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Welcome to
Starry Monday at Otterbein
Astronomy Lecture Series
-every first Monday of the monthMay 2, 2005
Dr. Uwe Trittmann
Today’s Topics
• The Night Sky in May
• History of Astronomy
Feedback!
• Please write down suggestions/your interests on the
note pads provided
• If you would like to hear from us, please leave your
email / address
• To learn more about astronomy and physics at
Otterbein, please visit
– http://www.otterbein.edu/dept/PHYS/weitkamp.asp (Obs.)
– http://www.otterbein.edu/dept/PHYS/ (Physics Dept.)
The Night Sky in May
• The sun is getting higher -> shorter nights!
• Virgo contain lots of galaxies, Hercules some of
the best globular star clusters
• Jupiter is bright, Saturn sets early
Moon Phases
• Today (Waning crescent, 33%)
• 5 / 8 (New Moon)
• 5 / 16 (First Quarter Moon)
• 5 / 23 (Full Moon)
• 5 / 30 (Last Quarter Moon)
Today
at
Noon
Sun at
meridian,
i.e.
exactly
south
10 PM
Typical
observing
hour, early
May
no Moon
Pluto
Jupiter
Saturn
South
Virgo and
Coma
with the VirgoComa
galaxy
cluster
VirgoComa
Cluster
• Lots of
galaxies
within a
few
degrees
M87, M88
and M91
Zenith
Big Dipper
points to the
north pole
East
– Hercules
– Corona
Borealis
– Bootes
Globular Star
Clusters:
•M3
• M 13
• M 92
M13: Globular Cluster
SouthWest
Spring
Constellations
- Cancer
- Leo
- Hydra
Deep Sky
Objects:
- Beehive
Cluster (M44)
A Short History of Astronomy
• Ancient (before 500 BC)
• Egyptians, Babylonians, Mayans, Incas, Chinese
• Classical Antiquity (500 BC-500 AD)
• Greeks, Romans: Plato, Aristotle, Ptolemy
• Middle Ages (500-1450 AD)
• Arabic astronomers
• Renaissance (1450-1550 AD)
• Copernicus
• Baroque (1550-1700 AD)
• Brahe, Kepler, Galilei, Newton
Ancient Astronomy
Stonehenge, England
Pyramids,
Gizeh near Cairo, Egypt
The Babylonians
• Made systematic measurements as early as
~2000 BC
• By ~ 800–400 BC
– State support for the calendar and astrology
– Compiled the first star catalogs and began longterm records of planetary motions
– Were able to predict lunar and solar eclipses
• May also have invented astrology
The Greeks
Plato (428 BC)
• Introduces the celestial
sphere. The stars are
fixed to a sphere that
rotates around the Earth
• introduces prejudice in
favor of circles
• values theory over
observation
The Greeks
• Aristotle (384–322 BC)
– Argued that the planets move on spheres around the
Earth (“geocentric” model)
– Argues that the earth is spherical based on the shape of
its shadow on the moon during lunar eclipses
• Aristarchus (310–230 BC)
– Attempts to measure relative distance and sizes of sun
and moon
– Proposes, nearly 2000 years before Copernicus, that all
planets orbit the Sun, including the Earth
(“heliocentric” model)
The Greeks
• Eratosthenes (ca. 276 BC)
– Measures the radius of the earth to about 20%
(Works also between Detroit and Columbus)
The Greeks
Hipparchus (~190 BC)
– His star catalog a standard
reference for sixteen
centuries!
– Introduces coordinates for
the celestial sphere:
• Declination (dec)
• Right Ascension (RA)
(analogous to latitude and
longitude, respectively)
Ptolemy (~140 AD)
• Puts forth a complete
geocentric model
• dominates scientific
thought during the
Middle Ages
• Longest lasting (wrong)
theory ever: 1000 yrs
Major Work: Almagest
Retrograde Motion
Epicycles
• Ptolemy’s
explanation of
retrograde motion
• About 40(!) epicycles
necessary to explain
all observations
complicated theory
The Medieval Setting
•
•
•
•
Dominant Church
1000 years of relative stagnation
Experimental research greatly reduced
To answer a question:
“Study the Bible or Aristotle!”
The Renaissance Setting
• Invention of the print (1450) by Gutenberg
Books widely available!
(Think: Manuscripts vs Amazon.com)
•
•
•
•
End of Middle Age Church Domination
Back to the roots (renaissance=rebirth)
Study of Arabic astronomers
Intellectual movement
Nicolas Copernicus (1473–1543)
• Rediscovers the heliocentric
model of Aristarchus
• Planets on circles
needs 48(!!) epicycles to explain
different speeds of planets
• Not more accurate than Ptolemy
Major Work : De
Revolutionibus
Orbium Celestium
(published posthumously)
The heliocentric Explanation of
retrograde planetary motion
See also: SkyGaze
The Scientific Method
• Systematized by Francis
Bacon, Descartes and
Galileo in the 17th
century
• Not the only way of
knowing, but a very
successful one
• A method to yield
conclusions that are
independent of the
individual
• Conclusions are based
on observation
Rene Descartes – The Rationalist
Described the method to do science,
known for his mind-body dualism
Major Works:
• Discourse (1637) [full title: Discours
de la méthode pour bien conduire sa
raison et chercher la vérité dans les
sciences, Discussion of the method for
correct reasoning and for searching
the truth in the sciences.]
• Meditations on first Philosophy (1641)
Rene Descartes (1596-1650)
Rene Descartes’
Discourse
Describes the method to do science
in a straightforward way (see below)
Major points:
• Science must be based on
correct reasoning (logic)
• Science must be formulated in
mathematical language
Starting line:
“Good sense is the most evenly distributed thing in the world, for all people suppose
themselves so well provided with it that even those who are the most difficult to satisfy
in every other respect never seem to desire more than they have.”
