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History of Astronomy
• How did we get to where we are today?
– We stand on the shoulders of giants.
• Greek, Egyptian, Asian, European, Islamic, Indian scientists
all observed the heavens and tried to explain what they saw
• What did the ancients know?
– And how did they know it?
• 4 distinct periods of astronomical advances
Lecture Outline
• 4 periods of astronomy
– Prehistoric: motions of sun, moon, and stars
used to benefit mankind.
– Classical: astronomers use simple geometry
to try to explain what they see.
– Renaissance: the boom of observational
astronomy.
– Modern: search for underlying physical laws
that describe the heavens.
Prehistoric Astronomy
• Before 500 B.C.
• Cyclic motion of sun, moon, and stars
– Time-keeping
– Direction determination
– Fortelling the future?
• Prehistoric astronomers identified
recurring patterns in the night sky
Great Pyramid of Giza
To Orion
To Thuban
Astronomical alignments built in to the pyramid
Prehistoric Astronomy
• Celestial Sphere
– Easy way to visualize the observable universe
– Stars embedded in spherical shell that rotates
around the Earth
• Describes movement of stars, but totally unrealistic
• No sense of distance from earth
• North Star (Polaris) is the top of the C.S.
– All stars appear to rotate around Polaris over the course
of a night.
Prehistoric Astronomy
• Constellations
– Star-patterns on the C.S.
– Used for navigation
• North star
• Southern cross
– keeping track of seasons
• winter, summer, planting season, harvesting
season
Prehistoric Astronomy
• Motion of Sun & Stars
– Daily/Diurnal: like Sun, stars rise in east, set
in west
• Ancients believed this due to rotation of C.S.
• from Earth rotating underneath C.S.
• To them, the Earth does not appear to move, so
easier to believe Earth is stationary
– Annual: new patterns emerge and old ones
dissapear
• Earth’s orbit around sun changes visible
constellations
– Earth itself blocks ½ of C.S. from view
Prehistoric Astronomy
• Motion of the Planets
– Ancients observed bright “stars” that seemed
to move through the sky faster than others
• These are the planets of the solar system
• Observed to move through same narrow band of
sky  the “Zodiac” (“little animal”)
– Because planets lie in roughly same orbital plane (but
ancients did not know this)
17° swath
Prehistoric Astronomy
• Sometimes, retrograde motion of these
Zodiac “stars” was observed!
– What is going on?
Prehistoric Astronomy
• Observations of the planets led to the
downfall of many early ideas about the
nature of the universe and sky
– Thought to be calm and unchanging
– Rigid C.S.
– …but planets move differently from everything
else, so C.S. cannot be static and unchanging
• Also, observations of comets (which were first
thought to be phenomena in the Earth’s
atmosphere)
Prehistoric Astronomy
• The Moon & Eclipses
– Solar eclipse: sky becomes dark as night for a
short while
– Lunar eclipse: moon turns red and/or
disappears for a while
– Occur infrequently…need near perfect
alignment of sun, moon, and earth
• Prehistoric astronomers did not know
• seen as omens (good or bad, depending on the
circumstances)
Prehistoric Astronomy
Classical Astronomy
• 500 B.C. - 1400 A.D.
• The “Big Names” of classical astronomy
– Pythagoras: Greek philosopher, geometry
whiz (because, well, he invented it as we use
it today)
– Aristotle: Greek philosopher, student of Plato
– Eratosthenes: geographer & astronomer
• Pioneers in determining the size and
shape of the earth
Classical Astronomy
• Pythagoras
There is geometry in the humming of the strings
... there is music in the spacing of the spheres.
– His ideas would influence generations of
scientists and philosophers for centuries
– The solid body we call the “sphere” is perfect.
• What is meant by this?
• So naturally, the Gods would model the earth as a
sphere during the Creation…
– True, to some extent…
• Equatorial bulge…Earth is “pear-shaped”
• Small-scale deviations from sphericity…mountains,
canyons, deep-sea trenches, etc.
Classical Astronomy
• Aristotle
– More quantifiable evidence than Pythagoras
– Observations of curved shadow on the moon
during lunar eclipses
• It’s earth’s shadow, so earth must be curved like a
sphere!
– Observations of night sky
• Travelers moving south begin to see “new”
(unfamiliar) stars.
– Only possible if a horizon exists, therefore, earth must be
curved! (There is no horizon on a flat earth)
Classical Astronomy
• Eratosthenes
– Brilliant experiment to determine the size of
the earth
– A large obelisk in Alexandria casts a ~7°
shadow when the sun is at it’s highest point in
the sky…
– 5000 stadia (~500 miles) away in Egyptian
city of Syene, the sun casts no shadow at it’s
highest point in the sky…
• There must be ~7° between the lines joining
Alexandria and Syene to the center of the
(spherical) earth
Eratosthenes (cont.)
• Therefore, the distance from Alexandria to
Syene must be ~ 1/50th of the distance
around the earth (circumference)
– 360°/7° ≈ 50
• Cearth = 50 x 5000 stadia = 250,000 stadia ≈
25,000 miles
• Accepted value today: 24,044.5 miles
– amazingly accurate for that time…although the
precise value of the stadia is disputed
• some claim E’s estimate off by ~ 16%
Classical Astronomy
• Motion of the planets
– Geocentric universe = earth-centered
– Sun, moon, and each planet occupied its own
transparent, revolving sphere, nested
between the stationary earth and the celestial
sphere
– Ptolemy
• Attempt to describe retrograde planet motion by
epicycles: frisbee on a bike wheel
– circles moving in circles moving in circles…
• Too complicated, required many nested epicycles
for even smallest amount of accuracy in
predictions…
How did classical astronomers explain this strange backwards motion?
