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The Earth’s Motion
Chapter 2
Does the Earth Move
 How do we know that the earth is
moving?
 How do you know that when in a car that
the car is moving?
 If the earth is moving why don’t we fall
off?
The Geocentric
Theory
 The earth is the center and the universe
moves around the earth.
 Ancient astronomers found seven
heavenly objects that were different:
 Moon, Mercury, Venus, Sun, Mars, Jupiter
and Saturn (in that order)
 These objects moved slowly among the
stars, which were the outermost layer
The Geocentric Theory
 Ancient astronomers believed that the sun’s
sphere turned once each day.
 The other planets turned nearly once each day,
but at slightly different rates.
 This explained the independent paths of the planets.
 The moon’s sphere turned much more slowly
allowing it to change position rapidly.
 Explained why the moon rises about 50 minutes
later each night.
Geocentric Scientists
 Hipparchus – Greek astronomer around
190-120 BC.
 Created one of the earliest catalogs of 850
stars.
 Developed a mathematical geocentric
model.
Geocentric
Scientist
 Ptolemy – a.k.a Cladius Ptolemaeus –
AD 85-165 - Alexandrian (Egyptian)
astronomer.
 Presented the geocentric theory in a large
collection of writing titled The Greatest
Compilation – a.k.a. Almagest
 Geocentric theory is sometimes referred to
as the Ptolemaic theory because he played
such a large role in attempting to improve
the theory.
Problems with Geocentric
Theory
 The Planets sometime appear to be large,
bright, and close compared to other times
when they seem smaller, dimmer, and
farther away.
 Ptolemy tried to shift each planet so that the earth
was no longer the center causing an eccentric
(off-center circle).
 Astronomers were offended by this because it was
not a symmetrical and in their eyes not a
beautiful…but it did make the model closer to the
observations.
Problems with Geocentric
Theory
 Model also could not explain why it
appeared that planets had retrograde
motion.
 Planets slowed down, stopped, and then
appeared to back up.
 The planets would trace a backward loop
and then resume their forward motion.
Problems with Geocentric
Theory
 Ptolemy explained
 retrograde motion by imagining that each
planet moved in small circles called
epicycles.
 The planets continued to revolve around the
earth also.
 He called the normal forward path the
deferent so that it did not get confused with
the epicycle.
 http://videos.howstuffworks.com/hsw/20230-distance-and-time-ptolemy-video.htm
Major Objections to
Geocentric Theory
 Inaccuracy – astronomers tried MANY
different values for the distances of the
planets as well as their speeds to explain
the was observed.
 Complexity – the more they tried to
change for the explanation the more
confusing it became.
The Heliocentric
Theory
 The Renaissance
 Fourteenth, Fifteenth, and Sixteenth centuries
brought the great rebirth of learning.
 Renewed interest in every area of culture: art,
literature, music, drama, geography, science, and
medicine.
 Also, a reawakening for the Bible.
 Gutenberg printing press allowed for books to be
reprinted more easily.
The Heliocentric Theory
 Sun centered as opposed to Earth
centered.
 The sun remains still and everything
revolves around it with the exception of
the Earth’s moon.
 The Earth’s moon revolves around the
Earth.
Heliocentric Scientists
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Copernicus
Aquinas
Galileo
Kepler
Newton
Tycho
Copernicus
 Polish – formulated the heliocentric
theory
 Born during the renaissance.
 Disillusioned by the many “tricks” that must
be played to get the Geocentric Theory to
work.
 Author of “The Six Books Concerning the
Revolutions of the Heavenly Spheres” or
“The Revolutions” for short.
 Answered many arguments that critics tried.
Copernicus’ Arguments
 Critics
 Spinning earth would fly
apart
 Birds would be left behind
 If earth revolved around
sun then the nearer stars
would shift their position
compared to distant stars
when viewed from
different places
 Answers
 Rotating sky more
likely to fly apart
because of size
 Earth drags along the
atmosphere including
birds
 They do indeed shift
and it can be seen
with a telescope
Copernicus’ Arguments
 Also addressed the issue of planets
appearing to move backwards.
 Proved that as the earth caught up to a
slower moving planet such as Mars or
Jupiter, it would appear to make the planet
go backwards as the earth went by.
 Heliocentric Theory is also called the
Copernican Theory because of his
advances.
Tycho Brahe
 Danish nobleman
 One of the most accurate astronomical
observers is history.
 He did not agree with Copernicus, but his
observations helped to establish the
Heliocentric theory as superior.
 Built an observatory on the Isle of Hven near
Copenhagen and studied for the next 20 years
the positions of stars and planets.
Johannes Kepler
 German astronomer, Outstanding
Christian and brilliant mathematician
 Joined Tycho’s staff in 1600 in Prague
after protestants were persecuted and
forced to leave Austria.
 He recognized the brilliance of Tycho’s
work.
 Devised three laws that described the
planetary motion.
Johannes Kepler
 First two laws published in a book “New
Astronomy” in 1609.
 Third law came in 1618.
 His major contribution to the Copernican
theory was his mathematical proof that
the orbits of the planets were actual
ellipitical.
 Very important that his work glorify God.

http://videos.howstuffworks.com/hsw/23314-the-history-of-astronomy-laws-ofplanetary-motion-video.htm
Galileo
 First to use a telescope for astronomy
 First to observe that Venus has phases
like the moon.
 First to observe Saturn’s rings, but he did
not identify them as rings.
 Discovered the four largest moons of
Jupiter.
 Jupiter’s moons helped solidify the sun
centered and earth rotating around it
theory
Galileo
 Galileo’s book Dialogue on the Two Chief
World Systems was banned by the Catholic
church.
