Post on 18-Dec-2015
Eclipses, Tides & a big Whack –The Earth-Moon System
An Earth-Week Lecture
April 16, 2007
Dr. Uwe Trittmann
Welcome to
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.)
Features of the Earth & Moon
• Mass: Earth: 6 1024 kg Moon: 1/81 Earth’s• Radius: Earth: 6400 km Moon: 1/4 Earth’s ra• Density: Earth: 5500 kg/m3 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 (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?
Annular Eclipse1
3
2
If the moon just isn’t big enough …(Actually, it appears smaller, since it is further away due its eccentric orbit)
Totality
Totality
A: Because the sky on Earth is blue. The Earth’s atmosphere’s air moleculesfilter out red & orange frequencies by scattering them into Earth’s night side. This“rest light” illuminates the Moon (weakly).
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
The idea behind Tides• There are two tidal
bulges, i.e. two high tides per day, because:– 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
France
Iceland
Italy
Continental Continental
PlatformPlatformEngland
ÁFRICA
Already night time here.
Spain
Atlantic
Ocean
Cabo Verde Island
Canary IslandsCanary Islands
Islas de la Madera y Azores
Grande São Paulo
Rio de Janeiro
Belo Horizonte
Salvador
Atlantic Ocean
Brazilian Continental Platform.
NIGHTFALL IN BRAZIL
NIGHTFALL IN USA
A: During summer in the northern hemisphere, because it gets dark later the more north you go!
The biggest concentration of lights (from top to bottom) are the cities of Boston, New York, Philadelphia and Washington.
MiamiHouston
Dallas
Chicago
Still daylight in California.
Puerto Rico
Columbus
Structure of the Earth• Core– 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
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!
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 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!
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!)
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.
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.)