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Lecture PowerPoint
Chapter 2
Astronomy Today,
5th edition
Last revised:
© 2005 Pearson Prentice Hall
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Chapter 2
The Copernican Revolution
Harlow Shapley
Edwin Hubble
Annie Jump Cannon
Karl Jansky
Units of Chapter 2
Ancient Astronomy (Archaeoastronomy)
The Geocentric Universe – Aristotle & Ptolemy
The Heliocentric Model of the Solar System
The Foundations of the Copernican Revolution
Copernicus and Galileo
The Birth of Modern Astronomy
Units of Chapter 2, cont.
The Laws of Planetary Motion
Kepler’sThree Laws
Some Properties of Planetary Orbits
The Dimensions of the Solar System
Newton’s Laws – Laws of Motion & Gravity
The Moon Is Falling!
Weighing the Sun
2.1 Ancient Astronomy
• Ancient civilizations observed the skies
• Many built structures to mark astronomical
summer solstice
Summer solstice sunrise at
There are
hundreds of
these type of
constructs all
over Europe
sunrise over heelstone
The Dawn of Astronomy
Native American Astronomy in the SW US
2.1 Ancient Astronomy
Spokes of the Big Horn Medicine Wheel (WY)
are aligned with rising and setting of Sun and
other stars
2.1 Ancient Astronomy
Mayan Astronomy
The Caracol temple
at Chichen Itza has
many windows that
are aligned with
astronomical events
There are
hundreds of
various types of
observatories all
over the Americas
2.2 The Geocentric Universe
Ancient and Medieval
astronomers observed:
Five planets: Mercury,
Venus, Mars, Jupiter,
Muslim Astronomers
Middle Ages
For more information on the
contributions of different world cultures
to astronomy see the “History of
Astronomy” Study Guide
For more details on ancient
observatories and astronomers go to the
same Study Guide
Ancient Western World
(Greek Philosopher-Scientists)
Claudius Ptolemy
Early Greek
2.2 The Geocentric Universe
Early observations:
• Inferior planets never too far from Sun
• Superior planets not tied to Sun; exhibit
retrograde motion
• Superior planets brightest at opposition
• Inferior planets brightest near inferior
2.2 The Geocentric Universe
Sun, Moon, and stars all have simple
movements in the sky
• move with respect to fixed stars
• change in brightness
• change speed
• have retrograde
2.2 The Geocentric Universe
Earliest models had Earth at center of solar
Needed many
computations to
accurately track
planetary motions:
This is the Ptolemaic
Cosmos using the
Epicycle model of
Hipparchus to
account for
retrograde motions
Epicycle Model for Retrograde Motion Hipparchus
How epicycles
retrograde loops
The Ptolemaic Cosmos
Refinement of Epicycle Model
Heliocentric Cosmos
• Still used circles & uniform
circular motion
2.2 The Heliocentric Universe
• Inferior planets: Mercury, Venus
• Superior planets: Mars, Jupiter, Saturn
Now know:
Inferior planets have
orbits closer to Sun
than Earth’s
Superior planets’
orbits are farther
Solar System
2.3 The Heliocentric Model of the
Solar System
The Sun is at center of solar system
Only Moon
orbits around
Earth; planets
orbit around
Here retrograde
motion explained
as a projection
Retrograde Motion in the Heliocentric Cosmos
A Projection Effect
Stellar Background
Earth moves faster
outer planet moves slower
retrograde loop
Planet only appears to make a loop
Ancients really thought it was
2.4 The Birth of Modern Astronomy
Telescope invented around 1600 (NOT by
Galileo but Hans Lippershay)
Galileo built his own, made the following
• Moon has mountains and valleys
• Sun has sunspots, and rotates
• Venus has phases
• Jupiter has 4 moons
Astronomy as Physics
(The Beginning of the
Scientific Revolution)
2.4 The Birth of Modern Astronomy
Phases of Venus, in particular, cannot be
explained by the geocentric model
Phases of Venus
