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History of Astronomy
Astronomy 115, Spring 2013
Prehistoric Astronomy
Introduction
People of antiquity most likely began
studying the heavens many thousands of
years ago.
Early astronomical observations certainly
revealed the obvious:
Rising of the Sun in the eastern sky and its
setting in the west
Changing appearance of the Moon
Eclipses
Planets as a distinct class of objects different
from the stars
Prehistoric Astronomy
Introduction
Many astronomical phenomena are cyclic
on a day-to-day and year-to-year basis and
consequently gave prehistoric people:
Methods for time keeping
Ability to predict and plan future events
Incentive to build monumental structures such
as Stonehenge
Stonehenge (southern England)
built roughly 4500 years ago, used as a burial site, gathering
area and timekeeping
Prehistoric Astronomy
The Celestial Sphere
Vast distances to stars prevents us from
sensing their true 3-D arrangement
Naked eye observations treat all stars at the
same distance, on a giant celestial sphere
with the Earth at its center
Prehistoric Astronomy
Constellations
Constellations are fixed arrangements of
stars that resemble animals, objects, and
mythological figures
Stars in a constellation are not physically
related
Positions of stars change very slowly;
constellations will look the same for
thousands of years
Origin of the ancient constellations is
unknown although they probably served as
mnemonic devices for tracking the seasons
and navigation
Prehistoric Astronomy
• Motion of the Sun and the Stars
– Daily or Diurnal Motion
• Sun, Moon, planets, and stars rise in the east
and set in the west
• Daily motion can be explained by the rotation of
the celestial sphere about the north and south
celestial poles located directly above the
Earth’s north and south poles
• The celestial poles can act as navigation aides
and astronomical reference points
• The celestial equator, which lies directly
above the Earth’s equator, provides another
astronomical reference marker
Chichen Itza (Yucatan Peninsula, Mexico)
built between 1000 to 1500 years ago; used for rituals, timekeeping
Chaco Canyon (New Mexico, USA)
built between 800 and 1250 years ago; trade, timekeeping
Introduction
Western astronomy divides into 4 periods
Prehistoric (before 500 B.C.E.)
Cyclical motions of Sun, Moon and stars observed
Keeping time and determining directions develops
Classical (500 B.C.E. to C.E. 1400)
Measurements of the heavens
Geometry and models to explain motions
Renaissance (1400 to 1650)
Accumulation of data lead to better models
Technology (the telescope) enters picture
Modern (1650 to present)
Physical laws and mathematical techniques
Technological advances accelerate
Early Ideas of the Heavens
Ancient Greek Astronomers
Through the use of models and observations,
they were the first to use a careful and
systematic manner to explain the workings of
the heavens
Limited to naked-eye observations, their idea
of using logic and mathematics as tools for
investigating nature is still with us today
Their investigative methodology is in many
ways as important as the discoveries
themselves
Early Ideas of the Heavens
• The Shape of the Earth
– Pythagoras taught as early as 500 B.C. that
the Earth was round, based on the belief that
the sphere is the perfect shape used by the
gods
– By 300 B.C., Aristotle presented naked-eye
observations for the Earth’s spherical shape:
• Shape of Earth’s shadow on the Moon during an
eclipse
• A traveler moving south will see stars previously
hidden by the southern horizon
Early Ideas of the Heavens
• The Size of the Earth
– Eratosthenes (276-195 B.C.) made the first
measurement of the Earth’s size
– He obtained a value of 25,000 miles for the
circumference, a value very close to today’s value
– His method entailed measuring the shadow length of
a stick set vertically in the ground in the town of
Alexandria on the summer solstice at noon,
converting the shadow length to an angle of solar
light incidence, and using the distance to Syene, a
town where no shadow is cast at noon on the
summer solstice
Measuring the Earth’s circumference
Early Ideas of the Heavens
• Distance and Size of the Sun and Moon
– The sizes and distances of the Sun and Moon
relative to Earth were determined by Aristarchus
about 75 years before Eratosthenes measured the
Earth’s size
– These relative sizes were based on the angular
size of objects and a simple geometry formula
relating the object’s diameter, its angular size, and
its distance
– Aristarchus realizing the Sun was very large
proposed the Sun as center of the Solar System,
but the lack of parallax argued against such a
model
– Once the actual size of the Earth was determined,
the absolute sizes and distances of the Sun and
Moon could be determined
Aristarchus
• About 310 to 230 BCE
• We have no writings that
have survived but
Aristarchus is referred to in
the writings of other
philosophers
• Most noted for proposing
the idea of a heliocentric
universe with the earth as
one of the planets moving
around the Sun.