A Classical Example
• Aristotle observes that during lunar eclipses the
Earth’s shadow on the moon is curved
• He assumes it will be curved for all eclipses
• A hypothesis that explains this: the earth is round
• A prediction of this theory is that the location of
the stars in the sky should be different for
observers at different latitudes
• This is confirmed by additional observations
– E.g. Canopus is visible in Egypt but not further north
Scientific Theories
• Must be falsifiable
– There must be some way the theory could fail
• Should make predictions
– The more, the better!
• Theories that are very well tested and have the
widest applicability are often known as “laws of
nature”
• Always subject to revision or modification
• Occam’s Razor
Performing Experiments
• Experiments must be repeatable – requires careful
control over variables
• Possible outcomes of an experiment:
– The experiment may support the theory
• We then continue to make predictions and test them
– The experiment may falsify the theory
• We need a new theory that describes both the original data and
the results of the new experiment
• Since we cannot do every possible experiment, a
theory can never be proven true; it can only be
proven false
The Baroque Setting
• In the 1600s church through counterreformation much stricter
• G. BRUNO (Italian; 1548) proposes that the
Sun is just one star out of an infinite number
 burned at the stake for heresy 1600
• 30 Years War (1618-1648) between religions
• New inventions: telescope, air pump, etc.
Tycho Brahe – The Data Taker
• Key question:
Where are things?
• Catalogued positions of planets
in Uraniborg and Prague
• Working without telescope
• Data ten times as accurate as
before
• Died at banquet binge drinking
Tycho Brahe (1546–1601)
Tycho Brahe
• collects detailed and accurate (1-2’
accuracy) observations of stellar and
planetary positions over a period of
20 years
• His research costed 5-10% of Danish
GNP
• shows that comets and novas are
extralunar contrary to Aristotle
• Shows that stars can change
(Supernova of 1572)
Tycho Brahe observing
Johannes Kepler–The Phenomenologist
• Key question:
How are things happening?
Major Works:
• Harmonices Mundi (1619)
• Rudolphian Tables (1612)
• Astronomia Nova
• Dioptrice
Johannes Kepler (1571–1630)
Kepler’s Beginnings
• Astrologer and Mystic
• Tried to find “music in
the skies”
• Tried to explain
distances of the
5 known planets by
spheres resting on the
5 mathematical bodies
 pre-scientific
Johannes Kepler
Manuscript: trying to disentangle
The mystery of Mars’ orbit 
Kepler’s First Law
The orbits of the planets are ellipses, with
the Sun at one focus
Ellipses
a = “semimajor axis”; e = “eccentricity”
Conic Sections
From Halley’s book (1710)
Kepler’s Second Law
An imaginary line connecting the Sun to any planet sweeps
out equal areas of the ellipse in equal times
Kepler’s Third Law
The square of a planet’s orbital period is proportional to the cube of its
orbital semi-major axis:
P 2  a3
a
P
Planet Orbital Semi-Major Axis Orbital Period
Mercury
0.387
0.241
Venus
0.723
0.615
Earth
1.000
1.000
Mars
1.524
1.881
Jupiter
5.203
11.86
Saturn
9.539
29.46
Uranus
19.19
84.01
Neptune
30.06
164.8
Pluto
39.53
248.6
(A.U.)
(Earth years)
Eccentricity
0.206
0.007
0.017
0.093
0.048
0.056
0.046
0.010
0.248
P2/a3
1.002
1.001
1.000
1.000
0.999
1.000
0.999
1.000
1.001
Galileo Galilei – The Experimentalist
Did experiments (falling bodies) rather
than studying Aristotle
Major Works
• Siderius Nuntius (1610)
• Dialogue concerning the Two Chief
World Systems (1632)
The latter discusses Copernicus vs
Ptolemy ban by Church (1633)
– revoked by pope 1992
(1564–1642)
Siderius Nuntius (1610)
Dialogo (1632)
Galileo Galilei (1564–1642)
• Astronomical observations that
contradict Aristotle:
– Observed mountains on the Moon,
suggesting that the Earth is not unique
– Sunspots; suggests that celestial bodies
are not perfect and can change
– Observed four moons of Jupiter; showed
that not all bodies orbit Earth
– Observed phases of Venus (and correlation of apparent
size and phase); evidence that Venus orbits the Sun
• Also observed
– the rings of Saturn
– that the Milky Way is made of stars
Phases of
Venus
Heliocentric
(observed)
Geocentric
(not observed)
Isaac Newton – The Theorist
• Key question:
Why are things happening?
• Invented calculus and physics
while on vacation from college
• His three Laws of Motion,
together with the Law of
Universal Gravitation, explain all
of Kepler’s Laws (and more!)
Isaac Newton (1642–1727)
Isaac Newton (1642–1727)
Major Works:
• Principia (1687)
[Full title: Philosophiae naturalis
principia mathematica]
• Opticks [sic!](1704)
• Major findings:
– Three axioms of motion
– Universal gravity
Law of Universal Gravitation
Mman
MEarth
R
Force = G Mearth Mman / R2
Orbital Motion
Cannon “Thought Experiment”
• http://www.phys.virginia.edu/classes/109N/more_stuff/Appl
ets/newt/newtmtn.html
Mark your Calendars!
• Next Starry Monday at Otterbein: June 6, 2005, 7 pm
(this is a Monday
• Web pages:
– http://www.otterbein.edu/dept/PHYS/weitkamp.asp (Obs.)
– http://www.otterbein.edu/dept/PHYS/ (Physics Dept.)
)
Mark your Calendars II
• Physics Coffee is every Wednesday, 3:30 pm
• Open to the public, everyone welcome!
• Location: across the hall, Science 256
• Free coffee, cookies, etc.