further reading, here are some of my references that cover the development
of science using historical records and also cover science's philosophical
underpinnings using philosophy tools.
- Owen Barfield Saving the Appearances pp. 46--54.
- Paul K. Feyerabend Galileo and the Tyranny of Truth in
The Galileo Affair: A Meeting of Faith and Science ed. Coyne, Heller,
Zycinski (Vatican City: 1985), pp. 155-166 and other papers from that symposium
held at the Vatican.
- George S. Johnston 1995, The Galileo Affair (Scepter Press:
Princeton, NJ). HTML version version available here.
- Thomas S. Kuhn The Copernican Revolution: Planetary Astronomy in the
Development of Western Thought (Cambridge, Mass: Harvard Univ. Press,
1957).
- Ronald Pine Science and the Human Prospect (Wadsworth Publ. Co:
Belmont, CA, 1989) esp. ch. 5: pp. 130-162.
- A prelude to Copernicus is Owen Gingerich's Scientific American
article Astronomy in the Age of Columbus Nov. 1992, pp. 100-105.
last updated 04 Dec 95
Nick Strobel --
Email:
strobel@astro.washington.edu
(206) 543-1979
University of Washington
Astronomy
Box 351580
Seattle, WA 98195-1580