Note change in the angular size with Venus’s
distance from Earth
Near inferior
Near superior
The Waning of
2.5 The Laws of Planetary Motion
Kepler’s laws were derived by using the pre-telescopic
observations made by
Tycho Brahe
Brahe’s Giant Quadrant
at Uraniborg Observatory
on the Isle of Hveen
2.5 Kepler’s Laws of Planetary
1. Planetary orbits are ellipses, Sun at one focus
2.5 Kepler’s Laws of Planetary
2. Imaginary line connecting Sun and planet
sweeps out equal areas in equal times
The Law of Equal Areas
Kepler’s Second Law
2.5 Kepler’s Laws of Planetary Motion
3. Square of period of a
planet’s orbital motion
is proportional to the
cube of the semimajor axis
P2 = ka3
P2 = ka3
Note: these are log
Orbital Speed vs. Distance
Comparing the Models:
Copernican vs. Kepler
2.6 The Dimensions of the Solar System
Astronomical unit (AU): mean distance from
Earth to Sun (93 x 106 mi =150 x 106 km)
First measured during
transits of Mercury
and Venus, using
Now done using radar
2.6 The Dimensions of the Solar System
Now measured using radar (Arecibo Obs.):
Ratio of mean
radius of Venus’s
orbit to that of
Earth very well
2.7 Newton’s Laws
•Newton’s laws of motion explain
how objects interact with the
world and with each other.
•Newton published his
work in the famous
Principia Mathematica
in 1687
Isaac Newton
•Sir Edmund Halley was the
driving force to get Newton to
publish his work to the world
•Halley used it to predict the
return of the comet that now
bears his name
2.7 Newton’s Laws
Newton’s First Law (The Principle of Inertia):
An object at rest will remain at rest, and an object
moving in a straight line at constant speed will
not change its motion, unless an external force
acts on it.
at rest
external force
2.7 Newton’s Laws
Newton’s second law (F = ma)
When a force is exerted on an object, its
acceleration is inversely proportional to its mass:
a = F/m
Newton’s third law (Action = Reaction)
When object A exerts a force on object B, object
B exerts an equal and opposite force on object A.
Illustrations of Newton’s 3 Laws of Motion
F = ma
Axn = Rxn
2.7 Newton’s Laws
On the Earth’s
surface, acceleration
is approximately
constant (g), and
directed toward
center of Earth:
g = GME
Wt. = mg
2.7 Newton’s Laws
For two massive
objects, gravitational
force is proportional to
the product of their
masses divided by the
square of the distance
between them:
Electricity, magnetism
and intensity of light also
follow inverse square laws
2.7 Newton’s Laws
The constant G is called the Universal
Gravitational Constant; it is measured
experimentally and found to be:
G = 6.67 × 10–11 N m2/kg2
2.7 Newton’s Laws
Kepler’s laws are a
consequence of
Newton’s laws
Possible Orbital Paths in Newton’s Theory of
2.7 Newton’s Laws
Circular velocity:
minimum speed needed
to go into a circular orbit
Escape velocity: speed
necessary for a
projectile to completely
escape a planet’s
gravitational field
“Newton’s Cannon”
How fast to orbit or escape Earth?
vcirc = 7900 m/s = 7.90 km/s = 4.9 mi/s = 17,700 mi/hr
Anything that can fly this fast will not hit the ground.
This is the speed for low Earth orbital objects like the
Space Shuttle, the ISS (International Space Station) and spy
How fast to orbit the Moon or to escape from it?
Summary of Chapter 2
• First models of solar system were geocentric,
but couldn't easily explain retrograde motion
• Heliocentric model does; also explains
brightness variations
• Galileo's observations supported heliocentric
• Kepler found three empirical laws of
planetary motion from observations
Summary of Chapter 2, cont.
• Laws of Newtonian mechanics explained
Kepler’s observations
• Gravitational force between two masses is
proportional to the product of the masses,
divided by the square of the distance
between them