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Stoev1r, Edward, C., Si.Measuring Continental Drift: The Laser RangingExperiment. Crustal Evolution Education Project.Teacher's Guide [and] Student Investigation.National Association of Geology Teachers.National Science-FOundation, Washington, D.C.CEEP-MOD-TX6-7-5; ISBN-0-89873-036-8;ISBN-0-89873-0377679SED-75-20151; SED-77-08539; SED-78-2510416p.Ward's Natural Science Establishment, Inc., P.O. Box1712, Rochester, NY 14603 (or P.O. Box 1749,Monterey, CA 93940.)

MF01Plus Postage. PC Not Available from EDRS.*Earth Science; Geology; Instructional Materials;Measurement; *Motion; *Science Activities; *ScienceCoatse Improvement Projects; Sci'ence Curriculum;Science Education; Science Instruction; SecondaryEducb.tion; *Secondary School Science; Te.achingGuides; Teaching Methods

IDENTIFIERS 1, Continental Drift; *Crustal Evolution EducationProject; Earthquakes; National Science Foundation;*Plate Tectonis

ABSTRACTCrustal Evolution Education Project (CEEP)

were designed to: (1) provide students-with the methods and resultsof continuing investigations into the composition, history, andprocesses of the earths crust and the application of this knowledgeto man's activities and (2) to be used by.teachers with little or.noprevious background in the modern theories of sea-floor spreading,continentl,ldrift, and plate tectonics. Each module consists of twobooklets: a teacher's guide and student investigation. The teacher'sguide contains all'of the information present in the studentinvestigation booklet as well as: (1) a general introduction; (2)prerequisite student background; (3) objectives; (4) list.of required'materials; (5) background information; (6) suggested approach; (7)procedure, recommending two 45-minute class periods; (8) summaryquestions, (with answers); (9) extension activities; and 110) list ofreferences. The activity in this module (requiring knowledge ofPythagorean theorem) is designed to answersuch questions' as: (1) Arecrustal plates moving ?'(2) How fast and in what direction do theymove? (3) Are motions constant? (4) Do plates stretch? (5) Does anentire plate move as a unit? (6) How is e earth's rotation linkedto plate motions? (Author/JN)

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MeasuringContine-`al Drift:

The LoserRanging Experime-A

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s TEACHER'S GUIDECatalog No. 34W1019

For use with Student Investigation 34W1119Class time: two 45-Minute periods

Developed*THE NATIONAL ASSOCIATION OF GEOLOGY TEACHERS

Produced and Distributed byWard's Natural Science Establishment, Inc. Rochester, NY Monterey, CA

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NAGT Crustal Evolution Education ProjectEdward C. Stoever, Jr., Project Director

Welcome to the exciting world of current researchinto the composition. history and processes of theearth s crust and the application of this knowledgeto man s activities The earth sciences arecurrently experiencing a dramatic revolution inour understanding of the way in which the earthworks CEEP modules are designed to bring intothe classroom the methods and results of thesecontinuing investigations The Crustal EvolutionEducation Project began work in 1974 underthe auspices of the Natidnal Association ofGeology Teachers CEEP materials have beendeveloped by teams of science educato,rs,classroom teachers, and scientists Prior topublicatiqn the materials were field tested bymore than 200 teachers and over 12,000 students

Current crustal evolution research is a breakingstory that students are hying through today

About CEEP Modules ...Most CEEP rj-iodules consist of two booklets aTeacher's Guide and a Student Investgation TheTeacher's Guide, contains all the infieFmationand illustrations in the Student Investigation,'plus sections printed in color, intended only for theteacher as welt as answers to they questions thatare included in the Student InvestigationIn some modules there are illustrations thatappear only in the Teacher s Guide, and these aredesignated, by figure letters instead of the numbersequence used in the Student Investigation

