Activity 2 The Earth–Moon System - THUNDERRIDGE...

GoalsIn this activity you will:

• Investigate lunar phases using amodel and observations in yourcommunity.

• Investigate the general idea of tidalforces.

• Understand the role of the Earth,the Moon, and the Sun in creatingtides on Earth.

• Understand the Earth–Moon systemand the Moon’s likely origin.

• Compare the appearance of theMoon to other solar-system bodies.

Think about It

Think about the last time that you gazed at a full Moon.

• What happened to make the Moon look the way it does?• What is the origin of the Moon? • How does the Moon affect the Earth?

What do you think? Record your ideas about these questions inyour EarthComm notebook. Be prepared to discuss yourresponses with your small group and the class.

Activity 2 The Earth–Moon System

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Earth System Evolution Astronomy

Ch 1 Astronomy 9/20/04 2:42 PM 4

Investigate

Part A: Lunar Phases

1. Attach a pencil to a white Styrofoam®

ball (at least 5 cm in diameter) bypushing the pencil into the foam. Setup a light source on one side of theroom. Use a lamp with a bright bulb(150-W) without a lampshade or havea partner hold a flashlight pointed inyour direction. Close the shades andturn off the overhead lights.

2. Stand approximately 2 m in front ofthe light source. Hold the pencil andball at arm’s length away with yourarm extended towards the lightsource. The ball represents the Moon.The light source is the Sun. You arestanding in the place of Earth.

a) How much of the illuminated Moonsurface is visible from Earth? Drawa sketch of you, the light source,and the foam ball to explain this.

3. Keeping the ball straight in front ofyou, turn 45° to your left but staystanding in one place.

a) How much of the illuminatedMoon surface is visible from Earth?

b) Has the amount of lightilluminating the Moon changed?

c) Which side of the Moon isilluminated? Which side of theMoon is still dark? Draw anotherdiagram in your notebook of thefoam ball, you, and the light sourcein order to explain what you see.

4. Continue rotating counterclockwiseaway from the light source whileholding the ball directly in front ofyou. Observe how the illuminatedportion of the Moon changes shapeas you turn 45° each time.

a) After you pass the full Moonphase, which side of the Moon isilluminated? Which side of theMoon is dark?

b) How would the Moon phasesappear from Earth if the Moonrotated in the opposite direction?

Part B: Observing the Moon

1. Observe the Moon for a period of atleast four weeks. During this timeyou will notice that the apparentshape of the Moon changes.

a) Construct a calendar chart torecord what you see and whenyou see it. Sketch the Moon, alongwith any obvious surface featuresthat you can see with the nakedeye or binoculars.

b) Do you always see the Moon inthe night sky?

c) How many days does it take to gothrough a cycle of changes?

d) What kinds of surface features doyou see on the Moon?

e) Label each phase of the Mooncorrectly and explain briefly thepositions of the Sun, the Earth,and the Moon during each phase.

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Activity 2 The Earth-Moon System

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Tell an adult before you go outside toobserve the Moon.

Be careful not to poke the sharp end ofthe pencil into your skin while pushing thepencil into the foam. Use caution around

the light source. It is hot. Do not touch theStyrofoam to the light.

Ch 1 Astronomy 9/20/04 2:42 PM Page 15

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Part C: Tides and Lunar Phases1. Investigate the relationship between

tides and phases of the Moon.

a) On a sheet of graph paper, plotthe high tides for each city andeach day in January shown inTable 1. To prepare the graph,look at the data to find the rangeof values. This will help you planthe scales for the vertical axis (tideheight) and horizontal axis (date).

b) On the same graph, plot theMoon phase using a bold line.Moon phases were assigned valuesthat range from zero (new Moon)to four (full Moon).

2. Repeat this process for low tides.

3. Answer the following questions inyour EarthComm notebook:

a) What relationships exist betweenhigh tides and phases of theMoon?

b) What relationships exist betweenlow tides and phases of theMoon?

c) Summarize your ideas about howthe Moon affects the tides. Recordyour ideas in your EarthCommnotebook.

