Astronomy 1021 Chapter Notes - Chapter 3: Nicolaus Copernicus, Machu Picchu, Celestial Navigation

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16 Aug 2016
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Chapter 3: The Science of Astronomy
Astronomy is an OBSERVATIONAL science: we cannot perform experiments on stars, we can
only study what we see by recognizing patterns and understand those observations
ANCIENT ASTRONOMY
Ancient cultures were curious about patterns in the world around them, recorded what they
observed
Thousands of years B.C., Central Africa: predicted seasons by watching the crescent moon,
position of ecliptic in the sky changes with seasons
ASTRONOMY & SOCIETY
Civilizations that can keep track of time and seasons had an advantage
Purposes reflected in many archaeological remains of different civilizations: structures or
buildings that are built with special orientation with respect to the sun, the equinoxes, etc
ANCIENT ASTRONOMY
Timekeeping
Our 24 hours clock comes from the Egyptians, who divided the night in 12 intervals
according to the rising of bright stars
Even the names of the days of the week come from ancient astronomy
Stonehenge
Templo Mayor
Scotland: 4,000-year-old stone circle; Moon rises as shown here every 18.6 years
Peru: Lines and patterns, some aligned with stars
Machu Picchu, Peru: Structures aligned with solstices
South Pacific: Polynesians were very skilled in art of celestial navigation
France: Cave paintings from 18,000 B.C. my suggest knowledge of lunar phases (29 dots)
Babylonian Clay tablet
Anasazi rock painting
1054 CE: powerful supernova recorded by the Chinese, visible during the day for 23 days
OBSERVATIONS & SCIENCE
Important element in modern science: carefully recorded, repeatable observations
ANCIENT GREEKS
Greeks were the first people known to make models of nature
Tried to explain patterns in nature without resorting to myth or the supernatural
Tried to explain motions of astronomical objects using logical reasoning and observations
Sought to explain properties of matter and motion of celestial objects
Scientific models are conceptual representations to explain and predict phenomena
EARLY COSMOLOGY
Flat earth was a common assumption among most early cultures, earth separated from the sky
by air
First model of Earth: flat
Observations seem to suggest that Earth (land and ocean) is basically flat
Sky is a dome (blue during day, dark and starry at night)
Greeks were the first to introduce the idea of a spherical earth (Aristotle)
Eratosthenes measured Earth's diameter to within 2% with no modern tools
Thales (624 – 546 B.C.): first known model of Universe (Earth is flat) that did not rely on
supernatural forces.
Anaximander (610 – 546 B.C.): introduced concept of celestial sphere
Pythagoras (560 – 480 B.C.): taught that the Earth is a sphere.
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Plato (428—348 B.C.): all heavenly objects move in perfect circles at constant speeds, and
reside on huge spheres encircling Earth.
Pythagoreans had mystical interest in mathematical perfection, and sphere was considered
geometrically perfect
ERATOSTHENES
Heard report that at noon on longest day of the year, no shadows in Syene, Egypt indicating that
Sun was directly overhead
He realized it did not occur in Alexandria where he lived
Postulated that the earth must be curved
Measured angle of shadow to estimate distance
So the distance between Syene and Alexandria is 7/360 of the Earth circumference.
Measuring distance between the two cities: based on estimated average speed of a caravan of
camels!
Ended up with distance of about 42,000 km
ARISTARCHUS
Used primarily observations of the moon to estimate distance to the Moon and Sun and sizes for
both
Also put planets in correct order of distance around the sun
Used lunar eclipses to determine sizes and distances
From ratio of Earth-Moon and Earth-Sun distances, we can determine distance to Moon in Earth
radii
Found that Sun is much larger than the earth
HIPPARCHUS
Star catalog & brightness system (magnitudes)
Measured parallax of Sun and Moon and determined accurate Earth-Moon distance
Discovered precessions of equinoxes
ARISTOTLE
Persuasive arguments for geocentric view dominated Western thought for almost 2000 years
Geo-centric= Earth-centered view
All stars, etc are fixed, permanent and do not change
Only sun, earth, moon and planets change appearance
PTOLEMY
Quantitative treatment of cosmos models
Culminated in work by Ptolemy: Almagest
Based on Aristotle's view and perfect circles, but offered explanation for retrograde motions of
planets: Each planet turns around Earth on small circle (EPICYCLE) that turns upon larger circle
(DEFERENT)
Problems with Ptolemy's model
Model couldn't fit observations
Epicycles upon epicycles were required to match observed motions precisely
More than 100 epicycles were eventually added
Errors in predicted positions of planet accumulated to several degrees by 1400 A.D
Preserving ideas of the Greeks
Muslim world preserved and enhanced knowledge they received from Greeks while Europe was
in its Dark Ages
After fall of Constantinople (Istanbul), scholars headed west to Europe, carrying knowledge that
helped ignite European Renaissance
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