For some modules maps, rulers and othercommon classroom materials are needed, and in

t

Tr n,ateriNiwas Dfc.'Dared with thes,ir.,-r--,r; of National SciencE Foundation

Nr5s SED 75 20151 SED 77-08539and SED.78 25104 However any opinionstindlogs conc*Alons ,-)frecommendations0,orc.ssr<irr,1,1 are tnose of the authorisean': do not necessarily reflect the views')f N5F

s-irir!r tr) r mply with US Public Law

94 86 every school district in the U S AJsmq these materials agrees to make

for inspection by parents orguarolans of chlidron engaged ined attonal programs or protects of theschool 1.str,c1

Copyright 1970 Pi Southeast Missouri Stale University

Teachers and students alike have a uniqueopportunity through CEEP modules to share in theunfolding of these educationally important andexciting advances CEEP modules are designedto provide students with appealing firsthandinvestigative experiences with concepts which areat or close to the frontiers of scientific inquiry intoplate tectonics Furthermore, the tEEP modulesare designed to be used by teachers with little orno previous background in the modern theoriesof sea-floor spreading, continental drift and platetectonics

We know that you will enjoy using CEEPmodules in your classroom Read"On and beprepared to experience a renewed enthusiasm forteaching as you learn more about the living earthin this and other CEEP modules

varying quantities according to the methCSd ofpresentation Read over the module beforescheduling its use in class and refer to tl)e list ofMATERIALS in the module

Each module is individual and self contained incontent, but some are divided into two or moreparts for convenienle The recommended lengthof time for each module is indicated Some modulesrequire prerequisite knowledge of some aspectsof basic earth science, this is noted in theTeacher's Guide

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Measuring Continental Drift:The Laser Ranging, ExperimentINTRODUCTIONThis activity deals with a very complexexperiment. imagine trying to measure a change

'In distance of 1 cm between two points severalthousand kilometers'apart. Consider for a momentsome of the reasons for doing this experiment.Continental drift has never been directlymeasured. All such motions have beert iqferredfrom scientific evidence. So, a direct measurementwould be a clinching point to settle the argumentbetween ftirifters" and "nondrifters".

The experiment is designed to answer muchquestions as: 1) Are the crustal plates moving?,2) How fast are the plates moving?, 3) In whatdirection is motion taking place?, 4) Are themotions of the plates constant?, 5) Do the platesstretch?, 6) Does an entire plate move as a unit?and 7) How is the earth's rotation, linked toplate motions?

Cle Fly, the answers to"these questions Willmake major contributions to the new theories ofglobal tectonics.

PREREQUISITE STUDENT BACKGROUNDStudents should krdw how to use the PythagoreanTheorem. They should know how to measurewith a meter stick and plot points on a graph.

Scientists are interested in knowing how farthey can see on the earth's curved 'surface Thereason scientists want to see a long way isbecause they want to measure the distancebetween fixed points on two or more continentsIf they can measure this distance accurately, theywill be able to answer questidns like these1. Are the crustal plates,mciving? How fast arethey moving? In what direction are they moving?

Are the motions of the plates constant?1. Do the plates stretch?4. Does a whole plate move as one piece?

Scientists have not answered these questionsyet, but they are trying. It is not easy to measureslow movements accurately lust centimetersper yearbetween continents

OBJECTIVESAfter you have completed this activity, youshould be abld to1. Calculate how far on the earth you cap see

Tfrom a mountaintcip2. Explain what is meant by the phrase"line-of-sight."3. Explain how the curvature oi the earth affectshow far we can see along the earth's eurface.

MATERIALSMasking tapeMeter stickone for each group of studentsString

4. Discuss what happens to the distance betweena point on the earth and a point on the moon,in terms of the earth's rotation5. Graph data on a gridand draw a curve tofit the data.6. Explain how scientists might accuratelymeasure the distance between two pointslocated on different crustal plates

Figure 2 in PARR' Ei can be.constructed out Ofcardboard or heavy construction paper, but it issuggested that it bit marked orr the,floor withmasking tape.-Use the dimensions showni inFigure 2. Take seven pieces of string and stretchthem from point Y,on the moon to points A,-B, C, D, E, F. and G on the circle. The, strings will -

help the students to measure in a straight fine.The strings, should be between land 3,m long,

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BACKGROUND INFORMATIONScientists are de_visin6 instruments that can beplaced almost ahywhere on earth and thenestablish the precise location of that spot on theearth. The instrument shoots a laser beam atthe moon. The beam Is reflected from a reflectorplaced on the moon by the astronauts.

