Download Lecture 1: The Universe: a Historical Perspective

Lecture 1: The Universe:
a Historical Perspective
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Babylonian astronomy
Egyptian astronomy
Chinese astronomy
Arabic astronomy
Mayan astronomy
Greek astronomy
the Copernican Revolution
Olber's Paradox
the Problem of Longitude
the Curtis–Shapley debate
Edwin Hubble
the celestial sphere
Observables for early astronomers
sun, moon, planets move
● lunar phases
● constellations
● stars vary in color
● stars slowly move across sky
● stars do not appear to move
relative to each other
● comets
● eclipses
● new stars (“novae”)
● the night sky is dark
● sun-spots
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Babylonian astronomy
star catalogs and planetary motions
recorded from as early as 1800 B.C.E.
● continuous, careful records from 747 B.C.E.
● 360 degrees in circle from 450 B.C.E.
● surviving records show impressive
calculations of celestial motions, but little
discussion of 'why'; Greeks good at asking
'why', but poor measurements until late
● goal was good (solar) calendar
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Sun's track
er
k
a
we
o
zer
strong
force of Sun
on Moon
Moon's track
full
Moon
half
Moon
new
Moon
Earth
Berossos' theory of the Moon's phases (290 B.C.E.)
Egyptian astronomy
poor astronomers
● 'Catalogue of the Universe' compiled by
Amenliope about 1100 B.C.E. lists five
constellations and omits the planets
● obviously some astronomical knowledge
based on pyramid alignments, but overall
not a very impressive record
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Chinese astronomy
Imperial astronomers, assigned “to deduce from the
twelve winds the state of harmony of heaven and earth”
● first human record of a solar eclipse: 2136 B.C.E.
● recognized moon reflects the sun's light c. 400 B.C.E.
● By 20 B.C.E., Chinese knew how eclipses caused;
predicting eclipses by 8 B.C.E.
● sun-spots recorded as early as 29 B.C.E.
● 'celestial sphere' theory replaced in Later Han dynasty
with Hsuan Yeh teaching of 'infinite, empty space'
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Jesuits replace with Western astronomy in 16th century
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sun
stars
Thai Yang (the Greater Yang)
Hsaio Yang (the Lesser Yang)
moon
planets
Thai Yin (the Greater Yin)
Hsaio Yin (the Lesser Yin)
Earth
measuring distance of the Sun
(at noon on the summer solstice)
Arabic astronomy
well-developed mathematics, and a long history of
records
● better measurements than either Greeks or Chinese
for (1) Earth's obliquity relative to the ecliptic and
(2) length of the year
● key importance for preserving Greek thought
● many star names are Arabic (e.g. Deneb, Algol,
Betelgeuse, Aldeberan, ...)
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Mayan astronomy
300 – 900 C.E.
● Aztecs thought Earth was the back of huge alligator;
Mayans may have thought the same
● accurate calendars, with 365 day year, as well as the
'long count', starting with Day 0 = August 13th
(Gregorian), 3114 B.C.E.
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Greek astronomy
Greek city-states vs. Babylonian/Chinese empire
● preferred theorizing to accurate predictions or exact
explanations – addressed “why”
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Pythagoreans (6th century B.C.E.) try to use math to
describe Nature; “the Music of the Spheres”
● early (c. 400 B.C.E.) recognition
of moon reflecting solar light
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measurement of the diameter of
the Earth by Eratosthenes, the
librarian in Alexandria
(3rd century B.C.E.)
Alexandria
Syene
Earth
parallel rays
of light from
the Sun
the Aristotelian Universe
voluminous works of Aristotle (384 – 322 B.C.E.) became
basis for medieval & Renaissance thought
● synthesized & expanded Greek thought on logic,
astronomy, physics, math, biology, medicine, rhetoric...
● Earth immobile, at center of Universe
● concentric spherical shells sitting in
the aether
● heavens are immutable
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Problems with the Aristotelian Universe
annular vs. total solar eclipses
● eclipses last different lengths of time
● Herakleides: Mercury and Venus sometimes behind Sun,
sometimes in front of Sun
● Mercury and Venus orbit Sun?
● motion of celestial sphere due to
rotation of the Earth?
