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Chapter 2 (2.3): The Earth/Moon/Sun
Topics
1.
2.
3.
4.
Moon phases
Eclipses: shadows and visualizing the earth/moon/sun
Solar calendars vs Lunar calendars.
Distances and angles
* Visualize in 3D!
* Ask “How do we know?”
1- Moon phases
• What determines the appearance of the moon?
– What is moonlight?
– Why does the moon rise in the east, set in the west?
– Why does the moon’s appearance change?
1- Moon phases
• What determines the appearance of the moon?
– What is moonlight? Reflected sunlight!
– Why does the moon rise in the east, set in the west?
– Why does the moon’s appearance change?
1- Moon phases
• What determines the appearance of the moon?
– What is moonlight? Reflected sunlight!
– Why does the moon rise in the east, set in the west? The earth spins!
– Why does the moon’s appearance change?
1- Moon phases
• What determines the appearance of the moon?
– What is moonlight? Reflected sunlight!
– Why does the moon rise in the east, set in the west? The earth spins!
– Why does the moon’s appearance change? Because it ORBITS the EARTH!
Background for i>clicker Quizzes: Siderial vs Solar Time
1. The earth orbits around the Sun and spins on its own axis in
the same sense (i.e. both clockwise or both anticlockwise)
•
A siderial day is defined as the time it takes the Earth to
make a complete spin on its axis relative to distant stars
•
A solar day is defined as the
time it takes the Earth to do
a complete spin on its axis
relative to the Sun
•
The length of a siderial day
is about 4 minutes shorter
than the length of a solar day
1- Moon phases
1- Moon phases
phases:
new
waxing crescent
first quarter
waxing gibbous
full
waning gibbous
third quarter
waning crescent
new
1- Moon phases
When does a FULL MOON rise?
When does a NEW MOON rise?
Does an astronomer on the moon see the
Earth “rise” or “set”?
Does he/she see phases of the Earth?
1- Moon phases
When does a FIRST quarter MOON rise?
When does a NEW MOON rise?
For the picture at right, what is the phase of the
Moon as seen from Earth?
1- Moon phases
When does a FIRST quarter MOON rise?
When does a NEW MOON rise?
For the picture at right, what is the phase of the
Moon as seen from Earth?
1- Moon phases
When does a FIRST quarter MOON rise?
When does a NEW MOON rise?
For the picture at right, what is the phase of the
Moon as seen from Earth? Crescent
The Earth/Moon/Sun
Topics
1. Eclipses: shadows and visualizing the earth/moon/sun
* Visualize in 3D!
2- Eclipses
• Lunar eclipse = Earth casts shadow on Moon (E between M & S)
• Solar eclipse = Moon casts shadow on Earth (M between E & S)
NOTE!
Earth, moon, and sun are rarely perfectly aligned!
2- Eclipses – Lunar
Partial vs Full shadows … the sun is not a dot!
• Full = Umbra
• Partial = Penumbra
(our vantage point for
these drawings is looking
DOWN on the last slide.)
2- Eclipses – Lunar
Total Lunar Eclipse, Jan 9/10, 2001
2- Eclipses – Lunar
• Why does the moon appear red during a full lunar eclipse?
• Blue light is scattered more efficiently Earth’s atmosphere
(Daytime sky looks blue.)
• Red light is scattered less efficiently (setting sun looks red)
And the path of light (all colors) is bent by mass (general relativity!)
2- Eclipses – Solar
• The Sun and moon are coincidentally same angular size when
seen from Earth.
2- Eclipses – Solar
• The Sun and moon are coincidentally roughly the same angular
size when seen from Earth.
Why are Solar Eclipses so
much rarer than Lunar eclipses?
2Eclipses
–
Solar
As day progresses, moon moves in between earth and sun..
Practice
questions
As day progresses, moon moves in between earth and sun..
During a solar eclipse:
A- The Earth’s shadow falls on the Sun
B- The Moon’s shadow falls on the Earth
C- The Sun’s shadow falls on the Moon
D- The Earth’s shadow falls on the Moon
E- The Earth stops turning
F- The moon falls out of the sky.
G- Birds fall from the sky
H- The Sun falls from the sky.
Practice
questions
As day progresses, moon moves in between earth and sun..
During a solar eclipse:
A- The Earth’s shadow falls on the Sun
(You can’t cast a shadow onto the light source.)
B- The Moon’s shadow falls on the Earth
Moon does cast a shadow on Earth.