Since The astronomers know precisely' wherethe moon is located at all times, they can use thecomputer to solve from 30 to 40 simultaneousequations necessary to locate the laser instrumenton the earth. These.equations include the variablesthat astronomers use to determinje the preciseposition of the instrument. Some of theseinstruments are mobile and can be placed atmany different locations on the earth.

SUGGESTED APPROACHStudents can work individually or in small groups.The mathematics in PART A should bechecked periodically.

1

PROCEDUREPART A How far on the earthl,can we see froma mountaintop'?Students use the Pythagorean Theorem to find .

distances.Key words; right triangle, line-of-sightTime required: clne 45-minute periodMaterials...noneThe Greek mathematician Pythagoras (6thCentury B.0 ), worked out a formula for findingthe length of the sides of all right triangles. (Aright triangle is one that has one angle of 900.)Every right triangle has two sides shorter than thethirJ. Pythagoras said to take each short side,multiply it by itself, and add these two products/together. The answer will equal the longestWe multiplied by itself.

Lets apply Pythagoras' idea to our problem withthe mountain. Look at Figure 1. Find right triangleAUT. Its sides are AT, AU and UT,1. When you stand on the surface of the earth,you are 6400 km from the center of the earth. -Howfar are you from the center of the earth if youstand on a mountaintop that is 2 km above thesurface? 6402 km'. This is side AT of the

Astronomers will make mankojbservations fromthese same points during thrnext ten or moreyears. Changes of distance between any two pointswill be measured, and the rate and direction ofmovements can be established.

This complex problem involves a rotatingearth and a moving moon. However, the moon'sposition is known mathematically and is con-sidered as being fixed at the moment the moonis on the earth's meridian passing through theobserving site. Similarly, in' the activity the moonis thought of as being fixed and tie earth's turningjs the only variable being observed.

triangle AUT.2., The other sides of the triangle are AU and UT.AU js 6400 km long.You now want to find out how long UT is..Hereis how you do it:3. First multiply the length of side AT by itself.Your answer is 40,985,604 sq km.4. Second, multiply the length of side AU by itself.Your answer is 40,960,000 sq km.

2

Figure 1.

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5. Now subtract these two numbers. Your answeris 25,604 sq km6: Side UT has a length equal to the square root ofyour answer to question 5. Side UT equals

160 km. (Round off this number to the nearestones place ) This is the answer to the first questionin PART A7. XT is twice the length of side UT. Therefore,the length of XT is 320 km. This number is thedistance efttw.qen two mountains that are 2 kmhigh, if you could stand on top of one and Justbarely see the other over the curvature of theearth The line XT, along which you can see, is

PROCEDUREPART B. How can you tell when the moon anda point on the earth are closest together?Students make measurements using meter sticksand plot the data on a graph.Key words: none r

Time required: one 45-minute periodMaterials. meter sticksStart by measuring somethrig.you can touchand see In or near your classroom, your teacherhas'made a'model that looks like Figure 2

called your line-of-sight. Finding a high place fromwhich to obserVe makes your line-of-sight longer.Np mountains on earth are high enough to enableyou to see a place which is several thousandkilometers away Therefore, you,cannot measuredistances across continental plates, or platemovements, from a mbuntaintpp. You must findsome new wayto make measurements over very -long distances. This new way uses points onthe earth and some other place8. List some places from which you mighttake measurements.The moon, satellites, and other planets.

Y represents the moon and the letters A, B, C,D, E, F and G are positions on the rotating earthNotice the arrows on the arc The arrows indicatethe direction that the earth is turning,1. Using the meter stick, measure the distancealong each string from your point on the arcto point Y Record your results below.

Distance(meters)

A-Y B-Y C-Y D-Y E-Y F-Y G-Ye

Earth's surface

Figure 2. Earth-moo rlmodel.

Moon

3

2. Plot the data you collected in question 1 o)n the

'graph below.

