Eclipses, Tides & a big Whack –The Earth-Moon System

An Earth-Week Lecture

April 16, 2007

Dr. Uwe Trittmann

Welcome to

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: ¼Earth/Moon mass: 1/81Earth-Moon distance: 384,000 km

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

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

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)

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 moleculesfilter 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

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

SANDSTORM LEAVING NORTH AFRICA TOWARDS THE ATLANTIC – CANARY ISLAND.

The Surfaces of Earth & Moon

GIBRALTAR STRAITS

NIGHT ARRIVES BETWEEN EUROPE & AFRICA

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

RED SEA

Earth at night (computer processed – it’s never dark all over the Earth)

Grande São Paulo

Rio de Janeiro

Belo Horizonte

Salvador

Atlantic Ocean

Brazilian Continental Platform.

NIGHTFALL IN BRAZIL

NIGHTFALL IN USA

Q: During which season wasthis picture taken?

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

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

2

3

4

5

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

• Coma-Virgo Cluster

• Globular Star Clusters– M3, M5

East

Virgo and

Comawith the

Virgo-Coma galaxy cluster

Virgo-Coma

Cluster

• Lots of galaxies within a few degrees

M87, M88 and M91

East

– Hercules– Corona

Borealis– Bootes

Globular Star Clusters:• M 3• 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.