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Welcome to
Starry Monday at Otterbein
Astronomy Lecture Series
-every first Monday of the monthMay 5, 2007
Dr. Uwe Trittmann
Today’s Topics
• From Galileo to Newton - The Birth of
Modern Astronomy, Part II
• The Night Sky in May
On the Web
• To learn more about astronomy and physics at
Otterbein, please visit
– http://www.otterbein.edu/dept/PHYS/weitkamp.a
sp (Observatory)
– http://www.otterbein.edu/dept/PHYS/ (Physics
Dept.)
From Galileo to Newton - the Birth of
Modern Astronomy, Part II
1610
1687
Sunspots
• MPEG video
from Galileo
Project (June 2 –
July 8, 1613)
Saturn
• Sketch of 1616
• Engraving in “The Assayer” (1623)
Galileo and his Contemporaries
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Elizabeth I. (1533-1603) – Queen of England
Tycho Brahe (1546-1601) – Danish Astronomer
Francis Bacon (1561-1626) – English Philosopher
Shakespeare (1564- 1616) – Poet & Playwright
Galileo Galilei (1564-1642) – Italian PAM
Johannes Kepler (1571-1630) – German PAM
Rene Descartes (1596 - 1650) – French PPM
Christiaan Huygens (1629-1695) – Dutch PAM
Isaac Newton (1643-1727) – English PM
Louis XIV (1638-1715) – French “Sun King”
Epochs
• Renaissance: 1450-1600
– “Rebirth”, back to the roots
• Baroque: 1600-1715
– Epoch of the religious wars
– Later: Louis XIV and Newton
• Rococo: 1715-1775
Tycho Brahe Johannes Kepler
Galileo Galilei
Observations
 Data
Experiment
 test predictions
Phenomenology/Theory
 Predictions
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
Francis Bacon (1561-1626)
Contemporary of Shakespeare,
Elizabeth I, Kepler and Galilei
Rejects Aristotelianism and
Scholasticism
Major Work:
• Novum Organum (1620)
(“New Tools”, the old Organum was Aristotle’s)
Francis Bacon
(1561-1626)
Novum Organum Title:
Allegory on the daring
mind: A Ship passes
through the “pillars of
Hercules” (the straight
of Gibraltar, the “end
of the world”), beyond
old knowledge.
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; Discourse on the Method of
Rightly Conducting the Reason, and
Seeking Truth in the Sciences]
• Meditations on first Philosophy (1641)
[6 Meditations: Of the Things that we may doubt;
Of the Nature of the Human Mind; Of God: that He
exists; Of Truth and Error; Of the Essence of
Material Things; Of the Existence of Material
Things; Of the Real Distinction between the Mind
and the Body of Man]
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: the simplest theory wins
Common Misconceptions
• Theories can be proven
– Fact: they cannot! If an experiment agrees 10
billion times with predictions, maybe the next
experiment disagrees
– However: one disagreeing experiment can
falsify the theory
– Most probably, the sun will rise tomorrow – but
no guarantees!
Common Misconceptions
• Theories are just “theories”, i.e. hypotheses
– Fact: scientists call a hypothesis a theory if it is
very well tested, e.g. Einstein’s theory of
Relativity, Darwin’s theory of evolution
– There may be hundreds of thousands of
reviewed publications that independently lend
evidence to the theory’s correctness
Common Misconceptions
• Scientists will defend their theories against any
new and/or unconventional approach
– Fact: scientists love new ideas that explain everything
known to date – and make new predictions
– However, since so much is known, it is very unlikely
that someone comes up with a better description of
Nature than the previous, well-tested one
– Unless the new theory is simpler, or makes correct
predictions at odds with the old theory, it is quite
efficient to stay with the old theory
Common Misconceptions
• Scientists want to explain everything
– Fact: scientists want to come up with an
accurate description of Nature, i.e. of how the
fundamental entities in the universe interact
– Naturally, science cannot (and does not want to)
say anything about topics like God, Religion,
Freedom, Ethics, Art, etc.
Common Misconceptions
• Scientists can predict everything that will happen
in the Universe (Laplace’s Demon)
• Fact: even in principle (if you would know the
position and position of every particle) this is not
possible because of
– Quantum Mechanics: the laws of QM are inherently
stochastic; QM predicts the probability of what might
happen, not what will actually happen
– Possibility of chaotic behavior: very small differences
in the initial conditions can have vastly different effects
(a butterfly’s wing movements can stir up a hurricane,
in principle)
The New Physics & Astronomy in a
Nutshell: Newton’s Principia
• Newton’s 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!)
