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ASTR100 (Spring 2008)
Introduction to Astronomy
The Science of Astronomy
Prof. D.C. Richardson
Sections 0101-0106
How did astronomical observations
benefit ancient societies?
Keeping track of time and seasons.
For practical purposes, including
agriculture.
For religious and ceremonial
purposes.
Aid to navigation.
Ancient people of central Africa (6500
BC) could predict seasons from the
orientation of the crescent moon.
Days of week were named for Sun, Moon, and visible planets
Planets Known in Ancient Times
The “Wanderers”
• Mercury
– difficult to see; always
close to Sun in sky
• Venus
– very bright when visible
— morning or evening
“star”
• Mars
– noticeably red
• Jupiter
– very bright
• Saturn
– moderately bright
What did ancient civilizations
achieve in astronomy?
•
•
•
•
•
•
daily timekeeping
tracking the seasons and calendar
monitoring lunar cycles
monitoring planets and stars
predicting eclipses
and more…
 Egyptian obelisk:
shadows tell time
of day, like a
modern-day
sundial.
England: Stonehenge (completed around 1550 B.C.)
SW United States: “Sun Dagger” marks
summer solstice
Wyoming: Big Horn Medicine Wheel
South Pacific: Polynesians were very skilled in
art of celestial navigation
Modern science traces its roots to
the Greeks
• Greeks were the first
people known to make
models of nature.
• They tried to explain
patterns in nature without
resorting to myth or the
supernatural.
Greek geocentric model (c. 400 BC)
How did the Greeks explain planetary motion?
Underpinnings of the Greek geocentric model:
• Earth at center of Universe.
• Heavens must be “perfect”:
objects moving on perfect spheres
or in perfect circles.
Plato
Aristotle
But this made it difficult to explain apparent
retrograde motion of planets…
Over a period of 10 weeks, Mars appears to stop, back up, then
go forward again…
Explaining Apparent Retrograde
Motion
 Easy for us to explain: occurs when we
“lap” another planet (or when Mercury
or Venus lap us).
 But very difficult to explain if you think
that Earth is the center of the universe!
 In fact, ancients considered but
rejected the correct explanation…
The most sophisticated
geocentric model was
that of Ptolemy (A.D.
100-170) — the
Ptolemaic model:
• Sufficiently accurate
to remain in use for
1,500 years.
Ptolemy
• Copernicus (1473-1543)
proposed a Sun-centered
model, but still based on
perfect circles.
Copernicus (1473–1543)
• Copernicus (1473-1543)
proposed a Sun-centered
model, but still based on
perfect circles.
• Tycho Brahe (1546-1601)
made careful observations
but could not detect
Earth’s motion.
• Copernicus (1473-1543)
proposed a Sun-centered
model, but still based on
perfect circles.
• Tycho Brahe (1546-1601)
“If I had believed that
we could
made
carefulignore
observations
these eight minutes but
[of arc],
would
could Inot
detect
have patched up myEarth’s
hypothesis
motion.
accordingly. But, since it was not
• Kepler
Brahe’s data
permissible to ignore,
thoseused
eight
show
Copernican
minutes pointed theto
road
to the
a complete
model could work, but only
reformation in astronomy.”
if planetary orbits are
Johannes Kepler
ellipses, not circles!
(1571-1630)
Kepler’s three laws of planetary motion
Kepler’s First Law: The orbit of each planet around
the Sun is an ellipse with the Sun at one focus.
Kepler’s Second Law: As a planet moves
around its orbit, it sweeps out equal areas in
equal times.
This means that a planet travels faster when it is nearer to
the Sun and slower when it is farther from the Sun.
Kepler’s Third Law
More distant planets orbit the Sun at slower
average speeds, obeying the relationship
p2 = a3
p = orbital period in years
a = avg. distance from Sun in AU
Kepler’s 3rd Law
Thought Question
An asteroid orbits the Sun at an
average distance of a = 4 AU. How
long does it take to orbit the Sun?
A.
B.
C.
D.
4 years.
8 years.
16 years.
64 years.
(Hint: remember that p2 = a3.)
Thought Question
An asteroid orbits the Sun at an
average distance of a = 4 AU. How
long does it take to orbit the Sun?
A.
B.
C.
D.
4 years.
8 years.
16 years.
64 years.
(Hint: remember that p2 = a3.)
How did Galileo solidify the Copernican revolution?
Galileo (1564-1642) overcame
major objections to the
Copernican view.
• Using his telescope, Galileo saw:
 sunspots on Sun (“imperfections”)
 mountains and valleys on the Moon
(proving it is not a perfect sphere)
Galileo also saw four
moons orbiting
Jupiter, proving that
not all objects orbit
the Earth…
Ptolemaic View
Copernican View
… and his observations of phases of Venus
proved that it orbits the Sun and not Earth.
ASTR100 (Spring 2008)
Introduction to Astronomy
Newton’s Laws of Motion
Prof. D.C. Richardson
Sections 0101-0106
How do we describe motion?
Precise definitions to describe motion:
 Speed: rate at which object moves.
speed = distance/time (units: m/s)
Example: speed of 10 m/s.
 Velocity: speed and direction.
Example: 10 m/s due east.
 Acceleration: any change in velocity.
Units: speed/time (m/s2).
The Acceleration of Gravity
 All falling objects
accelerate at the
same rate (not
counting friction of
air resistance).
 On Earth, g ≈ 10
m/s2: speed
increases 10 m/s
with each second
of falling.
The Acceleration of Gravity (g)
 Galileo showed that g is the same for all
falling objects, regardless of their mass.
Apollo 15 demonstration
Momentum and Force
 Momentum = mass × velocity.
 A net force changes momentum.
 Often only velocity changes (not mass).
 The rotational momentum of a spinning
or orbiting object is known as angular
momentum.
Is there a net force? Y/N
1.
2.
3.
4.
5.
A car coming to a stop. 
A bus speeding up. 
An elevator moving at constant speed.
A bicycle going around a curve. 
A moon orbiting Jupiter. 

How is mass different from weight?
 mass – amount of matter in an object.
 weight – force that acts upon an object.
You are weightless
in free-fall!
On the Moon…
A.
B.
C.
D.
My
My
My
My
weight
weight
weight
weight
is
is
is
is
the same, my mass is less.
less, my mass is the same.
more, my mass is the same.
more, my mass is less.
On the Moon…
A.
B.
C.
D.
My weight is the same, my mass is less.
My weight is less, my mass is the same.
My weight is more, my mass is the same.
My weight is more, my mass is less.
Why are astronauts weightless in space?
• There IS gravity in space…
• Weightlessness is due to a constant state of free-fall…
How did Newton change our view of the Universe?
 Realized the same
physical laws that operate
on Earth also operate in
the heavens…
 one universe.
 Discovered laws of motion
and gravity.
 Much more: experiments
with light; first reflecting
telescope; calculus…
Sir Isaac Newton
(1642-1727)