Aristarchus measures the Earth-Sun
and Earth-Moon distance
• Aristarchus is the first to attempt to measure the
relative distance between the Earth-Moon and the
Earth-Sun without the aid of trigonometry.
• Actual angle = 89° 50” not 87°
Hipparchus of Nicaea
• 190 to 120 BCE
• Based on measurements during an eclipse, he is
able to place a range on the distance to the moon.
• He estimated it to be between 59 and 67 Earth
diameters. The actual number is 60.
• Writes the first catalog of stars, in which he
describes the brightest stars as “stars of the first
magnitude”, the next-brightest stars as “second
magnitude, and so on, a scale we still use in
modified form today.
Early Ideas of the Heavens
The Motion of the Planets
Because of the general east to west motion
of objects in the sky, geocentric theories
were developed to explain the motions
Eudoxus (400-347 B.C.) proposed a
geocentric model in which each celestial
object was mounted on its own revolving
transparent sphere with its own separate tilt
The faster an object moved in the sky, the
smaller was its corresponding sphere
This simple geocentric model could not
explain retrograde motion without appealing
to clumsy and unappealing contrivances
Aristotle (Athens)
– 384 to 322 BCE
– Founded his own school called
the Lyceum in Athens
– Made contributions to all areas of
philosophy but math was his
weakness
– He did not believe that empirical
evidence was necessary to prove
ideas.
Refines the “Geocentric Model” which places the
“imperfect” Earth at the center and all of the
“perfect” celestial objects go around us in perfect
circular motion.
The geocentric universe
Shell of “fixed” stars
Ptolemy of Alexandria
• Ptolemy is the most
influential astronomer in
his day and his models
of the universe will
prevail for the next 1400
years
• He is a great admirer of
Hipparchus and his
rigorous method of
verifying ideas with
empirical data.
Early Ideas of the Heavens
• Ptolemy (about C.E. 150)
– Ptolemy of Alexandria improved the geocentric
model by assuming each planet moved on a small
circle, which in turn had its center move on a
much larger circle centered on the Earth
– The small circles were called epicycles and were
incorporated so as to explain retrograde motion
– Ptolemy’s model was able to predict planetary
motion with fair precision
– Discrepancies remained and this led to the
development of very complex Ptolemaic models
up until about the 1500s
Geocentric or Heliocentric?
• Of the early philosophers, only Hipparchus
favored the heliocentric model
• Most philosophers thought that the evidence
supported the geocentric model more than the
heliocentric model
• What was the evidence?
Geocentric Evidence
• Everything appears to revolve around the
Earth each day (diurnal motion).
• There is no observable parallax of the
stars, planets, Moon, or Sun.
• The motions of the stars and heavens are
perfect circles.
• The heavens were unchanging but the
Earth was not.
Problems With the Geocentric Model
• The planets appear to change brightness,
implying a change of distance.
• The planets undergo retrograde motion (they
move backwards compared to the direction the
Sun moves).
• The Sun, Moon, and planets do not move at the
same speed all the time.
• Mercury and Venus are never seen at opposition
(they always appear close to the Sun, never in
the opposite part of the sky from the Sun).
Evidence of the Heliocentric Model
• The changing brightness of the planets is
explained by the Earth getting closer and farther
from the planets during our orbit of the Sun.
• Our passing planets explain the retrograde motion
of the outer (“superior”) planets.
• Mercury and Venus are not seen at opposition
because they orbit the sun, just as we do, but
closer to the Sun than us (“inferior” planets).
Problems with the Heliocentric Model
• The changing speeds of the Sun, Moon, and
planets is not explained by simply placing the
Sun at the center of the universe.
• We do not feel the Earth moving or the
atmosphere being pulled away as we fly
around the Sun.
• If the Earth were spinning, wouldn’t we be
thrown off into space?
• Why is there no parallax due to our spin or our
orbit?
So which is correct?
• The ancients favored the geocentric model
because it seemed impossible to believe
that the Earth was moving.
• During those times, the mathematical and
scientific tools were not available to
answer the question without dispute.