Download Early Astronomies

Early Astronomies
Astronomy 1-1
Lecture 03-1
Ancient Astronomies
Astronomies were often necessary for
determining the agricultural seasons
The flooding of the Nile, closely related to the
rising of the star Sirius
Stonehenge related to astronomical occurrences
such as the summer and winter solstices
Mayans had a calendar based on Venus
North American Indians had an astronomical
device based on the summer and winter
solstices
Astronomy 1-1
Lecture 03-2
Cast of Characters
Thales (c624-547BC)
Rational inquiry.
Pythagoras (c570.-500BC)
Round Earth.
Philolaus (c500-400BC)
Celestial Harmonies.
Anaxagoras (c500-428BC)
Moon reflects sunlight;
Eclipses.
Plato (c428-347BC)
Reason not observation.
Eudoxus (c408-356BC)
Mathematical cosmology.
Aristarchus (c310-230BC)
Size of sun, moon;
Heliocentric theory.
Eratosthenes (c273-?BC)
Size of earth.
Apollonius (c265-19OBC)
Epicycle.
Hipparchus (c200-100BC)
Epicyclic theory of sun and
planets.
Ptolemy (c1OO-200AD)
Elaborate epicycle theory;
Almagest.
Aristotle (c384-322BC)
Physical Laws; Round earth.
Astronomy 1-1
Lecture 03-3
Early Astronomies
Babylonia ~3000BC
Divided sky into 12 parts
Numbering system based on 60
Circle divided into 360 degrees
Chaldeans could accurately predict
Solar, lunar, and planetary motions
Lunar and solar eclipses
Greece >1600BC
Dome of Heavens.
Earth floating dish.
Astronomy 1-1
Lecture 03-4
Early Astronomies
Thales
Rational inquiry leads to understanding of universe
beyond simple description.
Pythagoras
First to suggest Earth was round and that all heavenly
bodies moved in circles.
Philolaus
Planetary distances correspond to lengths of vibrating
strings producing harmonious sounds.
Anaxagoras
Moon shines by reflected light. Lead him to deduce
eclipses caused by passage of one between the other
two. Views considered sacrilegious. Exiled
Astronomy 1-1
Lecture 03-5
Early Astronomies
Plato
What we see of material world is only imperfect
representation of ideal creation. Can learn more
about universe by reason than by observation, since
observations can only give incomplete picture.
Idealized assumptions.
Eudoxus
Put Plato's ideas on mathematical footing. Concentric
spheres on which Sun, Moon, and planets moved.
Astronomy 1-1
Lecture 03-6
Early Astronomies
Aristotle
Adopted physical laws and showed in terms of these why the
universe functioned the way it did.
Circular motion - the only natural motion.
Earth not spinning since no great wind - so Earth is stationary.
All circular motions centered about the Earth.
Four basic elements: earth, air, fire, and water.
Showed Earth and universe spherical. Three ways to show Earth
spherical:
1) Only on sphere do things fall inwards, everywhere.
2) View of stars change as go north to south.
3) Shadow of Earth on Moon during eclipse is curved.
Broke with Plato, in that he used data.
Astronomy 1-1
Lecture 03-7
Early Astronomies
Astronomy now moved from Greece to Alexandria in
Egypt.
Aristarchus
Showed Sun was many times further than Moon and that Moon
was smaller than Earth and that Sun was larger than Earth.
Developed first heliocentric (Sun centered) theory. Up until this,
all theories had been geocentric (Earth centered).
Suggested that Earth rotated about its own axis.
Astronomy 1-1
Lecture 03-8
Stellar Parallax
Evidence for heliocentric theory
could only come from stellar
parallax
What do we mean by parallax?
Astronomy 1-1
Lecture 03-9
Parallax
Parallax is an apparent
displacement or difference of
orientation of an object viewed
along two different lines of sight
Nearby objects have a larger
parallax that more distant objects
when observed from different
positions
Our brain uses parallax for gain
depth perception
Astronomy 1-1
Lecture 03-10
Stellar Parallax
Stellar parallax is very small, much too small for
instruments of the day
So heliocentric theory was forgotten
Astronomy 1-1
Lecture 03-11
Early Astronomies
Eratosthenes
Determined size of the Earth
39,690 km, an error of less than 1%
Astronomy 1-1
Lecture 03-12
Early Astronomies
Appollonius
Improved models of
concentric spheres with
introduction of epicycle
and deferent
With adjustment of sizes
of both, better match to
data, especially the
retrograde motion of some
of the planets
Astronomy 1-1
Lecture 03-13
Early Astronomies
Hipparchus
Refined work on epicycles
Used trigonometry
Refined instruments for measuring
Stellar magnitude system
Precession of star positions, by comparing his data
with that from 160 years previous
His refined data lead to an offset for the position of the
Earth with respect to the center of the deferent
Astronomy 1-1
Lecture 03-14
Early Astronomies
No further significant work until 2nd century AD
Ptolemy
Almagest - a thirteen volume set of books
Based partly on the works of Hipparchus
Detailed studies of the sun, star charts, and astronomical
instruments
Detailed model of planetary motions, accurate to be used for the
next thousand years
Extended the idea of deferents and epicycles. Assumed Earth not
at the center of the deferent
Unclear as to whether he viewed this system as a mathematical
tool or reality
His model however was taken to be the literal truth for the next
fourteen centuries.
Astronomy 1-1
Lecture 03-15
Early Astronomies
Alexandria remained intellectual center until its fall in
640AD. when library buildings were burned
Europe fell into The DARK AGES.
Other cultures, mainly Arabic, contributed to mainstream
astronomy as they also needed accurate calendars
Refined the works of Ptolemy and passed it back to Europe
through Spain
By thirteenth century serious discrepancies existed
between observations and predictions
New epicycles had to be added to obtain agreement again
MUCH DISPLEASURE with complexity of model
Astronomy 1-1
Lecture 03-16
Complexity
Astronomy 1-1
Lecture 03-17