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Table 1 Heights of High and Low Tides in Five Coastal Locations during January 2001 (All heights are in feet.)

Breakwater, Savannah, Portland, Cape New Delaware Georgia Maine Hatteras, London,

North ConnecticutCarolina

Date Moon Moon High Low High Low High Low High Low High LowPhase Phase

1/3/01 First 2 3.6 0.2 7.3 0.5 8.5 1 2.6 0.2 2.4 0.3Quarter

1/6/01 Waxing 3 4.5 0 8.2 0.5 9.7 0.1 3.4 –0.4 3 –0.2Gibbous

1/10/01 Full Moon 4 5.6 –0.9 9.4 –1.5 11.6 –1.9 4.2 –0.8 3.5 –0.7

1/13/01 Waning 3 5.1 –0.7 8.8 –0.9 11 –1.4 3.7 –0.6 3 –0.5Gibbous

1/16/01 Last 2 4.1 –0.1 7.9 –0.2 9.7 0.1 3 –0.2 2.7 0Quarter

1/20/01 Waning 1 4.3 0.1 7.3 0.2 9.4 0.2 3.2 0 2.8 0Crescent

1/24/01 New 0 4.6 0 8.1 –0.1 9.7 –0.1 3.3 –0.1 2.8 –0.1Moon

1/30/01 Waxing 1 3.7 0.1 7.4 0.1 8.7 0.6 2.6 0 2.4 0.2Crescent


4. Table 1 shows data from the monthof January 2001. At the EarthCommweb site, you can obtain tidal dataduring the same period that you aredoing your Moon observations. Selectseveral cities nearest your community.

a) Record the highest high tide and thelowest low tide data for each city.Choose at least eight different daysto compare. Correlate these recordsto the appearance of the Moonduring your observation period.Make a table like Table 1 showinghigh and low tides for each location.

b) What do you notice about thecorrelation between high and lowtides and the appearance of theMoon?

Part D: Tidal Forces and the EarthSystem1. Use the data in Table 2.

a) Plot this data on graph paper.

Label the vertical axis “Numberof Days in a Year” and thehorizontal axis “Years beforePresent.” Give your graph a title.

b) Calculate the rate of decrease in the number of days per 100 million years (that is,calculate the slope of the line).

2. Answer the following questions:

a) How many fewer days are thereevery 10 million years? everymillion years?

b) Calculate the rate of decrease peryear.

c) Do you think that changes in thenumber of days in a year reflectchanges in the time it takes the Earthto orbit the Sun, or changes in thetime it takes the Earth to rotate on itsaxis? In other words, is a year gettingshorter, or are days getting longer?How would you test your idea?

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Table 2 Change in Rotation of Earth Due to Tidal Forces

Period Date (millions Length of Year of years ago) (days)

Precambrian 600 424

Cambrian 500 412

Ordovician 425 404

Silurian 405 402

Devonian 345 396

Mississippian 310 393

Pennsylvanian 280 390

Permian 230 385

Triassic 180 381

Jurassic 135 377

Cretaceous 65 371

Present 0 365.25


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In this activity you used a simplemodel and observations of the Moonto explore lunar phases and surfacecharacteristics of the Moon. You alsoexplored the relationship betweentides and the phases of the Moon.The tides also have an effect thatdecreases the number of days in ayear over time. That’s because tides

slow the rotation of the Earth,making each day longer. You nowunderstand that tides slow therotation of the Earth, and how thishas affected the Earth. This will beuseful when describing the Earth’sgravitational relationships with theMoon for the Chapter Challenge.

Reflecting on the Activity and the Challenge

Digging Deeper

THE EVOLUTION OF THE EARTH–MOON SYSTEM

The Formation of the Earth and Moon

You learned in theprevious activity thatduring the formationof the solar system,small fragments ofrocky material calledplanetesimals stucktogether in a processcalled accretion.Larger and largerpieces then cametogether to form theterrestrial planets.Theleftovers became theraw materials for theasteroids and comets.Eventually, much ofthis material was“swept up” by thenewborn innerplanets. Collisionsbetween the planetsand the leftover

planetesimals were common.This was how the Earth was born and lived itsearly life, but how was the Moon formed?

