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Welcome to
Eclipses, Tides & a big Whack –
The Earth-Moon System
An Earth-Week Lecture
April 16, 2007
Dr. Uwe Trittmann
Today’s Topics
• Eclipses, Tides & a big Whack –
The Earth-Moon System
• The Night Sky in April
On the Web
• To learn more about astronomy and physics at
Otterbein, please visit
– http://www.otterbein.edu/dept/PHYS/weitkamp.a
sp (Observatory)
– http://www.otterbein.edu/dept/PHYS/ (Physics
Dept.)
The Earth-Moon System
Earth/Moon radius: ¼
Earth/Moon mass: 1/81
Earth-Moon distance:
384,000 km
Features of the Earth & Moon
• Mass: Earth: 6  1024 kg
• Radius: Earth: 6400 km
• Density: Earth: 5500 kg/m3
Moon: 1/81 Earth’s
Moon: 1/4 Earth’s ra
Moon: 3300 kg/m3
– 5.5 times that of water
– About 2 times that of a rock
• Gravity: Earth: 9.8 m/s2
Moon: 1/6 Earth’s
gravity
(about the same as in water)
Motion of the Moon
• Moon shines not by its own light but by reflected
light of Sun
 Origin of the phases of the moon
• Moon revolves around the Earth
• period of revolution = 1
month
Phases of the Moon
Phases of the
Moon (cont’d)
• Moon rotates around
earth in one month
• Moon rotates around
itself in the same time
•  always shows us the
same side!
•  “dark side of the moon”
(not dark at all!)
Eclipses
• One celestial object hidden by other or in
the shadow of another
• Solar eclipse: sun hidden by the moon
• Lunar eclipse: moon in earth’s shadow (sun
hidden from moon by earth)
• Also: eclipses of Jupiter’s moons, etc.
• Most spectacular because moon and sun
appear to be the same size from earth
Solar Eclipses
•
•
•
•
Umbra – region of total shadow
Penumbra – region of partial shadow
Totality lasts only a few minutes!
Why isn’t there a solar eclipse every month?
Solar Eclipse
Solar Corona – Only seen during
Eclipses
Inside the
Umbra
•
•
•
•
It gets dark
Temperatures drop
Birds stop singing
Nature slows down
1
Annular Eclipse
2
3
If the moon
just isn’t big enough …
(Actually, it appears smaller,
since it is further away due its
eccentric orbit)
Lunar Eclipses
Moon moves into
earth’s
shadow…
…and out of it
(takes hours!)
Partial Eclipse
Not an
Eclipse !
How do we
know?
Towards Totality
Almost total…
Totality
Totality
Q: Why does the Moon look orange in
total lunar eclipses
Totality
Totality
A: Because the sky on Earth
is blue.
The Earth’s atmosphere’s
air molecules
filter out red & orange
frequencies by
scattering them into Earth’s
night side. This
“rest light” illuminates the
Moon (weakly).
Question
Why isn’t there
an eclipse
every month ?
Tides
• Daily fluctuations
in the ocean levels
• Two high and two
low tides per day
• A result of the
difference in
gravitational pull
from one side of
the Earth to the
other
• There are two
tidal bulges, i.e.
two high tides
per day, because:
The idea behind Tides
– Moon pulls
water towards it
on one side
– Earth is pulled
towards the
Moon, away
from the water
on the other side
Force of Moon on
water (strong)
Force of Moon on Earth (weaker)
Force of Moon on “back-water” (much weaker)
Spring and Neap Tides
• Tides especially
pronounced when
Sun and Moon
“work together”
• Same direction
 Spring tide
Other direction
 Neap tide
The Surfaces of Earth & Moon
SANDSTORM LEAVING NORTH
AFRICA TOWARDS THE ATLANTIC –
CANARY ISLAND.
GIBRALTAR STRAITS
NIGHT ARRIVES
BETWEEN EUROPE & AFRICA
Continental
England
Platform
Iceland
Atlantic
Ocean
France
Islas de la Madera y Azores
Spain
Italy
Already night time
here.
Canary Islands
ÁFRICA
Cabo Verde Island
RED SEA
Earth at night (computer processed – it’s never dark all over the Earth)
NIGHTFALL IN BRAZIL
Brazilian Continental
Platform.
Atlantic
Ocean
Belo Horizonte
Salvador
Rio de Janeiro
Grande São Paulo
Q: During which season was
this picture taken?
NIGHTFALL IN
USA
NIGHTFALL IN
USA
A: During summer in the northern
hemisphere, because it gets dark later the
more north you go!
Columbus
Still
daylight in
California.
Chicago
The biggest concentration of
lights (from top to bottom)
are the cities of Boston,
New York, Philadelphia and
Washington.
Dallas
Puerto Rico
Houston
Miami
• Core
Structure of the Earth
– Mostly iron and nickel
– Inner core solid, outer
core liquid
• Mantle
– Mostly basalt, a heavy
mineral containing iron
and magnesium
– Soft; can flow even
though it is solid rock
• Crust
– Solid surface layer;
“floats” on the mantle
•Density and temperature both increase with depth
Surface Features & Dynamics
• Continental plates drift apart
• hot magma comes to surface  Volcanism
Earth’s Changing Surface
Earth’s Atmosphere
• 78% Nitrogen,
21% Oxygen,
1% Other
• Troposphere –
region of weather
• Stratosphere –
stable and calm
• Ionosphere –
gases charged by
interaction with
radiation from
space
Ozone Layer (O3)
• Absorbs most UV
radiation from the
Sun
• Hole over
Antarctic
– Chlorofluorocarbons
(CFC’s) – released by
spray cans,
refrigerators
Greenhouse Effect
• Earth absorbs energy
from the Sun and
heats up
• Earth re-radiates the
absorbed energy in
the form of infrared
radiation
• The infrared radiation
is absorbed by carbon
dioxide and water
vapor in the
atmosphere
Magnetosphere
• Magnetic
north pole
about 7° west
of geographic
north pole
• Driven by
motion of
molten metals
in core
Motion of Charged Particles
• Charged particles
“trapped” by
magnetic fields
• Origin of the Van
Allen radiation belts
 The Earth’s
magnetic field
protects us from
harmful radiation!