Ptolemy
Ptolemy attempts to explain retrograde
motion by hypothesizing that the planets
move in circles within circles…
…his theories remained in use for another
1500 years, until it became too complex to
be physically meaningful
Classical Astronomy
• Aristarchus
– Measured size of moon
– Estimated distance to Sun (but wrong by
factor of ~ 20)
• Proposed Sun was center of the heavens
• Heliocentric model first appears
• Problem: should see stars shift positions (stellar
parallax)
Distances to even nearest stars are so huge, stellar parallax is a VERY small
quantity, and so could not be measured until the middle of the 19th century!
Evidence for heliocentric model existed, but was just outside our grasp to
observe…
 “Absence of evidence is not evidence of absence.”
The Renaissance
• 1400 A.D. – 1650 A.D.
• Revolutions in astronomy
• The Big Names:
– Nicolas Copernicus
– Tycho Brahe
– Johannes Kepler
– Galileo Galilei
– Isaac Newton
The Renaissance
• Copernicus
– Polish doctor & lawyer
– Revisited ideas of Aristarchus’ 2000 year old
“heliocentric” model
• Now able to explain retrograde motion: planets on
different, sun-centered orbits pass each other…
• But still some discrepancies…
– Model had perfectly circular orbits
– Still no parallax observed
– Published his ideas shortly before his death
• First person to describe the details of the
heliocentric model
At last, a simple & accurate
model for predicting
retrograde motion
The Renaissance
• Tycho Brahe
– Danish Nobility
• utilized his wealth to study the sky
– Built most accurate pointing & measuring
instruments of his time
• Still could not observe parallax  one of the last to
hold on to the geocentric model
– Actually, built at least 4 of each, operated
simultaneously, directed by Brahe himself
• One of the first to establish importance of
repeatability in scientific measurement
• More data = more accurate results
Tycho Brahe (cont.)
• Large, fine-toothed “gear” system
– Brahe and/or his assistant would sight stars
along an “aiming” direction
– Assistant would record the angular positions
based on the scale of the gear-teeth
• Incredibly accurate star & planet positions
– Observations in favor of heliocentric model
– Revealed true shape of planetary orbits, but
not to Brahe himself…
The Renaissance
• Kepler
– One of Brahe’s assistants
• inherited his work when he died
– Superior data (by volume and precision)
showed that Mars’ orbit is not circular, but
elliptical
• Kepler noticed the sun was not at the center of the
calculated orbit, but off to the side, at a “focus” of
the ellipse
• Proceeding with this model, Kepler calculated the
orbits of the other planets, and found excellent
agreement with the actual observations!!
Ellipses
Kepler (cont.)
•
Kepler’s 3 Laws
1) Planets move in elliptical orbits, with the sun
at one focus of the ellipse
2) Planets do not move with constant speed.
They move faster when nearer the sun, and
slower when they are farther away
3) The amount of time it takes a planet to orbit
the sun exactly once is related to the size of
the orbit (semimajor axis):
P2 years = a3 AU
Kepler (cont.)
1st Law
elliptical orbits,
Sun at one focus
2nd Law
equal areas in
equal time
3rd Law
period-major axis
relation
Kepler’s Laws are empirical, which means they are based on actual data &
measurements, not just theory…
The Renaissance
• Galileo
– Italian scientist, fascinated by motion
– He used telescope to:
• Observe and draw surface features of the moon
– Concluded moon was ball of rock
• Observe changing sunspots (sun NOT a constant orb…)
• Observe 4 large moons of Jupiter, proving that some bodies
in the solar system did NOT orbit the earth
• Observe evidence of Saturn’s rings
• Observe phases of Venus as proof of the heliocentric model
• Establish large size of Milky Way galaxy
The phases of Venus,
As observed from Earth
No matter what configuration,
epicycles cannot explain
the phases of Venus…
Galileo (cont.)
• Galileo’s experiments with motion
– Falling bodies, pendulum, etc…
• Coupled with his observations of 4 large
moons of Jupiter
– Called “Galilean Moons” in his honor
• Raised questions about nature of
planetary motion and what held planets in
orbit
– Condemned for his findings…
– Enter British Scientist Sir Isaac Newton…
The Renaissance
• Newton
– Born same year Galileo died
– Attempts to understand motion of the moon
• Leads him to deduce the law of gravity (as we still
use it today!)
• Requires him to invent new mathematics
• Leads him to deduce the general laws of motion
More on Newton and his Laws later…
• these Renaissance astronomers are
pictured on currency & postage alongside
kings & queens
Modern Astronomy
• 1650 A.D. to the present
• New discoveries!
– More planets & more moons
– Binary stars, galaxies, exotic phenomena
• New technologies!
– Better optics
– bigger, better telescopes
– Photographic film
– CCDs
Modern Astronomy
• Nature of matter & heat
– Up to 100 years ago, no idea atoms existed
• Theorized to exist by greeks during 5th century
B.C.
• “atom” = “indivisible” or “uncuttable”
– Rutherford & the gold foil experiment
• Probe structure of the atom
– Link between heat/temperature and molecular
motion discovered
• Lord Kelvin, working on improving steam engines
Modern Astronomy
• The Kelvin Temperature Scale
– Temperature directly proportional to speed of
molecular motion…
•
•
•
•
H2O freezing point ≈ 273 K
Room temperature ≈ 300 K
H2O boiling point ≈ 373 K
Surface of sun ≈ 6000 K
• Absolute Zero = 0 K (NO molecular motion)