3

B Y0

1

ptY CY D Y FY G-Y

d. When you moved from A to D, did you movetoward or away from Y (the moon)? Toward .

Going from D to G did you move toward or awayfrom the moon? Away4. Loot( at your graph. Estimate the distanceto Y if you are located between B and C

m

The answer obtained by the students will dependon the size of the construction of Figure.2.5. Now try to draw a reasonable line through thepoints on your graph Ybu can see that the line iscurved What best explains why the line is curved'You were curve in going fromA to G arid nt Y is fixed:

Think about the earth and the Moon. The earthrotates, so a fixed point on the earth movesbeneath the moon. [The moon does move s4wly,but at thiS moment consider it fixed just as Y (themoon) was fixed j One moment you are at A.relative to the moon, the next moment at Dandthird moment at G. How can vu tell when themoon anel--.3 point on the earth are-closesttogether?

In Table 1 you have been given data to ploton a graph. Look over this table. This date is anexample of the distances to the moon which wouldbe measured by a laser ranging station locatedon the equator in South America. Notic"e that the.earth-moon relationship in Figure 3 is similar towhat you measured earlier.

4v.

4 TIME DISTANCE (km)

6, a.m. 384,000.

7 a.m. 382,344

.96 a.m. 380,800

9 a.m. 379,475

101.m. 378,458

1,5\1 a.m. 377,818

12 a.m. 377,600

1 p.m. 377,818

2 p.m. 378,458

3 p.m. 379,475

4 p.m. 380,800

5 p.m 382,344

6 p.m. 384,000

Table 1 Distance from a point on the earth'ssurface to the moon measured by laser rangingequipment at a South American laser rangingstation,

Figure 3. The sketch shows how the pointrotates beneath the moan.

7

384.000

382.000

380,000

378,000

376,0006

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7 8 9 10 11 12

6. Plot the data from liable 1 on the graph aboveNote that a particular point on tie earth's surface,the center of the earth, and the moon lie on astraight tine only 4Eitne particular time during theday At that time, thrs point is closer to the moonthan it is at any other time7.. On a Pacific island, on the equator, anotherclass of students is doing the same activity. Themoon passes their meridian exactly four hourslater Plot the curve of their observations on yourgraph in question 6 (Plot the same curve, budeverything should be moved to the right 4 hours.)Look at Figure 3 Nbtice the points when the moonis on the meridian. In South America the was12 noon, on a Pacific island the time was 4 hourslater. How far is the South American stationfrom the Pacific island' To answer this questionwe need to know how far a point on the equatorrotates in one hour Astronomers tell us this isabout 1700 lqn. So the distance from the SouthAmencan'staiion to the Pacific island stationis 1700 x 4 = 6800 km.

SUMMARY QUESTIONS1.lExplain why geologists cannot measure thedistan between continents by line-of-sightmeasu ents.Beca he earth's surface curves and a point agreat distance away is hidden behind the curve.We cannot see through the ear*.2. When a point on the earth is closest to themoon, the moon is on the meridian.

2 3 4 6 7 10

8. Suppose that astronomers continue to makethis same measurement for ten years. Suppose)that by ten years later, the time difference betweenwhen the moan is on the meridian at-the twolocations has become .001 second longer thanfour hours The equipment is operating perftly,what could account for that difference'?The two points hay) drifted apak9. How far does a point on the earth's surfacemove in 001 second'?Distance = rate x timeRate = 1700 km/hr or.about 500 m/secDistance = 500 m/sec x 001 sec =

.5 m or 50 cm in 10 years,5 cm in one year

This is about as fast as your toenails grow.Do continents drift apart at ttpt rate?Geologist 'suspect so but they don't know for sure.That's why they are trying the laser rangingexperiment

REFERENCESScience News, 1974, Tracking the restless crust:

astronomital interferometric earth Surveying,v. 106, no. 8 and 9 (Aug. 24 and 31), p. 136-137.