Aurora Borealis
• Charged particles bump into air molecules and excite them
• Subsequently the molecule falls back into its ground state,
giving off radiation  the aurora!
Aurora Borealis from Space
• A cosmic particle
shower
The Moon: Large-Scale Features
• “Maria”
– Dark areas resembling
oceans
– Plains of solidified lava
– Part of the lunar mantle
– About 3.2–3.9 billion years
old
• Highlands (“Terrae”)
– Light-colored, resemble
continents
– The lunar crust
– More than 4 million years
old
The Moon
– Far Side
• Can be seen
by satellites
only
The Mountains of the Moon
• Especially well visible near the terminator
– the borderline between light and shadow
Moon from our Observatory I
• Through 8 inch telescope, 40 mm eyepiece, Sony
DSC 717 digital camera, December 31, 2003
Moon from our Observatory II
• Through 8 inch telescope, 40 mm eyepiece, Sony
DSC 717 digital camera, December 31, 2003
Moon from our Observatory III
• Through 8 inch telescope, 40 mm eyepiece, Sony
DSC 717 digital camera, December 31, 2003
The Moon - Touchdown
• Note the soft edges
of the crater
Erosion!
• Traces of the
Apollo lunar rover
Structure of the Moon
• Also consists
of crust, mantle
and core
• No
hydrosphere,
magnetosphere
or atmosphere
• Little seismic
action
Lunar Craters
• Old scars from
meteoroid impacts
• Lots of them; all
sizes
– Copernicus ~ 90
km across
– Reinhold ~ 40 km
across
– Also craters as
small as 0.01 mm!
(Almost) Catastrophic Impacts
•Orientale Basin
•Almost 1000 km
diameter
•A somewhat larger
impact body could have
destroyed the moon!
How They Form
Moon’s Changing Surface
Ages of the Earth and Moon
• Determined by radioactive dating
– Compare amount of radioactive material with amount of
decay product
– Useful isotopes:
• Uranium-238 (half-life 4.5 billion years)
• Uranium-235 (half-life 0.7 billion years)
• For shorter time scales, Carbon-14 (5730 years)
• Oldest surface rocks on Earth (Greenland, Labrador)
about 3.9 billion years old
– When rocks solidified
• Lunar highlands: 4.1–4.4 billion years old
– Rocks from lunar maria slightly younger, more recently
melted
• Meteorites: 4.5 billion years old
– Date to origin of solar system
Creation of the Earth-Moon system
1. Sister theory: Earth and Moon formed at same
time in the same part of the solar system (but they
have different compositions??)
2. Capture theory: Earth captured the Moon as it
passed by; need not have the same composition
(but gravitational capture is improbable)
3. Daughter or fission: spinning Earth threw off the
Moon (but how did it get to be spinning that fast?)
4. Impact theory: large body hits the (molten)
Earth and is absorbed; part of Earth's mantle is
knocked out. (Plausible: supported by computer
simulations; but there's no direct evidence!)
Impact (“Big Whack”) Theory
1
4
2
5
3
6
Giant Impact Theory
• A massive object sideswiped Earth 4.5 billion years
ago
• The collision scattered crustal debris that later
formed the Moon.
• New calculations show how big the object would
have to have been to strike Earth with sufficient force
to generate the volume of debris required to create
the Moon.
• Their computer modeling indicates that such an
object must have been at least 2 1/2 to 3 times the
mass of Mars.
The Night Sky in April
• Nights still long, but EDT => later observing!
• Spring constellations are up: Cancer, Leo, Big
Dipper
• Saturn dominates the evening, Jupiter early
morning.
Today
at
Noon
Sun at
meridian,
i.e.
exactly
south
10 PM
Typical
observing
hour,
early
April
Saturn
Zenith
Big Dipper
points to the
north pole
West
Perseus and
Auriga
with Plejades
and the
Double
Cluster
West
The Winter
Constellations
–
–
–
–
–
Orion
Taurus
Canis Major
Gemini
Canis Minor
South
• Spring
constellations:
– Leo
– Hydra
– Crater
– Sextans
East
• Canes
Venatici:
– M51
• ComaVirgo
Cluster
• Globular
Star
Clusters
– M3, M5
East
Virgo and
Coma
with the VirgoComa
galaxy
cluster
VirgoComa
Cluster
• Lots of
galaxies
within a
few
degrees
M87, M88
and M91
East
– Hercules
– Corona
Borealis
– Bootes
Globular Star
Clusters:
•M3
• M 13
• M 92
M13: Globular Cluster
Mark your Calendars!
• Next Starry Monday: May 7, 2005, 8 (!!!) pm
(this is a Monday
• Observing at Prairie Oaks Metro Park:
– Friday, April 27, 2007, 8:30 pm
– Friday, May 25, 2007, 9:00 pm
• Web pages:
– http://www.otterbein.edu/dept/PHYS/weitkamp.asp (Obs.)
– http://www.otterbein.edu/dept/PHYS/ (Physics Dept.)
)
Mark your Calendars II
•
•
•
•
Physics Coffee is every Wednesday, 3:30 pm
Open to the public, everyone welcome!
Location: across the hall, Science 256
Free coffee, cookies, etc.