Figure 1 The Moon is the only natural satellite of Earth.

Geo Wordsaccretion: the processwhereby dust and gasaccumulated into larger bodieslike stars and planets.


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Scientists theorize that an object the size of Mars collided with and probablyshattered the early Earth.The remnants of this titanic collision formed a ringof debris around what was left of our planet. Eventually this materialaccreted into a giant satellite, which became the Moon. Creating anEarth–Moon system from such a collision is not easy. In computersimulations, the Moon sometimes gets thrown off as a separate planet orcollides with the Earth and is destroyed. However, scientists have createdaccurate models that predict the orbit and composition of both the Earthand Moon from a collision with a Mars-sized object.The Moon’s orbit (itsdistance from the Earth, and its speed of movement) became adjusted sothat the gravitational pull of the Earth is just offset by the centrifugal forcethat tends to make the Moon move off in a straight line rather than circlethe Earth. After the Earth–Moon system became stabilized, incomingplanetesimals continued to bombard the two bodies, causing impact craters.The Earth’s surface has evolved since then. Because the Earth is geologicallyan active place, very few craters remain.The Moon, however, is geologicallyinactive. Figure 2 shows the Moon’s pockmarked face that has preserved itsearly history of collisions.

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Figure 2 Impact craters on the Moon.


When the Earth was first formed, its day probably lasted only about sixhours. Over time, Earth days have been getting longer and longer. In otherwords, the Earth takes longer to make one full rotation on its axis. On theother hand, scientists have no reason to think that the time it takes for theEarth to make one complete revolution around the Sun has changed throughgeologic time.The result is that there are fewer and fewer days in a year, asyou saw in the Investigate section.Why is the Earth’s rotation slowingdown? It has to do with the gravitational forces between the Earth, theMoon, and the Sun, which create ocean tides.

Tides

The gravitational pullbetween the Earth and theMoon is strong.This forceactually stretches the solidEarth about 20 cm alongthe Earth–Moon line.Thisstretching is called theEarth tide.The water inthe oceans is stretched inthe same way.Thestretching effect in theoceans is greater than inthe solid Earth, becausewater flows more easilythan the rock in theEarth’s interior.Thesebulges in the oceans, calledthe ocean tide, are whatcreate the high and lowtides (see Figure 3). Itprobably will seem strange

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water surfaceN

S

A

second high tide

firsthigh tide

N

S

to Moon

B

Figure 3 Schematic diagram oftides. Diagram A illustrates howthe ocean surface would behavewithout the Moon and the Sun(no tides). Diagram B illustrates,that in the presence of the Moonand the Sun, shorelines awayfrom the poles experience twohigh tides and two low tides perday.


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to you that there are two bulges, one pointing toward the Moon and theother away from the Moon. If the tides are caused by the pull of the Moon,why is there not just one bulge pointing toward the Moon? The explanation isnot simple. If you are curious, you can pursue it further in the InquiringFurther section of this activity.

As the Earth rotates through a 24-h day, shorelines experience two hightides—one when the tidal bulge that points toward the Moon passes by,and once when the tidal bulge that points away from the Moon passes by.The tidal cycle is not exactly 24 h. By the time the Earth has completed onerotation (in 24 h), the Moon is in a slightly different place because it hastraveled along about 1/30 of the way in its orbit around the Earth in that24-h period.That’s why the Moon rises and sets about 50 min. later eachday, and why high and low tides are about 50 min. later each day. Becausethere are two high tides each day, each high tide is about 25 min. later thanthe previous one.

The gravitational pull of the Sun also affects tides. Even though it has muchgreater mass than the Moon, its tidal effect is not as great, because it is somuch farther away from the Earth.The Moon is only 386,400 km away fromthe Earth, whereas the Sun is nearly 150,000,000 km away.The Moon exerts2.4 times more tide-producing force on the Earth than the Sun does.Thechanging relative positions of the Sun, the Moon, and the Earth causevariations in high and low tides.

