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Announcements
First MIDTERM Exam: Mon, Sep. 22, covering
chapters 1–3 + tutorials (review later
today).
Interim grades online, coded by class ID.
See course website “Grades” tab.
Another great night for observing! Meet in
the lobby outside class around 9pm. Bring
your blue ticket! Friends/relatives
welcome.
APOD/2008–09–17
Last Time
SCIENCE: A human endeavor characterized by the
hallmarks of 1) reliance on natural causes, 2)
“Occam’s Razor”, 3) Verifiability, and 4)
Falsifiability.
Planets move on the celestial sphere (as does the
moon/sun). 5 “visible eye” planets: mercury/venus/
mars/jupiter/saturn.
Planets normally “slip” W to E (“prograde”: lazy
planets!). Sometimes move E to W (“Retrograde”).
A mystery to the ancients (who assumed earthcentered universe!).
Important distinction: Diurnal motion: E to W (rise,
transit, set), due to earth’s rotation. Motion on
the celestial sphere (much slower, W to E
“slipping”).
Last Time
Greeks took the celestial sphere
“literally”. One sphere needed for each
wandering object (sun, moon, + 5 planets),
in addition to the “fixed stars”.
Problem: couldn’t explain retrograde
motion.
Solution (Ptolemy): small circles upon
large (offset) circles.
Last Time
Greeks knew the earth is round as early as
500BC, and estimated the circumference
using angles of the sun from different
cities separated by hundreds of km.
Rejected earth moving around the sun, since
stars suffered no “parallax” (angular shift
of foreground compared to background
objects).
The Foundations
Ancient
structure in
many cultures
marking
progression of
sun, moon, and
planets.
Astronomy Beginnings
Aristotle (~400BC):
Earth is spherical
(simple/perfect objects),
and at center of the
universe.
Shape of earth’s shadow
during lunar eclipse.
Ships sailing over
horizon.
Refinements
Ptolemy (100-170 AD):
Greek astronomer in
Alexandria, Egypt
Circular paths (some
offset), add “epicycles” —
circles upon circles — to
produce retrograde motion.
Good within a few degrees,
but very complex!
Model used for 1500 years!
Parallax (Again)
Earth moves around sun
➠ nearby stars “shift”
compared to background
stars due to “parallax”.
The lack of apparent
parallax convinced
greeks that earth must
not move.
In reality, stars
distances are so great,
their parallax is too
small to see with the
eye.
Testing – the key to
science
Greeks
Rational thought was sufficient
Inconsistencies blamed on faulty perception
Modern Science
Models make predictions
Verification provides support for model
A “Theory” is a generally accepted model that
explains many different observations and has
withstood extensive testing of its predictions
The Troubled Ages
Most astronomical records
destroyed with loss of
library of Alexandria.
Greek/Roman empires fall.
Islamic scientists kept
record and further
developed models during
the dark ages.
Renaissance: Christian
Church adopts geocentric
cosmology.
Revolution
Copernicus (1473-1543):
Polish scientist.
Studied inaccuracies in
ptolemaic model.
Better model, sun-centered,
but kept “perfect circles”.
A true revolution, but not
widely adopted.
Investigation
Tycho Brahe (1546-1601):
Understood that better
data were needed.
Three decades of naked eye
observations (good to 1
arcmin): built an excellent
data set.
Still could not see
parallax.
Model of sun orbiting
earth, other planets
orbiting sun (not adopted).
Eureka
Johannes Kepler (1571–1630):
Hired by Tycho to make
sense of the data.
Found a “new” ptolemaic
model with circles, but 8
arcmin discrepancies
remained.
Trusted the data... (but
still clung to ideals of
“perfection”).
Keplerian Model
Kepler’s First Law: The orbit of each
planet around the Sun is an ellipse, with
the sun at one focus.
Keplerian Model
Kepler’s First Law: The orbit of each
planet around the Sun is an ellipse, with
the sun at one focus.
Finite
Galileo Galelei (1564–1642):
Italian scientist, overturned
final objections to Kepler’s
models.
1. Earth moves, birds fall off.
2. Non-circular orbits not
perfect.
3. No stellar parallax, so
earth can’t move.
Galileo’s Impact
Experiments with rolling balls: Objects in
motion stay in motion unless otherwise
acted.
Universe is imperfect. Using one of the
first telescopes, he observed spots on the
sun, and crates and valleys on the moon.
Galileo’s Sunspots
Galileo’s Impact
Using one of the first
telescopes, observed
Moons clearly orbiting
jupiter (not the earth!)
Galileo’s Impact
Saw phases of Venus.
Geocentric: only crescent
phases.
Heliocentric: All phases.
Galileo’s Impact
Showed that stars could be at great
distances.
Milky way resolves into many stars in a
telescope.
Galileo’s Price
Catholic church
brought galileo
before the roman
inquisition.
Forced to recant his
claim that earth
orbit’s the sun.
Church formally
apologized in 1992.
Mid-Term Exam #1
Review
Chapters 1–3 (skip Kepler’s 2nd and 3rd
laws).
Workbook Exercises.
General Physical concepts stressed.
Applied understanding of motions of earth/
moon/sun/stars.
Historical context, not date of birth/
favorite breakfast cereal/etc.
Suggestions
Review “Big Picture”, “Key Concepts” and
“Review Questions & Problems” at end of
each chapter.
Better way: use the “Study Area” on
mastering astronomy (nothing here is
graded!).
Know how to apply concepts.
Concepts Covered
Relative scales of the Universe.
Things in the universe: planet, moons,
comet, asteroid, star, solar system, galaxy,
clusters.
Age of the universe.
Concepts Covered
Constellations
The celestial sphere.
Diurnal motions of the stars (rise/set/
transit).
What stars we see based on our latitude on
earth. “Circumpolar” stars.
Our system of Time, based on the sun’s
position!
Concepts Covered
Angular Sizes: sun, moon, angles vs.
distance.
Motions on the celestial sphere (why?).
Sun’s motion on the celestial sphere (as a
consequence of earth’s orbit around the
sun).
Sidereal vs. solar day.
Ecliptic: circular path of sun through
Concepts Covered
“Special” locations on the ecliptic = special
times of year: solstices, equinox.
Tilt of the earth’s axis compared to plane of
orbit, and the reason for seasons.
Precession of the earth’s axis.
Explain the movement of the sun from S to
N and back again through the year.
The Moon
Motion of the moon on the sky.
Orbit of the moon around earth.
Phases of the moon (and when you see
them!).
Eclipses
Lunar vs. solar eclipses.
Different types of eclipses.
Why aren’t eclipses more common?
Near coincidence of moon’s and sun’s
angular sizes
Planetary motion
Where planets are found on the sky (“the
zodiac”).
The 7 “wandering” objects on the sky.
Retrograde motion: what causes?
Normal “slippage” motion of planets on the
celestial sphere.
History
Greek explanation of planetary motions.
Geocentric vs. heliocentric models of the
solar system.
How retrograde planetary motions were
explained.
The hallmarks of modern science.
History
Earth is round. Size estimate size from
sun’s position.
Parallax, and its importance in greek
theories of the solar system.
Ptolemaic model, Copernican model, Tycho
Brahe, Kepler, Galileo.
Reminders
First MIDTERM Exam: Mon, Sep. 22
Grades online now.
Observing open through Thursday: looking
good all week!
Still need tickets? Come see me.