C- The Sun’s shadow falls on the Moon
(The light source can’t cast a shadow of itself.)
D- The Earth’s shadow falls on the Moon
Earth casts a shadow on the Moon during a LUNAR eclipse.
E- The Earth stops turning
(I really hope not. What would stop it? What would restart it?)
F- The moon falls out of the sky.
(I really hope not…)
G- Birds fall from the sky
(Better get indoors!)
H- The Sun falls from the sky.
(Better find another planet to live on.)
2- Eclipses – Solar
Photo of eclipse from orbit
Solving the Mystery of Planetary Motion
(use of the Scientific Method)
* Visualize in 3D!
* Ask “How do we know?”
Kepler’s Three Laws of Planetary Motion
1.
2.
3.
4.
Observe / Question
Hypothesize / Explain
Predict
Test!
A heliocentric model where planets move on ellipses = excellent predictions
5.8_Planetary Orbit Simulator --Kepler's laws
Kepler’s Three Laws of Planetary Motion
Kepler’s 1st Law:
All planets have elliptical orbits w/ the sun at one focus.
(Eccentricity of Earth’s path = 1.7%… nearly perfect circle.)
Brief aside about ellipses:
The ellipse is completely defined by:
center, the eccentricity, and the length of the semi-major axis
The focii are just geometrically defined points.
The sun lies at one focus of elliptical orbit of each planet.
Brief aside about ellipses:
An ellipse is defined by:
center, the eccentricity, and the length of the semi-major axis
If eccentricity is 0… then the foci are at the center, and it’s a circle.
Kepler’s Three Laws of Planetary Motion
Keplers 2nd Law:
A planets sweeps out equal areas in equal times
(i.e. Moves fastest at perihelion and slowest at aphelion.)
5.8 Planetary orbit simulator kepler's 2nd law
Kepler’s Three Laws of Planetary Motion
Kepler’s 3rd Law:
The ratio of
(a planet’s average distance from the Sun)3 to (its orbital period)2
is a constant for all the planets.
distance3 = distance * distance * distance
(time to orbit)2 = (time to orbit) * (time to orbit)
A planet that is close to the Sun,
completes an orbit in a shorter period
of time than a planet that is farther
from the Sun.
Light and Energy
Topics
1. How light (=energy) and matter interact
How light (= energy) and matter interact
A. What is the structure of matter?
B. How is energy stored in atoms?
C. What is light?
How light (= energy) and matter interact
A. What is the structure of matter?
Atoms = Nucleus + Electron cloud
Nucleus contains protons (p) and neutrons(n)
Electrons (e) sort of “orbit” the nucleus
How light (= energy) and matter interact
A. What is the structure of matter?
Atoms = Nucleus + Electron cloud
Nucleus contains protons (p) and neutrons(n)
Electrons (e) sort of “orbit” the nucleus
• these particles have “charge”
electrons ….. -1 e- (defines a fundamental unit of charge)
protons ……. +1 eneutrons ………0 (neutral)
• a neutral atom has net charge = 0 (#p’s = #e’s)
How light (= energy) and matter interact
A. What is the structure of matter?
Atoms = Nucleus + Electron cloud
Nucleus contains protons (p) and neutrons(n)
Electrons (e) sort of “orbit” the nucleus
FEM

Kq q
 12 2
r
You won’t need to use this formula.
Just notice the similarity to gravitational Force.
q= charge
r = distance between
• like charges repel each other, opposite charges attract
• atoms will attract electrons until net charge = 0 (#p’s = #e’s) !
How light (= energy) and matter interact
A. What is the structure of matter?
• Atomic Number = # of protons in nucleus
• Atomic Mass Number = # of protons + neutrons
• Molecules: consist of two or more atoms (H2O, CO2)
How light (= energy) and matter interact
A. What is the structure of matter?
• Isotope: same # of protons but different # of neutrons. (4He, 3He)
How light (= energy) and matter interact
A. What is the structure of matter?
B. How is energy stored in atoms?
Excited Electron States
If there is a FORCE (gravitational or
electromagnetic) , there can be
STORED ENERGY.
STORE energy = go to high potential energy.
RELEASE energy = “fall” back down
Key point:
Ground State
The states available to electrons in atoms are
QUANTIZED
Electrons in an ATOM can only have “sit” at specific energy levels,
which are determined by the #n’s and #p’s in the nucleus..