 The church had accepted the geocentric
theory and centered their doctrine around it.
 He was tried before the Catholic Inquisition
and placed on house arrest for the rest of
his life.
Newton
 English mathematician and physicist that
described the force between the sun and
the celestial objects around it.
 Was inspired to formulate the law of
gravity by an apple falling from a tree.
 Described that the more mass an object
has the more force it exerts on other
objects.
Newton
 Law of Universal Gravitation – Any two bodies
attract each other with a force proportional to
the product of their masses and inversely
proportional to the square of the distance
between them.
 Gravity provides the force that holds the
planets in orbit around the sun and the moons
around the planets.
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http://videos.howstuffworks.com/hsw/19121-exploring-gravity-the-law-of-universalgravitation-video.htm
Foucault
 French physicist who determined the speed of
light.
 Hung a pendulum in the Pantheon to prove
that the earth rotates.
 The swing of the pendulum changed as the
earth rotated.
 http://www.classzone.com/books/earth_science
/terc/content/visualizations/es0403/es0403pag
e01.cfm?chapter_no=visualization
Aquinas
 Italian philosopher
 Thought he could reconcile the natural
philosophy and thinking of the ancient
pagans with Christianity.
Chapter 2 part B
Evidence That the Earth
Moves
 Two motions:
 Rotates – spins around an imaginary line
(axis)
 Revolves – moves around the sun in its orbit
 Both motions can be proven from
observations
Evidence of Rotation
 The sun, moon, planets, and stars rise in
the east and set in the west.
 Northern stars move counterclockwise in
circles centered on the North Star
Earth’s Shape
 Earth’s Shape – slightly flattened at the
poles and bulging at the equator; 7928
miles around equator but 7901 miles
around the poles.
 Jupiter and Saturn spin the fastest and
show this effect the greatest; can see
through a telescope that these planets
are not round.
Earth’s Shape
 Mercury and Venus, the slowest spinning
planets appear the most spherical.
 Inertia – the tendency of moving matter
to continue moving in the same direction.
 The planet’s surface is moving fastest at
the equator which explains the bulge at
the equator.
Spinning Demo
Wind Patterns and
Projectile Paths
 Wind patterns would be much simpler if
the earth did not rotate; the warm air from
equator would move toward poles and
cooler air from higher latitudes would
move toward equator; straight north and
south.
 Coriolis effect – tendency for objects
moving in straight path to curve
 Surface of the earth moves at different
speeds at different latitudes.
Foucault Pendulum
 Understood inertia – an moving object
continues moving in the same direction
unless acted upon by an outside force.
 At the poles the earth turns at a rate of
15° per hour, 90° per six hours, 360° in
24 hours
 At the equator the you would not see the
degree changes because the earth is
moving with the point of attachement.
Direct Observation
 Astronauts on the moon saw the entire
surface of the earth within 24 hours.
Since the moon does not revolve around
the earth each day it could only mean
that the earth was rotating.
Evidence of the Earth’s
Revolution
 Evidence that supports both theories
includes the sun’s apparent path through
the stars and the earth’s seasons.
Sun’s APPARENT Motion Ecliptic
 When looking at an object in front of a
moving background it can appear that the
object is the one moving.
 The sun appears to be moving when you
look at the sun with the stars in the
background.
 Really the stars are the objects moving;
they rise approximately 4 minutes earlier
each day.
Seasons
 Why do we not experience summer when
the earth is closest to the sun?
 The tilt of the earth’s axis!
 Three reasons why summer is warmer:
 Day is longer so heat is absorbed longer
 Sun rays are more concentrated; absorb
more energy per square kilometer
 Higher angle sun rays lose less energy to
the atmosphere as the travel a shorter
distance
Solstices and Equinoxes
 Summer Solstice – June 21 –
Sun is directly over the Tropic
of Cancer; sun’s apparent
northward motion stops;
appears to move southward.
 Winter Solstice – December
21 – Sun is directly over the
Tropic of Capricorn; sun’s
apparent southward motion
stops; appears to move
northward.
Solstices and Equinoxes
 Vernal Equinox - ~March
21 – The sun is directly
over the equator.
 Autumnal Equinox - ~
September 22 – The
Sun is directly over the
equator.
 These dates can
fluctuate by a day
because of leap year.
Stellar Parallax
 Stellar – means stars
 Parallax – the
apparent shift of
nearby stars among
farther stars as the
earth’s position
around the sun
changes.
 Eye Demo:
Stellar Parallax
 Wilhelm Bessel first discovered a star’s
parallax in 1838.
 The closer the star the less parallax is
observable. The first star observed for
parallax was 67 trillion miles away.
Meteoric Evidence
 The meteors are generally brighter and
more plentiful after midnight.
 The observer is on the leading side of the earth
as it races along its orbit, logically causing more
collisions on the “front” side of the earth.
 If the earth were stationary there wouldn’t be an
explanation that could satisfy this observation.
Calendars
 The first recording of a time reference was
Genesis 1:5, “The evening and the morning
were the first day”
 Lunar – totally based on the moon’s phases
(Muslims)
 Lunisolar – based on both the moon’s phases
and the solar year (Hebrew and Chinese)
 Solar (Julian) – based only on the sun. Used
for all national governments.
 Gregorian – Currently used
Units of Time
 Day – 24 hours
 Week – 7 days – the most scheduling
done in weekly references
 Month – 30 or 31 days (exception
February)
 Season – 3 months – spring, summer,
autumn, winter
 Year – 12 months or 365 ¼ days