• Principia (1687)
[Full title: Philosophiae naturalis
principia mathematica] has his famous
three laws on page 19 of 443.
Isaac Newton (1642–1727)
Newton’s Laws of Motion
1. Every body continues in a state of rest or in a
state of uniform motion in a straight line unless it
is compelled to change that state by forces acting
on it (law of inertia)
2. The change of motion is proportional to the
motive force impressed (i.e. if the mass is
constant, F = ma)
3. For every action, there is an equal and opposite
reaction (That’s where forces come from!)
Newton’s
Laws
Always the same constant pull
a) No force: particle at rest
b) Force: particle starts moving
c) Two forces: particle changes
movement
Gravity pulls baseball back to earth
by continuously changing its velocity
(and thereby its position)
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
Applications
• From the distance r between two bodies and the
gravitational acceleration a of one of the bodies,
we can compute the mass M of the other
F = ma = G Mm/r2 (m cancels out)
– From the weight of objects (i.e., the force of gravity)
near the surface of the Earth, and known radius of Earth
RE = 6.4103 km, we find ME = 61024 kg
– Your weight on another planet is F = m  GM/r2
• E.g., on the Moon your weight would be 1/6 of what it is on
Earth
Applications (cont’d)
• The mass of the Sun can be deduced from the
orbital velocity of the planets: MS = rOrbitvOrbit2/G
= 21030 kg
– actually, Sun and planets orbit their common center of
mass
• Orbital mechanics. A body in an elliptical orbit
cannot escape the mass it's orbiting unless
something increases its velocity to a certain value
called the escape velocity
– Escape velocity from Earth's surface is about 25,000
mph (7 mi/sec)
Objections to the Heliocentric
Model Answered
• If the Earth is moving, why do dropped objects
appear to fall straight down?
– Dropped objects start with the velocity of Earth
(Galileo)
• If the Earth rotates, why don't we get thrown off?
– Earth's rotation isn't fast enough!
• If the Earth revolves around the Sun, why don't we
observe stellar parallax?
– It's there, but very small, because the stars are so far
away (Aristarchus)
• Why don't we feel the wind of our motion?
– The air moves along with the Earth
Problems of Both Models
• Lack of a fundamental explanation?
– Provided by Newton (but what explains Newton?!)
• Lack of direct evidence?
– Proof that the Earth rotates:
• Coriolis force (hurricanes are counterclockwise in the Northern
Hemisphere)
• Foucault pendulum
– Proof that earth and other planets revolve around the
sun:
• Aberration of starlight observed 1729
• Stellar parallax observed 1838
• Phases of Venus (Galileo)
The Night Sky in May
• Nights still long, but EDT => later observing!
• Spring constellations are up: Cancer, Leo, Big
Dipper
• Saturn dominates the evening, Jupiter early
morning.
Moon Phases
• Today: Waning Gibbous Moon
• 5 / 10 (Last quarter Moon)
• 5 / 16 (New Moon)
• 5 / 23 (First Quarter Moon)
• 5 / 31 (Full Moon)
Today
at
Noon
Sun at
meridian,
i.e.
exactly
south
10 PM
Typical
observing
hour,
early May
Jupiter
Saturn
Zenith
Big Dipper
points to the
north pole
South
• Saturn
near
Praesepe
(M44), an
open star
cluster
South
• Spring
constellations:
– Leo
– Hydra
– Crater
– Sextans
East
• Canes
Venatici:
– M51
• ComaVirgo
Cluster
• Globular
Star
Clusters
– M3, M5
East
Virgo and
Coma
with the VirgoComa
galaxy
cluster
VirgoComa
Cluster
• Lots of
galaxies
within a
few
degrees
M87, M88
and M91
East
– Hercules
– Corona
Borealis
– Bootes
Globular Star
Clusters:
•M3
• M 13
• M 92
M13: Globular Cluster
Mark your Calendars!
• Next Starry Monday: October 1, 2007, 7 (!!!) pm
(this is a Monday
• Observing at Prairie Oaks Metro Park:
– Friday, May 25, 2007, 9:00 pm
• 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.