The lunar phase that occurs when the Sun and the Moon are both on thesame side of the Earth is called the new Moon.At a new Moon, the Moon isin the same direction as the Sun and the Sun and Moon rise together in thesky.The tidal pull of the Sun and the Moon are adding together, and hightides are even higher than usual, and low tides are even lower than usual.These tides are called spring tides (see Figure 4). Don’t be confused by thisuse of the word “spring.” The spring tides have nothing to do with the springseason of the year! Spring tides happen when the Sun and Moon are ingeneral alignment and raising larger tides.This also happens at another lunarphase: the full Moon.At full Moon, the Moon and Sun are on opposite sidesof the Earth.When the Sun is setting, the full Moon is rising.When the Sunis rising, the full Moon is setting. Spring tides also occur during a full Moon,when the Sun and the Moon are on opposite sides of the Earth.Therefore,spring tides occur twice a month at both the new-Moon and full-Moonphases.

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Geo Wordsspring tide: the tides ofincreased range occurringsemimonthly near the times offull Moon and new Moon.


When the line between theEarth and Sun makes a rightangle with the line from theEarth to the Moon, as shown inFigure 4, their tidal effects tendto counteract one another.Atthose times, high tides are lowerthan usual and low tides arehigher than usual.These tides arecalled neap tides.They occurduring first quarter and third-quarter Moons.As with springtides, neap tides occur twice amonth.

The tide is like a kind of oceanwave.The high and low tidestravel around the Earth onceevery tidal cycle, that is, twiceper day.This wave lags behindthe Earth’s rotation, because it isforced by the Moon to travelfaster than it would if it werefree to move on its own.That’s

why the time of high tide generally does not coincide with the time that theMoon is directly overhead.The friction of this lag gradually slows the Earth’srotation.

Another way to look at tides is that the tidal bulges are always located on thesides of the Earth that point toward and away from the Moon, while the Earthwith its landmasses is rotating below the bulges. Each time land on the spinningEarth encounters a tidal bulge there is a high tide at that location.The mass ofwater in the tidal bulge acts a little like a giant brake shoe encircling the Earth.Each time the bulge of water hits a landmass, energy is lost by friction.Thewater heats up slightly. (This is in addition to the energy lost by waves hittingthe shore, which also heats the water by a small fraction of a degree.) Overlong periods of time, the tidal bulge has the effect of slowing down the rotationof the Earth, and actually causing the Moon to move away from the Earth.Thecurrent rate of motion of the Moon away from the Earth is a few centimeters ayear.This has been established by bouncing laser beams off of reflectors on theMoon to measure its distance.Although the Moon’s orbit is not circular and iscomplex in its shape, measurements over many years have established that the

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Geo Wordsneap tide: the tides ofdecreased range occurringsemimonthly near the times ofthe first and last quarter of theMoon.

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A. Spring Tide

B. Neap Tide

new Moonfull Moon

third quarter Moon

first quarter Moon

Earth

EarthSun

Sun

Figure 4 Schematic diagrams illustrating spring andneap tides.


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Moon is indeed moving away from the Earth. Special super-accurate clocks havealso established that the day is gradually becoming slightly longer as well,because of this same phenomenon, called “tidal friction.” The day (one rotationof the Earth on its axis) has gradually become longer over geologic time.As theEarth system evolves, cycles change as well.

In this activity you limited the factors that cause and control the tides to theastronomical forces.These factors play only one part.The continents andtheir different shapes and ocean basins also play a large role in shaping thenature of the tides. Although many places on Earth have two high tides andtwo low tides every day (a semidiurnal tide), some places experience onlyone high tide and one low tide every day (a diurnal tide).There are stillother places that have some combination of diurnal and semidiurnal tides(mixed tides). In these places (like along the west coast of the United States)there are two high tides and two low tides per day, but the heights of thesuccessive highs and lows are considerably different from one another.

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Figure 5 How do tides affect coastal communities?