4

3. If we know the time when the moon is on themeridian at two locations on the same parallel,what can we find?We can find the distance between the statiork4. The moon is on the meridian at an observatoryin the Hawaiian Islands at 12:00 noon. It is on themeridian at an observatory in Hong Kong at 6.00p.m Ten years later the time difference is .001

second shorter. What do you think is happening?The two locations are moving toward each other.

- 1=1Sullivan, W., 1974, Continents in motion. Flew

York, McGraw-Hill Publishing Company, 399 p.

4NAGT Crustal Evolution 'Education Project ModulesCEEP Modules are listed here*in alphabeticalorder. Each Module is designed for use inthe ntimber of class periods indicated For

_suggested sequences of CEEP Modules toof cover specific topics and for correlation

of CEEP Modules to standald earth sciencetextbooks, consult Ward's descriptiveliterature on CEEP. The Catalog Numbersshown here refer to the CLASS PACKof each Module consisting of a Teacher'sGuide and 30 copies of the StudentInvestitltion See Ward's descriptivelitevture for alternale order quantities.

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Copyright 1979 Except for the rights tornQterials reserved by others. thepublisher and the copyright °wrier herebygrant permission without Charge todomestic persons of the U S and Canadafor use of this Work and related materialsin the English language in the U S andCanada after 1985 For conditions of useqand-cermission to use the Work or any parttherebf for foreign publications orPublications in other than the Englishlanguage. apply to the copyright owner orpublisher

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,CEEP Module

A Sea-floor Mystery: MappingPolarity Reversals

Continents'And Ocean Basins:Floaters And Sinkers

Class _CLASS PACKPeriods Catalog No.

2 '3.44 Is/01201ix

. ,,3 . 34 W 1202

Crustal Movement: A Major Force 2 34 W 1203In Evolution

Deep Sea Trenches" And Radioactive 1 34 W 1204Waste

Drifting Continents And Magnetic 34 W 1205Eleldt

)Drifting Continents And WkriderIng 4 34 W 1206Poles

Earthquakes And Plate 1 34 W 1207Boundaries

Fossils As Clueg To Ancient 2 34 W 1208Continents

Hot Spots In The Earth's trust 3 34 W 1209

How Do Continents Spilt Apart? 1 34 W 1210tiow Do Scientists Decide Which Is 1 34 W 1211

,The Better Theory?How Does Heat Flow Vary In Thy 1 34 W 1217

Ocean Floor?How Fast is The Ocean Floor 2 34 W 1213

Moving?Iceland: The Case Of The Splitting 3 34 W 1214

PersonalityImaginary Continents: A Geological 1 11,- 34 V 1215

Puzzleintroduction To Lithospheric 1 34 W 1216

Plate Boundaries ---N

Lithospheric Plates And Ocean 1 34 W 1217Basin Topography

Locating Active Plate Boundaries 1 34 W 1248By Earthquake Data

Measuring Continental Drift: The , 2 34 W 1219Laser Ranging Experiment

Microfossils, Sediments And 3 34 W 1220Sea-floor Spreading

Movement Of The Pacific Ocean 1 34 W 1221floor

Plate BoundariestAnd Earthquakes 2 34 W 1222Predictions

Pion* The Shape Of The Ocean 1 34 W 1223Floor

Quake Estate (board game)Spreading Sea Floors And Fractured

The Rise And Fail Of The BeringLand Bridge .

,Troplcs In Antarctica?Volcanoes: Where And Why?What Happens When Continents

Collide?When A Piece Of A Continent

Breaks Ott,Which Way Is North?Why Does Sea Level Change? 0..

1-3 34 W 12241 34 W 1225

2 34 W 1227

2 34 W 12282 34 W 12291 34 W 1230

1 34 W 1?31

3 34 W 123234 W 1233

LORRIESWard's Nattiral Science Establishment, Inc.P.O., Box 1712, Rochester, New York 14603 P.O. Box 1749, Monterey, California 93940

-a

CUP MODULI NO. TX$ 741$1N 040.73-0314 10 PrInttI In U.S.A.