How light (= energy) and matter interact
A. What is the structure of matter?
B. How is energy stored in atoms?
Energy level transitions:
The only allowed changes
in energy
for an electron while it is
still trapped in the atom
are those corresponding
to a transition between
energy levels
Not Allowed
Allowed
How light (= energy) and matter interact
A. What is the structure of matter?
B. How is energy stored in atoms?
C. What is light?
Light = energy (sunlight feels warm!)
Energy unit:
Joule
Flow of energy: Watt = 1 Joule / second
The flow of energy is the rate that energy is
… moving
… delivered to earth
example: rate that energy is used in a lightbulb
rate that energy (aka photons aka sunlight)
hits the earth from the sun
How light (= energy) and matter interact
A. What is the structure of matter?
B. How is energy stored in atoms?
C. What is light: photons or waves?
Light = energy
(sunlight feels warm. The light that hits your skin delivers energy!)
You can think of light as WAVE or as a PARTICLE :
“wave/particle duality”
a particle (i.e. a photon) --- because it acts like a “packet” of energy.
a wave --- because it moves like a wave moves (mathematically convenient)
How light (= energy) and matter interact
A. What is the structure of matter?
B. How is energy stored in atoms?
C. What is light: photons or waves?
“wave/particle duality”
How light (= energy) and matter interact
A. What is the structure of matter?
B. How is energy stored in atoms?
C. What is light: photons or waves?
•
A wave is a pattern of motion that can
carry energy without carrying matter along
with it
•
Wavelength = 
distance between two wave peaks
Frequency = f
number of times per second that a wave
vibrates up and down
Speed of light = ALWAYS the SAME
•
•
wave speed = wavelength x frequency
wave speed =  * f
How light (= energy) and matter interact
A. What is the structure of matter?
B. How is energy stored in atoms?
C. What is light: photons or waves?
A particle of light, a photon, is like an energy packet.
The energy carried by the photon is related to its wavelength and frequency.
photon’s energy:
Energy = [Constant] * frequency
……………goes UP if frequency goes up!
Energy = [another constant] * 1/ wavelength … goes DOWN if wavelength goes up
(longer distance between peaks)
How light (= energy) and matter interact
A. What is the structure of matter?
B. How is energy stored in atoms?
C. What is light: photons or waves?
Our eyes are photon detectors!
Different energy photons are perceived as different COLORS
Prisms bend the path of photons according to their energy.
White light contains a continuum of energies (wavelengths).
Electromagnetic Spectrum:
high energy
(break down molecules, damage DNA, release e- in metals)
Moderate energies (“visible” bandpass, aka “optical” bandpass)
(the amounts of energy that release electrons from atoms)
low energy
(Can “shake” e-’s in metals, causing current in antennae, receivers, etc)
(“radio waves” are are photons, NOT sound!)
f unit: hertz = #/sec = s-1
E=hf unit: eV (= 1.6e-19 J) or Joules
2- Line emission
• From atoms -- electrons release photons with only certain
energies
– Each chemical (# p’s) has a unique set of energy levels that its
electrons can occupy. (quantized energy levels!)
– Electrons can move between levels:
Get energy = absorb a
photon, move to a higher
level
Lose energy = emit a
photon, fall to a lower
energy level
2- Line emission
• From atoms -- electrons release photons with only certain
energies
– Each chemical (specific # of p’s) has a unique set of energy levels
that electrons in its atoms can occupy (quantized energy levels!)
– Electrons can move between levels
– Each chemical element has its own “fingerprint” of energy levels
2- Line emission
• From molecules
– have additional energy levels because they can vibrate and rotate
– This complicates their spectra… large numbers of vibrational and
rotational energy levels
Note different appearance
of single lines vs “bands” of
lines.
… and then what? …
What happens to the photon after the atom/molecule in material
releases it? Answer depends DENSITY
If the material is TRANSPARENT ?
then the photons can travel freely out of the matter.
What is happening in this picture?
What could I learn from the specific lines that I see?
… and then what? …
What happens to the photon after the atom/molecule in material
releases it? Answer depends DENSITY
If the material is OPAQUE?
then the photons bounce around, sharing their energy.
They end up with a “thermalized” distribution of energies.
An analogy for “thermalized” photons (energy):
a single runner (photon)
running down an empty street:
her speed is whatever she wants
moving down a crowded street:
she bounces into the crowd, her speed gets closer and closer to the
average speed of the people in the crowd.