1. How did the Moonlikely form?

2. Describe the relativepositions of the Earth,the Moon, and the Sunfor a spring tide andfor a neap tide.

3. What effect have tideshad on the length of aday? Explain.

Check YourUnderstanding

Understanding and Applying What You Have Learned

1. Refer back to the graph of thechanging length of the day thatyou produced in the investigation.Think about the causes of tidalfriction and the eventual outcome

of tidal friction. Predict how longyou think the day will eventuallybe. Explain the reasoning for yourprediction.


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2. Think about the roles that the Sunand Moon play in causing theocean tides.

a) If the Earth had no Moon, howwould ocean tides be different?Explain your answer.

b) How would the ocean tides bedifferent if the Moon weretwice as close to the Earth as itis now?

c) What differences would therebe in the ocean tides if theMoon orbited the Earth half asfast as it does now?

3. Look at Figure 3B in in theDigging Deeper section. Pretendthat you are standing on ashoreline at the position of thedotted line. You stand there for 24 h and 50 min, observing thetides as they go up and down.

a) What differences would do younotice, if any, between the twohigh tides that day?

b) Redraw the diagram fromFigure 3B, only this time, makethe arrow to the Moon parallelto the Equator. Make sure youadjust the tidal bulge to reflectthis new position of the Moonrelative to the Earth. Whatdifferences would you now seebetween the two high tides thatday (assuming that you are stillat the same place)?

c) Every month, the Moon goesthrough a cycle in which itsorbit migrates from beingdirectly overhead south of theEquator to being directlyoverhead north of the Equatorand back again. To complicatethings, the maximum latitude atwhich the Moon is directly

overhead varies between about28.5° north and south, to about18.5° north and south (thisvariation is on a 16.8 yearcycle). How do you think themonthly cycle relates to therelative heights of successivehigh tides (or successive lowtides)?

4. Return to the tide tables for theocean shoreline that is nearest toyour community (your teachermay provide a copy of these toyou).

a) When is the next high tidegoing to occur? Find a calendarto determine the phase of theMoon. Figure out how tocombine these two pieces ofinformation to determinewhether this next high tide isthe bulge toward the Moon oraway from the Moon.

b) The tide tables also provide thepredicted height of the tides.Look down the table to see howmuch variation there is in thetide heights. Recalling that theSun also exerts tidal force onthe ocean water, try to draw apicture of the positions of theEarth, Moon, and Sun for:i) The highest high tide you

see on the tidal chart.

ii) The lowest high tide you seeon the tidal chart.

iii) The lowest low tide you seeon the tidal chart.

5. The questions below refer to yourinvestigation of lunar phases.

a) Explain why the Moon looksdifferent in the sky duringdifferent times of the month.

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Preparing for the Chapter Challenge

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Write several paragraphs explainingthe evolution of the Earth–Moonsystem, how mutual gravitationalattraction can affect a communitythrough the tides, and how the

changing length of the day couldsomeday affect the Earth system. Besure to support your positions withevidence.

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Inquiring Further

1. Tidal bulge

Use your school library or thelibrary of a nearby college oruniversity, or the Internet, toinvestigate the reason why the tidalbulge extends in the directionaway from the Moon as well as inthe direction toward the Moon.Why does the Earth have two tidalbulges instead of just one, on theside closest to the Moon?

2. Tidal forces throughout the solarsystem

Tidal forces are at workthroughout the solar system.Investigate how Jupiter’s tidalforces affect Jupiter’s MoonsEuropa, Io, Ganymede, andCallisto. Are tidal forces involvedwith Saturn’s rings? Write a shortreport explaining how tidalfriction is affecting these solar-system bodies.

3. Impact craters

Search for examples of impactcraters throughout the solarsystem. Do all the objects in thesolar system show evidence ofimpacts: the planets, the moons,and the asteroids? Are there anyimpact craters on the Earth, besidesthe Meteor Crater in Arizona?

b) What advantage is there toknowing the phases of theMoon? Who benefits from thisknowledge?

c) It takes 27.32166 days for theMoon to complete one orbitaround the Earth. The Moon

also takes 27.32166 days tocomplete the rotation about itsaxis. How does this explainwhy we see the same face of theMoon all the time?


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Activity 2 The Earth–Moon System - THUNDERRIDGE...

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