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CRUSTALEVOLUTIONEDUCATION

PROJECT

INTRODUCTIONScientists are interested in knowing how farthey can see on-the earth's curved surface. Thereason scientists want to see a long way is )t

because they want to measure the distancebetween fixed points on two or more continentsIf they can measure this distance accurately, theywill be able to answer questions like these.1. Are the crustal plates moving? How fast arethey moving? In what direction are they moving?2. Are the motions of the plates constant?3. Do the plates stretch?4. Does a whole plate move as one piece'?

Scientists have not answered these 4uestronsyet, but they are trying. It is not easy to measureslow movements accurately,justscentimetersper yearbetween continents

NAME .414 DATE

Th

Shident InvestigationCatalog No 34W1119

Measuring-ContinentalDrift: The Laser .

Rpnging Experiment

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OBJECTIVESAfter you have completed this activity, youshould be able to.1. Calculate how far on the earth you can seefrom a mol.intgintop.1,2. Explain what is meant by the,phrase"line-of-sight."3. E,k(plain how the curvature of the earth affectshow far we can see along the earth's surface,

Copyright 1979 by doutheaft Missouri State University

4. Discuss what happens to the distance betweena point on the earth and a,poirit on the moon,in terms of the easel's rotation.5. Graph data 9n a grid and draw a curve tofit the data.6. Explain how scientists- might accuratelymeasure the distance between two pointslocated on different crustal plates.

.1

1

PROCEDURE

PART A: How far on theearth can we see froma mountaintop"Materials: noneThe Greek mathematician Pythagoras (6th.Ceptury 15.C.), worked out a formula for findingthe length of the sides of, all right triangles. (Aright triangle. is one that has one angle of 90°.)Every right triangle has two sides shorter than thethird. Pythagoras said to take each short side,multiply it by itself, and add these two productstogether. The answer will equal the longest"Side multiplied by itself.

Let's apply Pythagoras' idea to our problem with$ the mountain. Look at Figure 1. Find right triangle

/WT. Its sides are AT, AU and UT.1. When ydu stand on the surface df the earth,you are 6400 km from the centert4 the earth. Howfar are you froM the center of the earth if youstand on a mountaintop that is 2 km above thesurface" km. This is side AT of thetriangle AUT.2. Thg other sides of the triangle are AU and UT.AU is km long.

You now want to find out how long UT is.is how y.ou do it:3- First multiply the length of side AT by itself.Your answer is sq km.4. Second, multiply fhe,length of side AU_by itself.Your answer is 'sq km.5. Now subtract these two numbers. Your answeris .sq km.

fit Side UT As alength equal to the square foot ofyour answer to question 5. Side UT equals

km. (Round off this number to the-nearestones plate.) This is the answer to the first questionin PART A7. XT is twice the length of side UT. Therefore,the length of XT is km. This number is thedistance between two mountains that are 2 kmhigh, if you could stand on top- of one andjustbarely see the other over the curvature of theearth. The li .e XT,Ailong which you can see, iscalled your H f-sight. Finding ahigh place fromwhich to o. your line -of- 'sight longer.No mountain- east' 'are high enough to enableyou to see a place which is several thousandkilometers away. Therefore, you cannot measuredistances across continental plates, or platemovements, from a mountaintop. You must findsome, new way to make measurements over verylong distances. This new way uses points on-the earth and some other place.

2

Figure 1.

8. List some places from which you mighttake measurements.

t

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PROCEDUREPART-B(How can you tell when the moon and

poirit on the earth are closest together?Materials: m'eter.sticksStart by measuring something you ian touchand see. In or near your c.Jassroom,,your teacherhas made a Model that looks like Figure 2.Y 'represents the moon and the letters A, B, C,

D,(E, F and G are positions on the rotating earth.1\iotice.the arrows on the arc. The arrows indicatethe direction that the earth is turning.

AI

1: Using the meter stick, measure the distanceaiongUch string.from your point on the arcto point Y. Record your results below.'

Distance(meters)

A' -N' B-Y C-Y b-Y E-Y 1 -Y.,. G-17

-47

O

,Earth's surface

Figure 2. Earth -moon model.

G

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c

Moon

A2. Plot the data you collected in question 1 on thegraph below.

