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A Brief History of
Astronomical Thought
From
Aristotle to Hawkings
From
Stonehenge to the
Hubble Space
Telescope
Ancient Astronomy
Religion and Myth
Astronomy is the oldest of the sciences. When Stoneage
humans turned to an agrarian way of life and began to
settle into communities, their interest must naturally have
turned to the "heavens":
1.The seasons became important; during different times of the year,
different stellar patterns appear in the sky. In the spring, Virgo and
her accompanying constellations signal the time to prepare the
earth, to plant crops, and to be wary of floods. In the fall, Orion
rises to indicate time to harvest and to prepare for winter.
2.The approximate equivalence of the human menstrual cycle and
the 30 day orbital period of the Moon which produces lunar phases
led to the belief that the heavens, and the Moon in particular, were
related to fertility.
3.To early humans facing an uncertain and changeable future, the
constancy of the heavens must have suggested perfection and
certainly led to deification in many cultures.
4.We may expect that eclipses would have been especially
frightening to early humans. After predicting the seasons, eclipse
prediction may have been one of the earliest astronomical activities.
Ancient Astronomy can be characterized as:
More Religious than Scientific
More Utilitarian than Explanatory or Theoretical
Stonehenge, constructed between 3100-2000 BCE on England's
Salisbury Plain, may be one of the earliest monuments to the
beginnings of Astronomy.
Classical Greek Astronomy
Plato to Ptolemy
The Universe is a rational place following
universal, natural laws.
Plato 427- 347 B.C.E.
Plato taught that there are absolute
truths and mathematics is the key.
Plato had Four Basic Beliefs:
1.There is certainty.
2.Mathematics gives us the power of
perception.
3.Though the physical applications
of mathematics may change, the
thoughts themselves are eternal and
are in another realm of existence.
4.Mathematics is thought and,
therefore, it is eternal and can be
known by anyone.
Western scientific history begins with the ancient Greek
civilization about 600 BCE.
Pythagoras of Samos (~580 - 500 B.C.E.)
Pythagoras’ developments in astronomy built upon those of
Anaximander from whom, apparently, came the idea of perfect
circular motion. The Pythagoreans believed that the planets were
attached to crystalline spheres, one for each planet, and a separate
sphere for the stars. These spheres were centered on the Earth,
which was itself in motion. Pythagoras is also credited with
recognizing that the "morning star" and "evening star" are both
the planet Venus.
Aristotle (384 - 322 B.C.E.)
Aristotle was a student of Plato,
founding his own school of Natural
Philosophy, the Lyceum, in Athens
about 335 BCE. Aristotle's
philosophy involved the qualitative
study of all natural phenomena,
pursued without the aid of
mathematics which was deemed to be
too "perfect" for application on an
imperfect terrestrial sphere.
In Aristotelian cosmology, the "imperfect" Earth was situated at the
center of the Universe (Solar System). It was composed of the four
elements: earth, air, water, and fire, each of which sought its natural
place in the Universe.
Aristotle adopted Pythagoras' model of concentric spheres for the
planets, but deduced that the Earth must not only be the center of
the Universe but must be immobile.
Aristotle's Natural Philosophy was embodied in the writings of St.
Thomas Aquinas and became the foundation of Church doctrine
and University instruction in medieval times.
Why did Aristotle believe the Earth was stationary?
At the time of Aristotle it was believed that “man” was a special
creation and therefore man’s home, the Earth, must be at the center
of the Universe.
Since the Earth was already in its proper location there was need
for the Earth to move.
Aristotle also could make observations that, to him, proved the
Earth did not move.
Did the Earth spin causing the surface of the Earth to move?
Did the Earth orbit around the Sun?
Parallax
Suppose we want to measure the distance across a river.
d=?
Parallax
Angle
If the distance from “A” to “B” is known and the parallax angle can
be estimated the distance “d” can be determined.
This is how with two eyes we can judge the distance to an object.
Another aspect of parallax is the apparent shift in position of an
object due to change in the observing position.
Background
Parallax
Angle
Object appears in
front of blue
background
Object in Foreground
Object appears in
front of green
background
Aristotle perceived that if the Earth traveled
around the Sun the observed position of a
star should shift when viewed at different
times of the year…stellar parallax.
Background Stars
*
Nearby Star
When Aristotle attempted to observe the stellar parallax he could
see none and so concluded that the Earth did not travel around the
6 months later
Sun.
Geocentric (Aristotelian) Model
1.The Earth is still; motionless.
2.Earth is at center of Universe.
3.Celestial bodies move in perfect circles at uniform speeds.
4.Stars were set in a rotating sphere that turned E to W once a day.
5.Planets, moon, sun also set in separate spheres that moved slower.
Aristarchus of Samos (310 BC - 230 B.C.E.)
Aristarchus was both a mathematician and
astronomer and he is most celebrated as the
first to propose a sun-centered universe. He
is also famed for his pioneering attempt to
determine the sizes and distances of the sun
and moon.
From his measurements and calculations
Aristarchus found that the Sun was many
times larger than the Earth.
It seemed more reasonable that a tiny Earth would orbit around
a large Sun rather than a large Sun orbit around a tiny Earth.
His sun-centered model failed to change the minds of the
Aristotelians. The earth-centered model persisted.
Hipparchus of Rhodes (190 - 120 B.C.E.)
Hipparchus compiled a star catalogue
containing about 850 stars. His
star catalogue, probably completed in
129 BC, has been claimed to have been
used by Ptolemy as the basis of his
own star catalogue.
The Hipparchus star catalog indicated
the brightness of stars and planets and
observed that the brightness of planets
vary from time to time.
Claudius Ptolemy (85 - 165 C.E.)
Ptolemy combined the best features of
the geocentric models that used
epicycles with the most accurate
observations of the planet positions to
create a model that would last for
nearly 1500 years.
These refinements were incompatible
with Aristotle's model and the
Pythagorean paradigm---a planet on an
epicycle would crash into its crystalline
sphere and the motion is not truly
centered on the Earth. Ptolemy’s model
was seen as only an accurate calculator
to predict the planet motions but the
reality is Aristotle's model.
The Ptolemaic system consisted of multiple circular paths to
explain such things as a planet’s varying brightness and retrograde
motion.
Ptolemy published his work in a large 13-volume series called the
“Almagest”, and his model and the geocentric view of the cosmos
was generally accepted for the next 1500 years.
Medieval Astronomy
Fall of the Roman Empire to the Renaissance
During the Middle Ages the Islamic civilization had flourished in
the Arabic countries. They had preserved and translated the Greek
writings and adopted the Greek ideals of logic and rational inquiry.
Islamic astronomers were careful observers of the sky and created
accurate star catalogs and tables of planet motions.
Aristotle's Natural Philosophy was embodied in the writings of
St. Thomas Aquinas (1224-1274 AD) and became the foundation
of Church doctrine and University instruction in medieval times.
Advances in the explanations of the motions of the stars and
planets were made by astronomers in Europe starting in the 16th
century.