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Lecture 4
• Eclipses
• Geocentric vs Heliocentric Theory
•The Nature of Scientific Theories
The Planets
Prof. Geoff Marcy
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Announcements
• Read Chapters 2 & 3 !
•Discussion Sections!
Homework Assignments:
- Assignment Chapters. 1&2: Due Fri (!) at 6pm
- Assignment Chapt. 3: Due Fri at 6pm
-Observation Project: Due this Thu. in class
Moonrise over Berkeley Hills: ~11pm
Tue, Wed, Thu 12-1: Disc. Sections Not full!
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Curiosity Landing on Mars
5 August 2012
Review of Last Lecture
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Last Time:
Useful Spherical Coordinate System
The Celestial Sphere
North Celestial Pole
Near the ”North Star”
• Stars appear “glued” to sphere
actually at different distances.
• Constellations: Apparent groupings
• North & South “Celestial poles”
• Celestial equator
• Ecliptic:
Sun’s path against the stars
= Orbital plane of Earth
• Celestial Sphere Rotates around us
every 24 hours: As Earth spins
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.
Motion of the Night Sky
as hours pass
1. From Berkeley
2. From the Equator
3. From the North Pole
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Berkeley Hills
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Moon: Going through phases
each 29.5 Days (one orbital period)
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If you stand on the Moon, does the Earth
go through “phases” ?
Yes !
When Earthlings see a crescent moon, what is the
phase of the Earth, as seen from the Moon?
a. New
b. Crescent
c. Quarter
d. Gibbous
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Section 1
Eclipses: Solar and Lunar
• What do they look like?
• Why do they happen?
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Eclipses
Solar Eclipse: Moon blocks the Sun
• Sun is behind the moon:
Only occurs at new moon
You are in the Moon’s shadow.
within umbra: total solar eclipse
within penumbra: partial solar eclipse
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2002 total Solar Eclipse
Ceduna, Australia Dec 4, 2002
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Solar Eclipse
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Solar Eclipse
1999 Aug 11 from the Russian Mir Space Station
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Eclipses
• The Earth & Moon cast
shadows.
• When either passes
through the other’s
shadow, we have an
eclipse.
• Why don’t we have an
eclipse every full & new
Moon?
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Sun
Total Solar Eclipse
Lusaka, Zambia 2001
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Solar Eclipse in India
24 October 1995
By: Solar Physicsts Wendy Carlos and Fred Espenak
India
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Solar Eclipse
May 20 2012
Just after Finals Last Spring
Sproul Plaza
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Solar Eclipse Predictions
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Total Solar Eclipse: 21 Aug 2017
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Solar Eclipse:
Aug 21, 2017
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Lunar Eclipses
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Section 2
A Model of the Solar System:
Geocentric vs Heliocentric
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We see only
one side of the Moon
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Why we always see
the same face of the Moon
Rotation period = orbital period
Not Correct Model
Correct Model
Moon
Earth
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Moon
Earth
model,
i.e. conceptual theory,
of the Solar System
Build a
Conceptual models must explain all
the motions of the planets:
i.e., the “data”. . .
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Planets: Moving relative to the Stars
Obvious to the eye
• Mercury
– Difficult to see; Always angularly close to Sun
• Venus
– Very bright. Always near Sun— morning or evening “star”
• Mars
– Noticeably orange. Usually moves west-to-east
– Sometimes backwards “retrograde” !
• Jupiter
– Very bright. Moves west-to-east against stars.
• Saturn
– Moderately bright. Moves more slowly west-to-east.
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Motion of the Planets Relative to the Earth
during several years
The Sun and planets
seem to orbit the earth
during months and
years .
Earth
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The Motion of Planets
• The Planets normally move West to East
against the background stars.
• Why do planets sometimes seem to move
backwards relative to the stars?
• Greeks concluded that the planets orbit the Earth.
Why did smart people conclude this?
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Observed Motion of Mars:
Normal & “Retrograde””
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To Explain Retrograde Motion
Two Models:
Geocentric
Heliocentric
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Ptolemy’s
Geocentric Model
• Earth is at center
• Sun orbits Earth
• Planets orbit on small
circles whose centers orbit
the Earth on larger circles
(The small circles are
called epicycles)
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In Heliocentric “model”
Retrograde Motion is a Natural Consequence
• Planets usually appear to move
eastward relative to the stars.
• But as we pass by them,
planets seem to move west
relative to the stars.
• Only noticeable over many
nights; on a single night, a
planet rises in east and sets in
west…
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Two Theories::
Earth-Centered Theory
Sun-Centered Theory
Which theory
Seems
“Best” ?
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Choosing the Best Model:
Explaining Retrograde Motion
• Natural result of Heliocentric Model
• Difficult to explain if Earth is at center
The Best “Model” or “Theory” explains
various data and phenomena with the fewest
assumptions.
“Occam’s Razor “: Choose the simplest model that
explains all the data.
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3D “model” of the Solar System
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Why did the Greeks reject the theory
that the Earth orbits the Sun?
• It ran contrary to their common sense:
Every day, the sun, moon, and stars rotates
around us. So, we “must be” at the center . . .
• If the Earth rotated, then there should be a
“great wind” as we moved through the air.
• Greeks knew that we should see stellar
“parallax” if we orbited the Sun – but they
could not detect it.
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Test the Theory that
Earth orbits the Sun:
Parallax:
Apparent shift of a star’s position due to
the Earth’s orbiting of the Sun.
Greeks didn’t
detect parallax
!
The nearest stars are
much farther away than
the Greeks thought.
The parallax angles of
the stars are so small,
that you need a telescope
to observe them.
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Two Possible reasons why
stellar parallax was not detected:
1. Stars are so far away that stellar parallax
is too small for naked eye to notice.
2. Earth does not orbit Sun; it is the center
of the universe.
Debate about theory:
Earth-centered vs. Sun-centered Planetary System.
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Section 3
Scientific Theory:
What is a good “Theory” ?
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Scientific Thinking
• It is a natural part of human curiosity:
Search for understanding and truths
that explain many facts.
• We draw conclusions based on our experiences.
• Progress is made through “trial and error.”
Hypothesize. Then test your hypothesis.
Eating pasta makes me get fat . . .
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Nicolaus Copernicus (1473-1543)
He thought Polemy’s
model was contrived
Yet he believed in
circular motion
De Revolutionibus
Orbium Coelestium
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Copernicus’ Heliocentric Model
•Sun
is at center
•Earth orbits like any other planet
•Inferior planet orbits are smaller
•Retrograde motion occurs when we “lap”
Mars & the other superior planets
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Johannes Kepler (1571-1630)
• Greatest theorist of
his day
• Imagined planets on
“heavenly spheres”
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Kepler’s Laws
1. Each planet’s orbit around the Sun is an
ellipse, with the Sun at one focus.
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Eccentricity of an Ellipse
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Kepler’s 2nd Law
A planet moves along its orbit with a speed that
changes in such a way that a line from the
planet to the Sun sweeps out equal areas in
equal intervals of time.
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Kepler’s 3rd Law
The cube of a planet’s average distance from the
Sun is equal to the square of its orbital period.
(Use units of years and AUs.)
3
a =P
2
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