13

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EC

.A-Y B-Y C-Y D-Y E Y F-Y G-Y

3

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3:. When'you moved from A to D, dia you movetoward or away from Y (the moon)?Going from D to G did you move toward or awayfrom the moon.2i ,

4. Look.at your graph. Bgtimate the distance 'to Y if you are located between B and C.

m

.5. Now try fo draw a reasonable line througtethepoints on your graph. You- can see that the line iscurved. What best explains why the line is curved?

. ,

4

ogatiink about the earth and the moon. The earth. rotates, so a fixed point on,theearth movesbeneath the moon. [The moon does move slowly,but al this moment consider it fixed just as Y (themoon) was fixed.] One momerit you are at Arelative to the moon, the next moment at D andthird moment at G. How can you tell wharl.themoon and a point on the earth are closest,together?

In Table 1 you have been given data to ploton a graph. Look over this table. This data is anexample of,the distances to the moon which wouldbe measured by a laser rangtmg station locatedon the equator in South America. Noticikthat theearth-moon relftionship in Figure 3 is similar towhat youmeasured earlier.

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TIME DISTANCE (km)6 a.m. 384,000

7 a.m. 3q,344 '8 a.m. -386,80Q

9 a.m. 4' 379,475

10 a.m. 378,458

11 a.m. 377,818

12 a.m. 377,600 ,

1 p.m. 377,818

2 p.m. 378,466

3 p.m. 379,475

4 p.m. 380,800

5 p.m. 382,344

6 p.m. 384,000..,

Tible 1 Distance from a / point on the earth'ssurface to the moon measured by laser ranging./ equipment at a South American laser rangingstation .

A A,...

4'

Figure 3. The sketch shows how the pointt 0 rotates beneath the moon.

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C

/

/384,000

382,000

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376 000

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6 7 8 9 - 10 11 12

6. Plot the data from Table 1 on the graph above.Note that a particular point on the earth's surface,the center of the earth, and the'moon lie on astraight line only at one particular time during theday At that time, this point is closer to,the moonthan it is at any other time.

,.

7. 4 a Pacific island, on the equator, anothercl s of students is doing the same activity. Themoon passes their meridian exactly four hourslater Plot the curve of their observations on your

'graph in question 6. (Plot the same curve, butever thing should be moved to the right 4 hours.)Look at Figure 3. Notice the points when the thoOnis on the meridian. In South America the time was12 noon, on a Pacific island the time was 4 hourslater How far is the South American stationfrom the Pacific island? To answer, this questionwe need to know how far a point on the equatorrotates in one hourAstronomers tell us this isabout 1700 km. So the distance from t South /-' --.American station to the Pacific island 'on 10

is 1700 x 4 =1300 km.8. Suppose that astronomers continue to makethis same measurement for ten years. Supposethat by ten years later the time difference Betweenwhen the moon is on the meridian at the twolocations has become .001 second longer than ,four hours. The equipment is operating perfectly;what could account for that difference?

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2 3 4 5 6 7 8 9 10

9. How far does a point on the earth's surfacemove in 001 second?Distance = rate x timeRate = 1700 km/hr or about 500 m/secDistance = 500 m/sec,x 001 sec =

m or cm in 10 years,cm in one year. ..

This is about as fast as your toenails growDo continents drift apart at that rate?Geologists suspect so but they don't know for sureThat's why they are trying the laser rangingexperiment

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SUMMARY QUESTIONS1. Explain why geologists cannot measure the 4. The moon is on the meridian at an observatory

distance between continents by line-of-sight in the Hawaiian Islands at 12.00 noon It is on themeasurements: meridian at an observatory in Hong'Kong at 6:00

p.m. Ten yearslater the time difference is .001,second shorter. What do ydu think is happening?

2. When a point on the earth is closest to themoon, the moon is on the3. If we know the time wheA the moon isbirttiC

34' meridian at two,locations on the same parallel,what pan we find?

REFERENCES

- Science News, 197.4, Tracking the restless crust:astronomical interferometric earth surveying.v. 106, no 8and 9 (Aug. 24 and 31), p. 136-137.

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