Lecture 1: The Universe: a Historical Perspective

● 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

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

Sun's track

Moon's track

force of Sunon Moon

zero

strongweaker

Earth

Berossos' theory of the Moon's phases (290 B.C.E.)

newMoon

halfMoon

fullMoon

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

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'● Jesuits replace with Western astronomy in 16th century

sun Thai Yang (the Greater Yang)stars Hsaio Yang (the Lesser Yang)

moon Thai Yin (the Greater Yin)planets 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, ...)

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.

Greek astronomy

● Greek city-states vs. Babylonian/Chinese empire● preferred theorizing to accurate predictions or exact explanations – addressed “why”● 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

Earth

parallel raysof light from the Sun

measurement of the diameter of the Earth by Eratosthenes, the librarian in Alexandria (3rd century B.C.E.)

Syene

Alexandria

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

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?

● Appolonios & Ptolemy expand theory of epicycles to all planets● 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

Total Solar Eclipse:

La Lava, Bolivia

Partial Solar Eclipse:

Toledo, Ohio

Earth

SunMoon

measurement of distance and size of Sun and Moon byAristarchus of Samos (2nd century B.C.E.):

at half moon, angle EMS = 90 degreesmeasure angle MES sets the geometry AoS measured MES = 87 degrees (correct value is 89 degrees 51 minutes)implies Sun 19x farther away than Moonsince Sun and Moon have same apparent size on the sky, Sun is 19x larger than the Moonlengths of lunar eclipses sets scale (relative to size of Earth)

the Ptolemaic Universe

● 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?

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?

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)

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

rb³Tb²

ra³

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)

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

Isaac Newton

● Newton (1642 – 1727)● created physics to describe Kepler's Laws● created calculus

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?”

● if the Universe were:(1) infinitely large, (2) infinitely old, (3) filled isotropically with stars,

then the night sky would not be dark

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● July 8th, 1714: Queen Anne posts £20,000 reward for “a practicable method of calculating longitude at sea”● astronomical solutions expected:

● 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

● magnetic solutions: finding true north● “powders of sympathy”

The Problem of Longitude

● 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

● 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

THE END

... or, rather, ...

THE BEGINNING