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The Puzzle of Planetary Motion
Finding Earth’s place in the Universe
versus
Observing the Planets
Five of the planets are bright enough to be seen by the unaided eye.
This view shows the sky after sunset on April 23, 2002.
This alignment of all 5 visible planets will not occur again until 2040. (Sorry, no truth to the 2012 hype!)
Observing the Planets
When and where we see the planets in the sky depends on their orbital positions relative to Earth.
Mercury & Venus are closer to the Sun than Earth and move faster in their orbits.
Mars, Jupiter, & Saturn are farther from the Sun and move more slowly.
Mars
Venus
Earth
Sun
Venus
Mars
Venus
MarsObserving the Planets
Earth
Sun
Several traditional terms related to the planets:
Inferior conjunction = planet aligns in front of Sun
Superior conjunction = planet aligns behind Sun
Opposition = planet appears opposite the Sun
Greatest Elongation = planet appears most separated from Sun
At what time does Venus risewhen it is in conjunction with the Sun?
A. Around sunset
B. Around midnight
C. Around sunrise
D. Around noon
At what time does Mars rise when it is at opposition?
A. Around sunset
B. Around midnight
C. Around sunrise
D. Around noon
Can you ever see Jupiter in the western skyin the early evening?
A. Yes, when Jupiter is closest to Earth.
B. Yes, when Jupiter is far from Earth.
C. No, Jupiter can never appear close to the Sun.
Can you ever see Venus in the eastern skyin the early evening?
A. Yes, when Venus is closest to Earth.
B. Yes, when Venus is farthest from Earth.
C. No, Venus can never appear far from the Sun.
At what time of day/night is Mercurynever above the horizon?
A. Around noon
B. Just after sunset
C. Around midnight
D. Just before sunrise
Claudius Ptolemaeus, PtolemyPtolemy was a Greek astronomer living in
Alexandria, Egypt in the 2nd century A.D.His huge publication, the Mathematical Syntaxis,
was published around 150 AD. He attempted to consolidate all of Greek astronomy and develop a more accurate theory to describe the motion of the planets.
While he didn’t invent an Earth-centered model, Ptolemy refined the idea and added details. He chose to explain planetary retrograde motion with an added motion: epicycles.
Ptolemy described the motion of the planets as due to a smaller circle riding along on a larger circle.
As the smaller circle turned, it would cause the planet to appear to move backwards on the sky (retrograde motion).
The smaller circles were called epicycles.
Ptolemy’s Model
Earth’s Place in the UniverseWhy place Earth at the center?Reason #1: No one can feel Earth rotating on its
axis or orbiting the Sun, so the natural conclusion was that Earth is stationary.
Reason #2: Earth’s motion should affect how we viewed the stars.
For example, hold your finger out in front of your face at arm’s length and look at your finger through each eye separately.
What do you notice?
Parallax of the Stars
The star’s location should shift back and forth relative to the background stars!
Now imagine viewing a nearby star from opposite sides of Earth’s orbit.
No Parallax? Earth Must Not Move.
In the ancient world view, the Universe was small so the stars were nearby.
To them, parallax should be easily visible to the unaided eye. But a parallax motion could not be observed for any star in the night sky.
This negative result was taken as proof by early astronomers that Earth doesn’t move!
Parallax Does ExistThe problem was early astronomers could not
conceive of the vast distances between stars.
Even the closest stars have very small parallaxes that require powerful telescopes to see.
A stellar parallax was first successfully measured in 1838 by Friedrich Bessel in Berlin, Germany.
Parallax is used by modern astronomers to measure the distances to nearby stars.
Ancient astronomers acceptedEarth was stationary because:
A. their religious beliefs were stronger than those of modern astronomers.
B. their observations of the natural world indicated a motionless Earth.
C. they were ignorant and didn’t know any better.
Nicolas Coppernic (1473-1543)
Copernicus studied astronomy and grew to question the underlying Ptolemaic theory.
While the overthrow of the Ptolemaic theory is named for Nicolas Coppernic, the idea that the Sun lies at the center of the Solar System dates back at least to Aristarchus of Samos in the 3rd century BC.
Copernican Retrograde Motion
Copernican model:
Planets closer to the Sun move faster than planets farther from the Sun.
All planets orbit Sun on circular orbits.
Copernicus worked decades before the invention of the telescope, so he had no observational evidence to support his theory. It was simply a logical replacement to Ptolemy’s theory.
On the Revolutions of the Heavenly Spheres was published just after his death in 1543 and only became widely read years later.
Copernicus’ Legacy
A Powerful New ToolAround 1608, Dutch spectacle-makers invented
the telescope by combining the power of two lenses held apart within a tube.
A telescope produces a magnified image of a distant group of objects.
This new instrument became an instant sensation and revolutionized human eyesight. Sailors, soldiers, surveyors, and explorers could now see farther than they could before.
The first person to use the new telescope for astronomy was an Italian named Galileo Galilei.
Galileo Galilei (1564-1642)Trained in medicine and mathematics, he became
a professor in Pisa, Italy.During this time he studied problems of physics.
While Galileo is best-known for his discoveries in astronomy, he also revolutionized our understanding of moving and falling objects.
While he was a professor, Galileo read of the Copernican theory and thought it was correct.
However, he was also aware of the controversysurrounding the Copernican model and made no effort to support the idea publicly.
New Observations of the SkyIt was his discoveries with the telescope that
made Galileo an vocal supporter of the Copernican theory.
After less than six months of observing the night sky, Galileo made 3 staggering discoveries. He described these discoveries in a small book called The Starry Messenger, published in 1610.
The 3 discoveries were:• The Moon has mountains and valleys• The Milky Way is made up of countless stars• Jupiter was accompanied by four companions
Jupiter’s FamilyGalileo was amazed when he turned his telescope to Jupiter on Jan. 7, 1610.
Rather than seeing just the planet, he also noticed 3 small “stars” next to Jupiter.
During the next week, Galileo saw a fourth “star” and realized that all four were always found next to Jupiter: they were moons.
A Modern ViewHere is a view of Jupiter and its moons using a
modern telescope and camera:
New DiscoveriesThe publication of The Starry Messenger made
Galileo famous across Europe and guaranteed him a lifetime position at the University of Pisa.
Also in 1610, Galileo observed:• Saturn - discovering giant moons on either side
of the planet (now identified as the rings)• the Sun - discovering that it rotated and had
imperfections that we now call sunspots• Venus - discovering a range of phases
Phases of Venus photographed in 2004
Photos by:
Statis Kalyvasof Greece
In the Ptolemaic model of the Solar System:
A. The planets orbited the Sun.
B. The planets and Sun orbited Earth.
C. The planets remained stationary while Earth and the Sun orbited one another.
In the Copernican model of the Solar System:
A. The planets orbited the Sun.
B. The planets and Sun orbited Earth.
C. The planets remained stationary while Earth and the Sun orbited one another.
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