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Appolonios & Ptolemy expand theory
of epicycles to all planets
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Aristarchus of Samos (2nd century
B.C.E.) proposes heliocentric theory
● not taken seriously; lack of parallax
implies immense distances
● dynamical issues for rapidly moving
Earth
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Total Solar Eclipse:
La Lava, Bolivia
Partial Solar Eclipse:
Toledo, Ohio
Moon
Earth
Sun
measurement of distance and size of Sun and Moon by
Aristarchus of Samos (2nd century B.C.E.):
at half moon, angle EMS = 90 degrees
measure angle MES sets the geometry
AoS measured MES = 87 degrees
(correct value is 89 degrees 51 minutes)
implies Sun 19x farther away than Moon
since Sun and Moon have same apparent size on
the sky, Sun is 19x larger than the Moon
lengths of lunar eclipses sets scale (relative to size
of Earth)
the Ptolemaic Universe
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Claudius Ptolemy (85 – 165 C.E.)
wrote the Almagest, a masterpiece of astronomy
describes the mathematics of epicycles
epicycles
the Copernican Revolution
Nicolas Koppernigk (1473 - 1543)
● De Revolutionibus Orbium Caelestium published 1543
● replaces Ptolemaic geocentric universe with a
heliocentric universe
● still based on epicycles
● last of the ancient astronomers or first of the modern
astronomers?
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the Copernican Revolution
Nicolas Koppernigk (1473 - 1543)
● De Revolutionibus Orbium Caelestium published 1543
● replaces Ptolemaic geocentric universe with a
heliocentric universe
● still based on epicycles
● last of the ancient astronomers or first of the modern
astronomers?
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recommened texts:
The Copernican Revolution: Planetary
Astronomy in the Development of Western
Thought, by Thomas Kuhn (1957)
The Sleepwalkers, by Arthur Koestler
Tycho Brahe & Johannes Kepler
Tycho Brahe (1546 - 1601)
● Johannes Kepler (1570 - 1629)
● Brahe: the most accomplished
observer prior to telescopes
● Kepler: used Brahe's
observations to realize
elliptical motions of the
planets (Kepler's Laws of
Planetary Motion)
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Kepler's First Law:
the orbit of a planet/comet about the Sun is
an ellipse with the Sun's center of mass at one
focus
Kepler's Second Law:
a line joining a planet/comet and the Sun
sweeps out equal areas in equal intervals of
time
Kepler's Third Law:
the squares of the periods of the planets are
proportional to the cubes of their semimajor
axes
Ta²
ra³
Tb²
r b³
Tycho Brahe & Johannes Kepler
Tycho Brahe (1546 - 1601)
● Johannes Kepler (1570 - 1629)
● Brahe: the most accomplished
observer prior to telescopes
● Kepler: used Brahe's
observations to realize
elliptical motions of the
planets (Kepler's Laws of
Planetary Motion)
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Galileo Galilei
Galileo (1564 – 1642); died year Newton was born
● first astronomical user of the telescope (1609); read
about the Dutch invention (1608) and made his own
● published The Starry Messenger (1610)
● lunar surface full of irregularities
● Milky Way composed of faint stars
● four moons around Jupiter
● phases of Venus
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Isaac Newton
Newton (1642 – 1727)
● created physics to describe
Kepler's Laws
● created calculus
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Olber's Paradox
First noted by Kepler (1610; Conversations with the
Starry Messenger):
“If this is true, and if there are suns having the same
nature as our Sun, why do not these suns
collectively outdistance our Sun in brilliance?”
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if the Universe were:
(1) infinitely large,
(2) infinitely old,
(3) filled isotropically with stars,
then the night sky would not be dark
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The Problem of Longitude
essential problem for navigation: latitude is easily
measured from height of noon Sun, but longitude is
tricky, making transoceanic travel dangerous
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July 8th , 1714: Queen Anne posts £20,000 reward for
“a practicable method of calculating longitude at sea”
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astronomical solutions expected:
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lunar distance method: measure separation of
moon and stars and compare to almanacs created
by the Royal Greenwich Observatory
moons around Jupiter
The Problem of Longitude
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magnetic solutions: finding true north
“powders of sympathy”
The Problem of Longitude
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John Harrison (1693-1776) built the first accurate
maritime clock
Harrison 1
Harrison 3
the Great Debate
Curtis-Shapley debate (1920)
● Harlow Shapley (1885-1972) used 60” telescope at
Mount Wilson to study globular clusters; used
Cepheid variable stars to get distances
● replaces heliocentric Universe with Universe centered
on distribution of globular clusters (Karl Bohlin, 1909)
● ignoring extinction, got center of “galaxy” at ~20 kpc
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Heber D. Curtis (1872-1942), of Lick Observatory,
believed that spiral nebulae constituted “island
universes” (but thought our Galaxy was 10x smaller)
Edwin Hubble
Edwin Hubble (1889-1953)
● ended the Great Debate at Winter meeting of the
American Astronomical Society meeting (Jan. 1925)
● using 60” and 100” telescopes at Mount Wilson
Observatory, measured Cepheids in spiral nebulae
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THE END
... or, rather, ...
THE
BEGINNING